A method for efficiently removing hexavalent chromium by using a PVDF@PVA-FeC2O4 / BWO composite membrane

Abstract

The application discloses a method for efficiently removing hexavalent chromium by using a PVDF@PVA-FeC2O4 / BWO composite membrane. The FeC2O4 / BWO composite material and the PVA hydrophilic polymer are used together to effectively improve the hydrophilicity and strength of the PVDF membrane, and significantly improve the reduction rate of Cr(VI), so that the recycling can be realized. After seven cycles, the reduction rate of Cr(VI) of the PVDF@PVA-FeC2O4 / BWO composite membrane can still reach 98%, which shows that the composite membrane has wide application value in the field of Cr(VI) reduction. The catalytic membrane has the advantages of simple preparation, low cost, high yield and high selectivity.

Description

Technical Field

[0001] This invention belongs to the field of environmental and chemical engineering technology, and relates to the preparation of a novel photocatalytic membrane PVDF@PVA-FeC2O4 / BWO, which improves the utilization value of BWO while achieving efficient recycling of hexavalent chromium in the water environment. Background Technology

[0002] In recent years, the demand for multifunctional and lightweight materials and equipment has presented numerous challenging problems. Despite improvements in water quality over the years, chromium (Cr) exhibits harmful effects due to its toxicity and non-biodegradability. It typically exists in the form of trivalent chromium (Cr(III)) or hexavalent chromium (Cr(VI)). Cr(VI) is toxic and mutagenic to organisms, while Cr(III) is one of the essential trace elements for the human body. Reducing Cr(VI) to Cr(III) is a commonly used strategy for Cr(VI) removal.

[0003] Photocatalysis technology has attracted widespread attention in the environmental field due to its advantages such as eco-friendliness, simple operation, low cost, low energy consumption, and high efficiency. Therefore, it is considered a promising strategy for effectively addressing Cr(VI) pollution in wastewater. However, powdered photocatalytic materials have consistently suffered from drawbacks during photoreduction processes, including the potential for secondary pollution, difficulty in recycling, and poor reproducibility. To improve this, a study proposes a method to firmly bond photocatalysts to a membrane surface to prepare photocatalytic membranes, solving the problem of powdered photocatalyst recycling while improving membrane performance. Polyvinylidene fluoride (PVDF) is immiscible with most organic solvents, making it an ideal carrier for addressing the difficulty of recycling powdered materials. However, its hydrophobicity reduces its resistance to fouling and biofouling, limiting its industrial applications.

[0004] Therefore, this invention employs a simple co-precipitation method to prepare a photocatalyst doped with ferrous oxalate (FeC2O4), forming a novel visible-light-responsive FeC2O4 / Bi2 group. 15 WO6 (FeC2O4 / BWO) composite materials were prepared to enhance the photocatalytic performance of BWO and improve its utilization value. Aiming to improve the recycling of powder materials, a PVA-assisted FeC2O4 / BWO composite PVDF film (PVDF@PVA-FeC2O4 / BWO) was constructed through surface chemical grafting, which also effectively improved the hydrophilicity of the PVDF film. However, we have not yet found any technologies or patents for the preparation of this type of photocatalytic film or its application in the Cr(VI) reduction field. Summary of the Invention

[0005] To achieve the objectives of this invention, the technical solution uses a PVDF membrane as the matrix material. The prepared FeC2O4 / BWO composite material is grafted onto the membrane surface under PVA-assisted conditions to construct a PVDF@PVA-FeC2O4 / BWO composite membrane, which then undergoes a photochemical reaction to reduce hexavalent chromium, exhibiting high efficiency and cycle stability. Furthermore, compared to pure PVDF membranes, the construction of the composite membrane significantly enhances hydrophilicity and strength. The technical solution adopted in this invention is carried out according to the following steps:

[0006] In a specific embodiment, the present invention provides a method for efficiently removing hexavalent chromium using a PVDF@PVA-FeC2O4 / BWO composite membrane. The method is characterized by comprising the following steps:

[0007] (a) Weigh out Bi(NO3)3·5H2O and Na2WO4·2H2O respectively, controlling the Bi / W molar ratio to be 2.15, and dissolve them sequentially in a certain amount of 1.2 mol L. -1 HNO3 and 1.2 mol L -1 In NaOH solution, the two solutions are completely mixed, stirred for 2-4 hours, heated and cooled, then repeatedly washed with anhydrous ethanol and distilled water, and finally dried for 13-20 hours to obtain bismuth tungstate powder, denoted as BWO.

[0008] (b) Prepare 0.1 mol L of each solution. -1 (NH4)2C2O4 solution and 0.1 mol L -1 FeSO4·7H2O solution, the two solutions are completely mixed, heated and stirred for 5-7 hours, cooled, then repeatedly washed with distilled water, and finally dried for 1-3 hours to obtain ferrous oxalate powder, denoted as FeC2O4.

[0009] (c) Weigh a certain amount of BWO powder obtained in step (a) and FeC2O4 powder obtained in step (b), dissolve them in 20 mL of H2O, stir continuously at room temperature for 15-20 h, and finally dry for 2-4 h to obtain FeC2O4 / BWO composite powder; (d) Place a certain amount of dimethylformamide (DMF) solution on a magnetic stirrer, and slowly add pre-dried polyvinylidene fluoride (PVDF) powder while stirring, and stir continuously for 5-7 h under certain temperature and speed conditions to obtain homogeneous casting solution;

[0010] (e) The casting solution obtained in step (d) is uniformly coated onto the surface of the glass plate using a small automatic coating machine;

[0011] (f) After the glass plate obtained in step (e) is left in an air bath for 9-15 seconds, it is immersed in a deionized water coagulation bath for 20-25 hours. The water is then changed every once in a while to remove the residual solvent, and a PVDF membrane is obtained.

[0012] (g) Store the PVDF membrane obtained in step (f) in deionized water for later use;

[0013] (h) Prepare 200 mL of 1 mol L -1 Potassium hydroxide (KOH) solution, to which 0.5g of tetrabutylammonium fluoride (TBAF) is added, the PVDF membrane obtained in step (g) is cut into a certain size and immersed in the above solution for reaction for 20-40 minutes;

[0014] (i) Prepare 200 mL of 1 mol L -1 A sodium bisulfite (NaHSO3) solution was prepared by adding 0.1 g of 98% concentrated sulfuric acid (H2SO4) to the solution, followed by placing the membrane treated in step (h) into the solution and reacting for 5-15 min to obtain a modified PVDF membrane.

[0015] (j) Prepare a 1 wt% glutaraldehyde solution, add 0.5 wt% H2SO4 as a catalyst, and immerse the modified PVDF membrane obtained in step (i) in the solution for 3-7 min.

[0016] (k) Weigh a certain amount of FeC2O4 / BWO composite powder obtained in step (c) and a certain amount of polyvinyl alcohol PVA powder and dissolve them in 30 mL of H2O. Stir continuously at a certain temperature for 1-3 h to obtain a uniform dispersion.

[0017] (1) The PVDF membrane obtained in step (j) is directly immersed in the dispersion obtained in step (k) and reacted for 5-10 min. After drying, a PVA-assisted FeC2O4 / BWO composite PVDF membrane is obtained, which is denoted as PVDF@PVA-FeC2O4 / BWO.

[0018] (m) Store the PVDF@PVA-FeC2O4 / BWO composite membrane obtained in step (1) in a cool and dry place for later use;

[0019] (n) A PVDF@PVA-FeC2O4 / BWO composite membrane of a certain size was placed in a photocatalytic reactor, and a certain amount of hexavalent Cr solution was added to control its concentration at 10-30 mg / L. -1 And adjust to a suitable pH value;

[0020] (o) Turn on the cooling device and the light source, and irradiate the reactor with a certain light intensity to carry out the photocatalytic reduction of hexavalent Cr. After the reaction has been going on for a period of time, turn off the photocatalytic reactor.

[0021] A method for efficiently removing hexavalent chromium using a PVDF@PVA-FeC2O4 / BWO composite membrane according to claim 1, characterized in that:

[0022] (1) The conditions for preparing the PVDF@PVA-FeC2O4 / BWO composite membrane are: FeC2O4 / BWO mass concentration is 0.06wt%, and PVA mass concentration is 6wt%.

[0023] (2) The conditions for reducing hexavalent Cr using the PVDF@PVA-FeC2O4 / BWO composite membrane are: pH = 2.0, and membrane thickness is 8*8 cm. 2 ;

[0024] (3) The reduction rate of hexavalent Cr can still reach 98% after the PVDF@PVA-FeC2O4 / BWO composite membrane is used seven times.

[0025] The purpose of this invention is to provide a method for preparing a PVDF@PVA-FeC2O4 / BWO composite membrane for the efficient recycling of hexavalent chromium in the environment, while effectively improving the hydrophilicity and mechanical properties of the PVDF membrane. The photocatalytic membrane material developed in this invention provides a data foundation and theoretical support for the treatment of Cr(VI)-containing wastewater, and has significant application prospects. Attached Figure Description

[0026] Figure 1 These are SEM images of PVDF (a, c) and PVDF@PVA-FeC2O4 / BWO composite films (b, d) (a and b are the top surfaces; c and d are cross-sections).

[0027] Figure 2 This is a graph showing the variation trends of Cr(III), Cr(VI), and total Cr in the PVDF@PVA-FeC2O4 / BWO membrane system under illumination.

[0028] Figure 3 This is a cycle experiment of photocatalytic reduction of Cr(VI) by PVDF@PVA-FeC2O4 / BWO membrane under illumination.

[0029] Figure 4 These are the (a) contact angles and (b) stress-strain curves of different films. Detailed Implementation

[0030] The implementation methods and steps of the present invention are described in detail below with specific embodiments.

[0031] Example 1:

[0032] (1) Weigh out 1.96g Bi(NO3)3·5H2O and 0.62g Na2WO4·2H2O respectively (controlling the Bi / W molar ratio to be 2.15), and dissolve them in 37.5mL of 1.2mol L⁻¹ water. -1 HNO3 and 1.2 mol L -1In NaOH solution, the two solutions were completely mixed and stirred for 2 hours. Then, the mixture was heated at 200°C for 10 hours and cooled. The mixture was then washed three times with anhydrous ethanol and distilled water. Finally, it was dried at 120°C for 4 hours to obtain bismuth tungstate powder, denoted as BWO.

[0033] (2) Prepare 20 mL of 0.1 mol L solution respectively. -1 (NH4)2C2O4 solution and 0.1 mol L -1 The FeSO4·7H2O solution was thoroughly mixed with the other two solutions, heated and stirred at 90°C for 6 hours, cooled, and then washed repeatedly with distilled water 3 times. Finally, it was dried for 2 hours to obtain ferrous oxalate powder, denoted as FeC2O4.

[0034] (3) Weigh 1g of BWO powder obtained in step (1) and 0.1g of FeC2O4 powder obtained in step (2), dissolve them in 20mL H2O, stir continuously at room temperature for 16h, and finally dry for 3h to obtain FeC2O4 / BWO composite powder.

[0035] (4) Place 35g of dimethylformamide (DMF) solution on a magnetic stirrer, and slowly add 4.5g of pre-dried polyvinylidene fluoride (PVDF) powder while stirring. After stirring continuously at 90°C for 6 hours, a homogeneous casting solution is obtained.

[0036] (5) The casting liquid obtained in step (4) is uniformly coated onto the surface of the glass plate using a small automatic coating machine;

[0037] (6) After the glass plate obtained in step (5) is left in an air bath for 12 seconds, it is immersed in a deionized water coagulation bath for 24 hours. Then the water is changed every once in a while to remove the residual solvent and a PVDF membrane is obtained.

[0038] (7) Store the PVDF membrane obtained in step (6) in deionized water for later use;

[0039] (8) Prepare 200 mL of 1 mol L -1 A potassium hydroxide (KOH) solution was prepared by adding 0.5 g of tetrabutylammonium fluoride (TBAF) to step (7).

[0040] The obtained PVDF film was cut into 8*8cm pieces. 2 And immerse it in the above solution and react for 30 minutes;

[0041] (9) Prepare 200 mL of 1 mol L -1 A sodium bisulfite (NaHSO3) solution was prepared by adding 0.1 g of 98% concentrated sulfuric acid (H2SO4), followed by...

[0042] The membrane treated in step (8) is introduced and reacted for 10 min to obtain a modified PVDF membrane;

[0043] (10) Prepare a 1wt% glutaraldehyde solution, add 0.5wt% H2SO4 as a catalyst, and immerse the modified PVDF membrane obtained in step (9) in the solution for 5 min.

[0044] (11) Weigh 0.4g of the FeC2O4 / BWO composite powder obtained in step (3) and 4g of polyvinyl alcohol (PVA) powder and dissolve them in 30mL of H2O.

[0045] In the middle, stir continuously at 95℃ for 2 hours to obtain a uniform dispersion;

[0046] (12) The PVDF membrane obtained in step (10) is directly immersed in the dispersion obtained in step (11) and reacted for 6 min. After drying at 70℃ for 20 min, a PVA-assisted FeC2O4 / BWO composite PVDF membrane is obtained, which is denoted as PVDF@PVA-FeC2O4 / BWO.

[0047] (13) Store the PVDF@PVA-FeC2O4 / BWO composite membrane obtained in step (12) in a cool and dry place for later use;

[0048] (14) 8*8cm 2 The PVDF@PVA-FeC2O4 / BWO composite membrane was placed in a photocatalytic reactor, and a certain amount of hexavalent Cr solution was added to control its concentration at 20 mg / L. -1 And adjust the pH to 2.0;

[0049] (15) Turn on the cooling device and light source, at 565mW cm -1 The reactor is irradiated with light intensity to carry out the photocatalytic reduction of hexavalent Cr. After a period of time, the photocatalytic reactor is turned off.

[0050] (16) After reacting for 10 min under the conditions of step (15), the reduction rate of Cr(VI) was measured to be as high as 100%.

[0051] The PVDF@PVA-FeC2O4 / BWO composite membrane prepared using this invention exhibits high efficiency in reducing Cr(VI), good recyclability, and high stability. Furthermore, the composite membrane construction improves the hydrophilicity and mechanical properties of the PVDF membrane, broadening its applications. In addition, the preparation method of this catalytic membrane is simple and safe.

[0052] The above description is an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent process transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for efficiently removing hexavalent chromium using a PVDF@PVA-FeC2O4 / BWO composite membrane, characterized in that, Includes the following steps: (a) Weigh out Bi(NO3)3·5H2O and Na2WO4·2H2O respectively, controlling the Bi / W molar ratio to be 2.15, and dissolve them sequentially in a certain amount of 1.2 mol L- 1 HNO3 and 1.2 mol L -1 In NaOH solution, the two solutions are completely mixed, stirred for 2-4 hours, heated and cooled, then repeatedly washed with anhydrous ethanol and distilled water, and finally dried for 13-20 hours to obtain bismuth tungstate powder, denoted as BWO. (b) Prepare 0.1 mol L of each solution. -1 (NH4)2C2O4 solution and 0.1 mol L -1 FeSO4·7H2O solution, the two solutions are completely mixed, heated and stirred for 5-7 hours, cooled, then repeatedly washed with distilled water, and finally dried for 1-3 hours to obtain ferrous oxalate powder, denoted as FeC2O4. (c) Weigh a certain amount of BWO powder obtained in step (a) and FeC2O4 powder obtained in step (b), dissolve them in 20 mL of H2O, stir continuously at room temperature for 15-20 h, and finally dry for 2-4 h to obtain FeC2O4 / BWO composite powder. (d) A certain amount of dimethylformamide (DMF) solution was placed on a magnetic stirrer. While stirring was started, pre-dried polyvinylidene fluoride (PVDF) powder was slowly added. The mixture was stirred continuously for 5-7 hours under certain temperature and speed conditions to obtain a homogeneous casting solution. (e) The casting solution obtained in step (d) is uniformly coated onto the surface of the glass plate using a small automatic coating machine; (f) After the glass plate obtained in step (e) is left in an air bath for 9-15 seconds, it is immersed in a deionized water coagulation bath for 20-25 hours. The water is then changed every once in a while to remove the residual solvent, and a PVDF membrane is obtained. (g) Store the PVDF membrane obtained in step (f) in deionized water for later use; (h) Prepare 200 mL of 1 mol L -1 Potassium hydroxide (KOH) solution, to which 0.5g of tetrabutylammonium fluoride (TBAF) is added, the PVDF membrane obtained in step (g) is cut into a certain size and immersed in the above solution for reaction for 20-40 minutes; (i) Prepare 200 mL of 1 mol L -1 A sodium bisulfite (NaHSO3) solution was prepared by adding 0.1 g of 98% concentrated sulfuric acid (H2SO4) to the solution, followed by placing the membrane treated in step (h) into the solution and reacting for 5-15 min to obtain a modified PVDF membrane. (j) Prepare a 1 wt% glutaraldehyde solution, add 0.5 wt% H2SO4 as a catalyst, and immerse the modified PVDF membrane obtained in step (i) in the solution for 3-7 min. (k) Weigh a certain amount of FeC2O4 / BWO composite powder obtained in step (c) and a certain amount of polyvinyl alcohol PVA powder and dissolve them in 30 mL of H2O. Stir continuously at a certain temperature for 1-3 h to obtain a uniform dispersion. (l) The PVDF membrane obtained in step (j) is directly immersed in the dispersion obtained in step (k) and reacted for 5-10 min. After drying, a PVA-assisted FeC2O4 / BWO composite PVDF membrane is obtained, denoted as PVDF@PVA-FeC2O4 / BWO. (m) Store the PVDF@PVA-FeC2O4 / BWO composite membrane obtained in step (1) in a cool and dry place for later use; (n) A PVDF@PVA-FeC2O4 / BWO composite membrane of a certain size was placed in a photocatalytic reactor, and a certain amount of hexavalent Cr solution was added to control its concentration at 10-30 mg / L. -1 And adjust to a suitable pH value; (o) Turn on the cooling device and the light source, and irradiate the reactor with a certain light intensity to carry out the photocatalytic reduction of hexavalent Cr. After the reaction has been going on for a period of time, turn off the photocatalytic reactor.

2. The method for efficiently removing hexavalent chromium using a PVDF@PVA-FeC2O4 / BWO composite membrane according to claim 1, characterized in that: (1) The conditions for preparing the PVDF@PVA-FeC2O4 / BWO composite membrane are: FeC2O4 / BWO mass concentration is 0.06wt%, and PVA mass concentration is 6wt%. (2) The conditions for reducing hexavalent Cr using the PVDF@PVA-FeC2O4 / BWO composite membrane are: pH = 2.0, and membrane thickness is 8*8 cm. 2 ; (3) The reduction rate of hexavalent Cr can still reach 98% after the PVDF@PVA-FeC2O4 / BWO composite membrane is used seven times.

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