Preparation method of lignin blended ultrafiltration membrane

Abstract

According to the lignin modified polymer separation membrane and the preparation method thereof provided by the invention, lignin is adopted to partially replace a polymer matrix, so that the material cost is reduced and the membrane performance is improved; meanwhile, post-treatment is performed on the membrane through an ethylene oxide cross-linking agent (such as polyethylene glycol diglycidyl ether, 1, 4-butanediol diglycidyl ether and the like), a cross-linked structure is formed on the surface and in pore channels of the membrane, lignin is fixed, and the stability and anti-scouring performance of the membrane are improved.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a method for preparing a lignin blended ultrafiltration membrane. Background Technology

[0002] Ultrafiltration and nanofiltration membranes play important roles in water treatment, often using petrochemical-based polymers such as polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polysulfone (PSF) as the main membrane-forming materials. These polymers are readily available, have good processing properties, and the resulting membranes typically exhibit excellent mechanical properties and chemical stability, thus enjoying widespread use in practical applications. However, these petrochemical-based polymer membranes still have certain limitations in terms of performance and cost, requiring further improvement.

[0003] Lignin is a byproduct of biomass refining and papermaking industries, and is abundant, widely available, and inexpensive. Its molecular structure contains numerous active functional groups such as phenolic hydroxyl and methoxy groups, endowing lignin with certain hydrophilicity and chemical reactivity. Introducing lignin into petrochemical-based polymer membrane matrices such as PVDF, PES, and PSF can introduce new functional groups onto the membrane surface or within the membrane, thereby significantly improving membrane performance. Despite the numerous advantages of lignin-modified separation membranes, some shortcomings still exist in practical applications, limiting their further performance improvement and wider application. These shortcomings are manifested in the following aspects: Insufficient stability of lignin in membrane matrix: Under the existing preparation method, lignin is introduced into the membrane matrix only by physical blending or weak interaction, which makes it easy to dissolve or leak during long-term operation or chemical cleaning, thus causing the membrane performance to decline and the life of the membrane to be shortened. Insufficient scour resistance and operational stability of membranes: Water treatment processes are often accompanied by high-speed water flow or backwashing operations. Simple lignin-modified membranes are prone to damage to the membrane surface structure due to lignin migration, resulting in poor scour resistance and insufficient durability. Lack of effective fixation strategies: Currently, there is a lack of efficient methods for achieving chemical fixation of lignin in and outside the membrane matrix. Most existing methods are limited to physical effects and are difficult to form stable covalent cross-linked networks on the membrane surface and within the pores, thus failing to balance the performance advantages of lignin modification with long-term structural stability.

[0004] Therefore, how to effectively solve the leakage problem of lignin in the membrane matrix and improve the stability and erosion resistance of the membrane while reducing costs and improving membrane performance has become a key technical challenge that urgently needs to be addressed in the research and application of lignin-modified separation membranes. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0006] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0007] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a lignin blend ultrafiltration membrane.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing a lignin-blended ultrafiltration membrane, characterized in that it includes: Lignin, polymer and organic solvent are ultrasonically stirred and mixed evenly under heating conditions, and then degassed to obtain casting solution; The casting solution is poured onto the substrate surface, and the membrane is scraped during the casting process; then it is immersed in a coagulation bath for phase transformation, and the substrate is detached to obtain the lignin-based ultrafiltration membrane material. A lignin-based membrane is obtained by immersing it in an aqueous or alcoholic solution containing an ethylene oxide crosslinking agent for crosslinking treatment.

[0009] As a preferred embodiment of the preparation method described in this invention, the lignin, polymer and organic solvent are ultrasonically stirred and mixed uniformly under heating conditions, wherein the heating temperature is 70~80℃, the stirring rate is 50~100 rpm and the stirring time is 9~12 h.

[0010] In a preferred embodiment of the preparation method described in this invention, the polymer is one of polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polysulfone (PSF); the lignin accounts for 5-40% of the total mass of the casting solution; and the organic solvent system is N,N-dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or a mixture thereof with a green solvent.

[0011] As a preferred embodiment of the preparation method described in this invention, the degassing treatment is ultrasonic degassing for 1-2 hours or degassing by standing for 10-15 hours.

[0012] In a preferred embodiment of the preparation method described in this invention, the thickness of the film is controlled to be 130~160 µm during the film scraping process.

[0013] As a preferred embodiment of the preparation method described in this invention, the coagulation bath is deionized water, and the product is soaked in the coagulation bath for 10-20 hours for phase inversion and demolding.

[0014] As a preferred embodiment of the preparation method of the present invention, the lignin-based membrane is immersed in an aqueous or alcoholic solution containing an ethylene oxide crosslinking agent for crosslinking posttreatment, wherein the reaction temperature is 40~80 ℃, the time is 0.5~4 h, and the concentration of the crosslinking agent is 5~15 wt%; wherein the ethylene oxide crosslinking agent is polyethylene glycol diglycidyl ether (PEGDGE).

[0015] In a preferred embodiment of the preparation method described in this invention, the crosslinking reaction involves the ring-opening reaction between epoxy groups and hydroxyl and / or carboxyl groups in lignin molecules to form a stable covalent bond network, thereby significantly enhancing the fixation effect of lignin in the membrane matrix.

[0016] Another objective of this invention is to overcome the shortcomings of the prior art and provide a lignin blend ultrafiltration membrane.

[0017] Another objective of this invention is to overcome the shortcomings of the prior art and provide an application of a lignin blend ultrafiltration membrane.

[0018] Beneficial effects of this invention: (1) This invention reduces the amount of petrochemical-based polymers by 10–40% by introducing lignin as a partial substitute component during membrane preparation. While maintaining the mechanical and separation properties of the membrane, it effectively reduces dependence on petrochemical raw materials and significantly lowers the cost of membrane materials. This not only alleviates the impact of fluctuations in the petrochemical market on the cost of traditional membrane materials, but also reflects the sustainability of resource utilization.

[0019] (2) The hydrophilic functional groups such as phenolic hydroxyl and methoxy groups abundant in lignin molecules can effectively improve the hydrophilicity of the membrane surface, enhance the interaction between water molecules and the membrane surface, reduce the adhesion of organic and biological pollutants on the membrane surface, and thus reduce the membrane fouling tendency. This modification enables the membrane to exhibit better antifouling performance in long-term operation, prolongs its service life and reduces cleaning and maintenance costs.

[0020] (3) The preparation process of this invention only involves the blending and post-processing steps of lignin with common petrochemical-based polymers. The process route is simple, the operating conditions are mild, and it is applicable to various polymer membrane systems such as PVDF, PES, and PSF, showing good versatility and scalability. This method is not only easy to promote and apply on a large scale, but also takes into account low cost and high performance, and has significant industrialization potential. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 The images show the FTIR spectra of the ultrafiltration membranes obtained in the embodiments and comparative examples of the present invention.

[0022] Figure 2 The images shown are SEM images of the surface and cross-section of the ultrafiltration membranes obtained in the embodiments and comparative examples of the present invention.

[0023] Figure 3 These are ultrafiltration performance test diagrams of the ultrafiltration membranes obtained in the embodiments and comparative examples of the present invention.

[0024] Figure 4 The UV spectrum of the ultrafiltration membranes obtained in the embodiments and comparative examples of this invention after immersion in 70% ethanol is shown. Detailed Implementation

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0027] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0028] The polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polysulfone (PSF) used in this invention were purchased from Solvay. N,N-dimethylacetamide (DMAC), polyethylene glycol diglycidyl ether (PEGDGE), bovine serum albumin (BSA), and lignin (L) were purchased from Aladdin; Sodium hydroxide was purchased from Shanghai Lingfeng.

[0029] The membrane performance evaluation method in this invention is as follows: The formula for calculating permeability is: Where Q is the pure water flux (liters), A is the effective area (square meters), and t is the filtration time (minutes). In this experiment, A is 9.616 cm⁻¹. 2 .

[0030] The formula for calculating the retention is: Where R is the retention rate (%), C P It is the permeate concentration (g / L), C f This is the concentration of the original solution (g / L).

[0031] Example 1 This embodiment provides a method for preparing a lignin blend ultrafiltration membrane, comprising the following steps: (1) Disperse 9 wt% lignin into 82 wt% DMAC solvent under ultrasonic conditions for 1 h; (2) After placing the PVDF powder in an oven at 60°C for 24 hours to remove moisture, dissolve it in the DMAC in step (1), and stir it with ultrasound for 12 hours to obtain the casting solution. (3) The membrane was cast onto the nonwoven fabric using a 150μm doctor blade and then immersed in deionized water to undergo phase inversion, thus obtaining a membrane sample.

[0032] (4) Weigh 0.5 g of NaOH and dissolve it in 200 mL of deionized water. Stir thoroughly until the NaOH is completely dissolved. Place the resulting solution in an oil bath and heat it at 50°C. When the solution temperature reaches the set value, immerse the membrane sample prepared in step (3) into the alkaline solution and react for 6 h. After the reaction is complete, take out the membrane sample and wash it thoroughly in deionized water. The resulting membrane sample is named PVDF-L9-P0.

[0033] Example 2 This embodiment provides a method for preparing a lignin blend ultrafiltration membrane, comprising the following steps: (1) Disperse 9 wt% lignin into 82 wt% DMAC solvent under ultrasonic conditions for 1 h; (2) After placing the PVDF powder in an oven at 60°C for 24 hours to remove moisture, dissolve it in the DMAC in step (1), and stir it with ultrasound for 12 hours to obtain the casting solution. (3) The membrane was cast onto the nonwoven fabric using a 150μm doctor blade and then immersed in deionized water to undergo phase inversion, thus obtaining a membrane sample.

[0034] (4) Weigh 0.5 g NaOH and dissolve it in 200 mL of deionized water. Stir thoroughly until the NaOH is completely dissolved. Then, slowly add PEGDGE solution dropwise, so that the mass ratio of PEGDGE solution is 5% of the total solution. Place the resulting mixed solution in an oil bath and heat it at 50°C. When the solution temperature reaches the set value, immerse the membrane sample prepared in step (3) in the alkaline PEGDGE mixed solution and react for 6 h to complete the crosslinking treatment. After the crosslinking is completed, take out the membrane sample and wash it thoroughly in deionized water. The resulting membrane sample is named PVDF-L9-P5.

[0035] Example 3 The difference between this embodiment and embodiment 2 is that the mass ratio of PEGDGE solution in step (4) is adjusted to 10% of the total solution, while the remaining steps are the same as in embodiment 2. The resulting membrane sample is denoted as PVDF-L9-P10.

[0036] Example 4 The difference between this embodiment and embodiment 2 is that the mass ratio of PEGDGE solution in step (4) is adjusted to 15% of the total solution. The remaining steps are the same as in embodiment 2. The resulting membrane sample is denoted as PVDF-L9-P15.

[0037] Example 5 This embodiment provides a method for preparing a lignin blend ultrafiltration membrane, comprising the following steps: (1) Disperse 9 wt% lignin into 82 wt% DMAC solvent under ultrasonic conditions for 1 h; (2) After removing moisture by placing PES powder in an oven at 60°C for 24 hours, dissolve it in DMAC in step (1) and react with ultrasonic stirring for 12 hours to obtain casting solution. (3) The membrane was cast onto the nonwoven fabric using a 150μm doctor blade and then immersed in deionized water to undergo phase inversion, thus obtaining a membrane sample.

[0038] (4) Weigh 0.5 g NaOH and dissolve it in 200 mL of deionized water, stirring thoroughly until the NaOH is completely dissolved. Then, slowly add PEGDGE solution dropwise, making the mass ratio of PEGDGE solution 10% of the total solution. Place the resulting mixed solution in an oil bath and heat it at 50°C. When the solution temperature reaches the set value, immerse the membrane sample prepared in step (3) in the alkaline PEGDGE mixed solution and react for 6 h to complete the crosslinking treatment. After the crosslinking is completed, take out the membrane sample and wash it thoroughly in deionized water. The resulting membrane sample is named PES-L9-P10.

[0039] Example 6 The difference between this embodiment and embodiment 5 is that the PES powder in step (2) is replaced with PSF powder, and the remaining steps are the same as in embodiment 5. The resulting membrane sample is denoted as PSF-L9-P10.

[0040] Comparative Example 1 (1) After placing PVDF powder in an oven at 60°C for 24 hours to remove moisture, dissolve it in DMAC and stir it with ultrasound for 12 hours to obtain casting solution. (2) The membrane was cast onto the nonwoven fabric using a 150μm doctor blade and immersed in deionized water to undergo phase inversion, and it was labeled as PVDF-UF.

[0041] Comparative Example 2 (1) Disperse 9 wt% lignin into 82 wt% DMAC solvent under ultrasonic conditions for 1 h; (2) After placing the PVDF powder in an oven at 60°C for 24 hours to remove moisture, dissolve it in the DMAC in step (1), and stir it with ultrasound for 12 hours to obtain the casting solution. (3) The membrane was cast onto the nonwoven fabric using a 150μm doctor blade and immersed in deionized water to undergo phase inversion, and it was labeled as PVDF-L9.

[0042] Figure 1 The images show the ATR-FTIR spectra of samples from Examples 1-4 and Comparative Example 2. As can be seen from the figures, at 877 cm⁻¹... -1 The absorption peak at 1072 cm⁻¹ corresponds to the C–C stretching vibration. -1 It is a C–F stretching vibration, 1401 cm. -1 The absorption peak at 836 cm⁻¹ corresponds to the C–H vibration, indicating the presence of PVDF polymer chain structures in the sample. -1 With 775 cm -1 The absorption peak at 3400 cm⁻¹ is attributed to the bending vibration of the C–H ring in the lignin aromatic ring, while the peak at 3400 cm⁻¹ represents the stretching vibration of the hydroxyl groups in the lignin molecule, indicating that lignin has been successfully incorporated into the membrane material. Furthermore, in the PVDF-L9-P0 sample, the absorption peak at 1650 cm⁻¹ is also significant. -1 With 1545 cm -1 The absorption peaks appearing at [location] correspond to the carbonyl and carboxylic acid or carboxylate structural signals generated after the thermal degradation and oxidation of lignin under alkaline heating conditions. Furthermore, in PVDF-L9-P5, PVDF-L9-P10, and PVDF-L9-P15 samples, an absorption peak at 720 cm⁻¹ was observed. -1 The long chain of PEGDGE –(CH2) appears at this location. nThe methylene rocking vibration peak indicates that PEGDGE undergoes a ring-opening reaction under high temperature and alkaline conditions, and reacts with the hydroxyl groups on the lignin chain to form intermolecular and intramolecular cross-links, thereby constructing a highly cross-linked three-dimensional network structure.

[0043] Figure 2 The following are the SEM characterization results of Examples 1-6 and Comparative Examples 1 and 2. As shown in Figure (a), the PVDF-UF membrane without lignin exhibits a typical finger-like pore structure. After lignin is introduced (Figure dh), the size of the finger-like pores in the membrane cross-section increases significantly, mainly due to the large amount of lignin precipitated during the non-solvent phase transformation. Compared to the uncrosslinked PVDF-L9 membrane (Figure d), the surface morphology of the membrane treated with PEGDGE (Figure fh) changes significantly. It can be seen that before crosslinking, the PVDF-L9-P10 membrane surface has a relatively obvious pore structure; with the increase of the crosslinking agent concentration, the pore size gradually decreases, indicating that the crosslinking reaction effectively regulates the membrane surface pore structure. However, in the PVDF-L9-P0 sample without added crosslinking agent, the membrane surface structure is damaged due to the destructive effect of the high-temperature alkaline environment. Similarly, the PES-L9-P10 and PSF-L9-P10 membranes (Figures bc) also exhibit a dense membrane surface after PEGDGE treatment.

[0044] Figure 3 The ultrafiltration performance test results of Examples 1-6 and Comparative Examples 1 and 2 are shown. The sample was cut to 9.616 cm... 2 The membrane sample was installed in the cross-flow filtration device, pre-pressed at 2 bar for 20 min, then adjusted to 1 bar and BSA solution was introduced for filtration testing.

[0045] The results showed that with the introduction of lignin, the pure water flux of the membrane was significantly higher than that of the PVDF-UF membrane without lignin. PEGDGE undergoes ring-opening under alkaline heating conditions, covalently cross-linking with the hydroxyl groups in the lignin molecules to form a stable three-dimensional network structure. The BSA rejection rate of the membrane treated with PEGDGE cross-linking was significantly improved. Specifically, the BSA rejection rate of the PVDF-L9-P10 sample increased from 22% of the uncross-linked PVDF-L9 membrane to 89.425%, indicating that PEGDGE cross-linking significantly improved the membrane's separation performance.

[0046] The flux rejection effects of the nanofiltration membranes obtained in Examples 1-6 and Comparative Examples 1 and 2 on bovine serum albumin aqueous solution are as described in the specification. Figure 3 As shown in the figure. The specific test results are shown in Table 1.

[0047] Table 1

[0048] Figure 4 The UV-Vis full spectrum of the samples from Examples 1-4 and Comparative Example 1 after soaking in 200 mL of 70% ethanol solution for different times (0, 1, 6, 24 h) is shown to characterize the leakage behavior of lignin. A characteristic absorption peak of lignin appears at 280 nm. For the untreated PVDF-L9 membrane, the absorbance at 280 nm continuously increases with prolonged soaking time, indicating that lignin gradually dissolves from the membrane. However, the absorbance at 280 nm of the sample treated with PEGDGE crosslinking significantly decreases, indicating that the crosslinking reaction effectively inhibits lignin leakage. Specifically, after soaking in 70% ethanol solution for 24 h, the absorbance at 280 nm of the PVDF-L9-P10 sample decreased from 0.075 to 0.012, indicating that after PEGDGE crosslinking, the lignin in the membrane forms a dense and stable crosslinked network structure with PEGDGE, thereby significantly improving the membrane stability and effectively preventing lignin migration.

[0049] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A method for preparing a lignin-blended ultrafiltration membrane, characterized in that: include, Lignin, polymer and organic solvent are ultrasonically stirred and mixed evenly under heating conditions, and then degassed to obtain casting solution; The casting solution is poured onto the substrate surface, and the membrane is scraped during the casting process; then it is immersed in a coagulation bath for phase transformation, and the substrate is detached to obtain the lignin-based ultrafiltration membrane material. A lignin-based membrane is obtained by immersing it in an aqueous or alcoholic solution containing an ethylene oxide crosslinking agent for crosslinking treatment.

2. The preparation method according to claim 1, characterized in that: The lignin, polymer, and organic solvent are ultrasonically stirred and mixed uniformly under heating conditions, wherein the heating temperature is 70~80℃, the stirring rate is 50~100 rpm, and the stirring time is 9~12 h.

3. The preparation method according to claim 2, characterized in that: The polymer is one of polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polysulfone (PSF); the lignin accounts for 5-40% of the total mass of the casting solution; the organic solvent system is N,N-dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or a mixture thereof with a green solvent.

4. The preparation method according to claim 1, characterized in that: The degassing treatment is performed by ultrasonic degassing for 1-2 hours or by standing for 10-15 hours.

5. The preparation method according to claim 1, characterized in that: The thickness of the film is controlled to be 130~160 µm during the scraping process.

6. The preparation method according to claim 1, characterized in that: The coagulation bath is deionized water, and the product is soaked in the coagulation bath for 10-20 hours for phase inversion and demolding.

7. The preparation method according to claim 1, characterized in that: The lignin-based membrane is impregnated in an aqueous or alcoholic solution containing an ethylene oxide crosslinking agent for crosslinking posttreatment, wherein the reaction temperature is 40-80℃, the time is 0.5-4 h, and the concentration of the crosslinking agent is 5-15 wt%; the ethylene oxide crosslinking agent is polyethylene glycol diglycidyl ether (PEGDGE).

8. The preparation method according to claim 7, characterized in that: The cross-linking reaction involves ring-opening reactions between epoxy groups and hydroxyl and / or carboxyl groups in lignin molecules to form a stable covalent bond network, thereby significantly enhancing the fixation effect of lignin in the membrane matrix.

9. The lignin blend ultrafiltration membrane prepared by any one of the preparation methods described in claims 1 to 8.

10. Application of the lignin blend ultrafiltration membrane as described in claim 9.

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