A lignin-based polyurethane porous nanofiltration membrane, its preparation method and application

Abstract

This invention discloses a lignin-based polyurethane loose nanofiltration membrane, its preparation method, and its application. The method includes the following steps: (1) drying lignin, mixing the dried lignin with dimethyl sulfoxide and stirring to fully dissolve the lignin, obtaining a lignin solution; (2) slowly adding hexamethylene diisocyanate to the lignin solution, followed by adding dibutyltin dilaurate, to obtain a casting solution containing lignin-based polyurethane; (3) degassing the casting solution, then uniformly pouring the casting solution onto a nonwoven fabric, scraping it into a film using a film scraper, and then transferring it to a deionized water coagulation bath for phase inversion. After the phase inversion is completed, a loose nanofiltration membrane with a nonwoven fabric as the support layer and a lignin-based polyurethane membrane as the separation layer is obtained. This invention uses lignin and hexamethylene diisocyanate as the main raw materials to prepare a polyurethane loose nanofiltration membrane, and the prepared membrane has a relatively high rejection capacity for anionic dye solutions.

Description

Technical Field

[0001] This invention relates to the field of polyurethane nanofiltration membrane technology, specifically to a lignin-based polyurethane porous nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Dyes are widely used in various industries, such as textiles and leather, and their production generates a large amount of dye-polluted wastewater. The textile industry alone accounts for 20% of global industrial wastewater discharge. Traditional water treatment methods are inefficient, costly, and complex to operate. Membrane separation technology, on the other hand, has shown great application potential in overcoming the shortcomings of traditional water treatment methods due to its advantages such as low energy consumption, high efficiency, simple process, and environmental friendliness.

[0003] Loose nanofiltration membranes possess advantages such as low operating pressure, high flux, large pore size, and effective dye retention. They can efficiently retain organic dyes and other organic matter while maintaining high permeability to monovalent and divalent salt ions, thus effectively separating and reusing organic matter and inorganic salts in high-salt organic wastewater. Their separation mechanism is mainly based on steric hindrance and charge action; uncharged dye molecules are retained through pore size sieving, while charged dye molecules are selectively separated by regulating the membrane surface charge. Therefore, loose nanofiltration membranes, achieving high inorganic salt permeability while efficiently removing organic matter, are gradually becoming an important separation medium for treating high-salt dye wastewater.

[0004] However, currently, loose nanofiltration membranes are mainly petroleum-based polymer membranes, whose production relies on fossil fuels, which are non-renewable resources. Their extraction and processing are energy-intensive and accompanied by carbon emissions, exacerbating resource depletion and climate change problems. Furthermore, they are difficult to degrade in the natural environment, which is inconsistent with the concept of green and environmentally friendly development. Based on this, this invention proposes a lignin-based polyurethane loose nanofiltration membrane, its preparation method, and its applications. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a lignin-based polyurethane loose nanofiltration membrane, its preparation method, and its application. The separation membrane is prepared using lignin as a matrix, reducing the amount of petroleum-based organic matter in the separation membrane and promoting green and sustainable development. Furthermore, the loose nanofiltration membrane of this invention has a high rejection rate for anionic dyes and can be used for dye wastewater treatment.

[0006] The technical solution adopted in this invention is as follows: In a first aspect, the present invention provides a method for preparing a lignin-based polyurethane porous nanofiltration membrane, comprising the steps of: (1) The lignin is dried, and the dried lignin is mixed with dimethyl sulfoxide and stirred to fully dissolve the lignin to obtain a lignin solution; (2) Hexamethylene diisocyanate is slowly added dropwise to the lignin solution obtained in step (1), and then dibutyltin dilaurate is added dropwise. The lignin and hexamethylene diisocyanate undergo an esterification reaction to obtain a casting solution containing lignin-based polyurethane. (3) The casting solution in step (2) is degassed, and then the casting solution is poured evenly onto the nonwoven fabric. The casting solution is scraped onto the nonwoven fabric using a film scraper until it forms a film. Then it is transferred to a deionized water coagulation bath for phase inversion and film formation. After the phase inversion is completed, it is taken out to obtain a loose nanofiltration membrane with nonwoven fabric as the support layer and lignin-based polyurethane membrane as the separation layer.

[0007] Furthermore, in step (1), the lignin is alkali lignin with an ash content of 1.0~1.5% and a total hydroxyl content of 3~4 mmol / g; and the lignin is dried in a vacuum oven at 50~70℃ for 15~30h.

[0008] Furthermore, the mass fraction of lignin in the lignin solution of step (1) is 14-18%.

[0009] Furthermore, in step (2), the mass ratio of lignin solids content to hexamethylene diisocyanate is 0.7~1.2:1.

[0010] In the above technical solution, the main components of the polyurethane loose nanofiltration membrane obtained by the esterification reaction of lignin and hexamethylene diisocyanate are lignin and hexamethylene diisocyanate, and the amount of lignin added reaches 40~55%.

[0011] Furthermore, in step (2), the dropping rate of hexamethylene diisocyanate is 3~8 s / drop.

[0012] Furthermore, in step (2), the mass ratio of the amount of dibutyltin dilaurate added to the lignin solid content is 0.005~0.2:1.

[0013] Furthermore, in step (2), the esterification reaction time of lignin and hexamethylene diisocyanate is 1-4 hours.

[0014] Furthermore, in step (3), the gap between the scrapers on the film scraper is 100~200μm; the temperature of the deionized water coagulation bath is 15~25℃; and the phase transformation time is 10~25min.

[0015] In a second aspect, the present invention provides a lignin-based polyurethane loose nanofiltration membrane, wherein the lignin-based polyurethane loose nanofiltration membrane is electronegative.

[0016] Thirdly, the present invention provides an application of a lignin-based polyurethane porous nanofiltration membrane in the separation of solutions containing anionic dyes.

[0017] The beneficial effects of this invention are as follows: (1) This invention provides a lignin-based polyurethane loose nanofiltration membrane, which is composed of a non-woven fabric support layer and a lignin-based polyurethane separation layer. The lignin-based polyurethane separation layer uses lignin and hexamethylene diisocyanate as the main raw materials. The amount of lignin used reaches 40-55%, which is a large amount and enhances the application of lignin in the separation membrane. The application of lignin materials is conducive to green and sustainable development. (2) This invention provides a lignin-based polyurethane loose nanofiltration membrane, which is prepared by a one-pot method and a phase inversion method. The operation is simple and the reaction conditions are mild. (3) The present invention uses a loose nanofiltration membrane prepared from lignin as a negatively charged membrane, which has a high rejection rate for anionic dyes but a low rejection rate for monovalent and divalent inorganic salts. When used for the treatment of wastewater containing anionic dyes, it can separate anionic dyes from inorganic salts, realize the separate recovery of dyes and inorganic salts in wastewater, not only reduce the pollution of dye wastewater to the environment, but also realize the recycling of resources. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in 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.

[0019] Figure 1 This is a schematic diagram of the reaction process between lignin hydroxyl groups and hexamethylene diisocyanate cyano groups in this invention; Figure 2 The attenuated total reflectance-Fourier transform infrared spectra and X-ray electron spectra of the membranes and lignin (SWKL) prepared in Examples 2-4 of this invention are shown, where (a) is the infrared spectrum and (b) is the X-ray electron spectrum. Figure 3 These are schematic diagrams of the cross-sectional morphology of the lignin-based polyurethane loose nanofiltration membranes prepared in Examples 2-4 of the present invention, wherein (a), (b), and (c) are cross-sectional morphology diagrams of the lignin-based polyurethane loose nanofiltration membranes prepared in Examples 2, 3, and 4, respectively. Figure 4 The results show the retention rates of the lignin-based polyurethane porous nanofiltration membranes prepared in Examples 1-5 of this invention for single salt solutions. Figure 5 The results show the retention rates of the lignin-based polyurethane loose nanofiltration membranes prepared in Examples 1-5 of this invention for single anionic dye solutions. Figure 6The graph shows the trend of the rejection rate of the lignin-based polyurethane loose nanofiltration membrane prepared in Example 3 of the present invention for a single anionic dye solution and a neutral polyethylene glycol (PEG) solution with different molecular weights. Figure 7 The calculation results of the contribution rate of the lignin-based polyurethane loose nanofiltration membrane prepared in Example 3 of the present invention to the pore size sieving and electrostatic repulsion of a single anionic dye. Figure 8 The graph shows the Zate potential variation trend of the lignin-based polyurethane loose nanofiltration membranes prepared in Examples 2-4 of this invention at different pH values. Figure 9 This is a schematic diagram of the retention performance of the lignin-based polyurethane loose nanofiltration membrane prepared in Example 3 of the present invention on the salt-dye composite pollution system, wherein (a) is a schematic diagram of the retention performance of the NaCl-CR composite pollution system; and (b) is a schematic diagram of the retention performance of the NaCl-MB composite pollution system. Detailed Implementation

[0020] This invention provides a lignin-based polyurethane porous nanofiltration membrane, its preparation method, and its applications. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] For ease of understanding, it should be noted here first that there are no special requirements for the non-woven fabric used in the following embodiments of the present invention; commercially available ordinary non-woven fabric can be used.

[0022] Example 1 This embodiment provides a method for preparing a lignin-based polyurethane porous nanofiltration membrane, including the following steps: (1) Alkali lignin with 1.2% ash and 3.4 mmol / g total hydroxyl content was placed in a vacuum oven and dried at 60°C for 24 h to remove moisture; the dried lignin was mixed with dimethyl sulfoxide and stirred at 40°C for 1 h to fully dissolve the lignin and obtain a lignin solution with a lignin content of 16 wt%. (2) Hexamethylene diisocyanate was slowly added dropwise to the lignin solution at a rate of 5s / drop through a constant pressure dropping funnel, and the mass ratio of lignin solid content to the added hexamethylene diisocyanate was 1.2:1. Then dibutyltin dilaurate was added dropwise, and the reaction was carried out for 2 hours. The amount of dibutyltin dilaurate added was 1wt% of the lignin solid content. During this process, lignin and hexamethylene diisocyanate underwent an esterification reaction, that is, the reaction of cyano group and hydroxyl group, to obtain a casting solution containing lignin-based polyurethane. (3) After the esterification reaction is completed, the casting solution is degassed and then the casting solution is poured evenly onto the non-woven fabric laid on the glass plate. A film scraper with a certain gap (150μm) is used to scrape the film at a constant speed. The scraped film is then immediately transferred into a deionized water coagulation bath at 25℃ for phase inversion to form a membrane. After 20 minutes, the phase inversion is completed. The non-woven fabric is removed to obtain a loose nanofiltration membrane with non-woven fabric as the support layer and lignin-based polyurethane membrane as the separation layer, which is denoted as M1. (4) Soak the lignin-based polyurethane loose nanofiltration membrane M1 in deionized water for 12 hours to remove residual solvent from the membrane pores.

[0023] Example 2 This embodiment provides a method for preparing a lignin-based polyurethane porous nanofiltration membrane, including the following steps: (1) Alkali lignin with 1.2% ash content and 3.4 mmol / g total hydroxyl content was placed in a vacuum oven and dried at 60°C for 24 h to remove moisture; the dried lignin was mixed with dimethyl sulfoxide and stirred at 40°C for 1 h to fully dissolve the lignin and obtain a lignin solution with a lignin content of 17 wt%. (2) Hexamethylene diisocyanate was slowly added dropwise to the lignin solution at a rate of 5s / drop through a constant pressure dropping funnel, and the mass ratio of lignin solid content to the added hexamethylene diisocyanate was 1.1:1. Then dibutyltin dilaurate was added dropwise, and the reaction was carried out for 2 hours. The amount of dibutyltin dilaurate added was 1wt% of the lignin solid content. During this process, lignin and hexamethylene diisocyanate underwent an esterification reaction, that is, the reaction of cyano group and hydroxyl group, to obtain a casting solution containing lignin-based polyurethane. (3) After the esterification reaction is completed, the casting solution is degassed and then the casting solution is poured evenly onto the non-woven fabric laid on the glass plate. A film scraper with a certain gap (150μm) is used to scrape the film at a constant speed. The scraped film is then immediately transferred into a deionized water coagulation bath at 25℃ for phase inversion to form a membrane. After 20 minutes, the phase inversion is completed. The non-woven fabric is removed to obtain a loose nanofiltration membrane with non-woven fabric as the support layer and lignin-based polyurethane membrane as the separation layer, which is denoted as M2. (4) Soak the lignin-based polyurethane loose nanofiltration membrane M2 in deionized water for 12 hours to remove residual solvent in the membrane pores.

[0024] Example 3 This embodiment provides a method for preparing a lignin-based polyurethane porous nanofiltration membrane, including the following steps: (1) Alkali lignin with 1.2% ash and 3.4 mmol / g total hydroxyl content was placed in a vacuum oven and dried at 60°C for 24 h to remove moisture; the dried lignin was mixed with dimethyl sulfoxide and stirred at 40°C for 1 h to fully dissolve the lignin and obtain a lignin solution with a lignin content of 16 wt%. (2) Hexamethylene diisocyanate was slowly added to the lignin solution at a rate of 5 drops / s through a constant pressure dropping funnel, and the mass ratio of lignin solid content to the added hexamethylene diisocyanate was 1:1. Then dibutyltin dilaurate was added, and the reaction was carried out for 2 hours. The amount of dibutyltin dilaurate added was 1 wt% of the lignin solid content. During this process, lignin and hexamethylene diisocyanate underwent an esterification reaction, that is, the reaction of cyano group and hydroxyl group, to obtain a casting solution containing lignin-based polyurethane. (3) After the esterification reaction is completed, the casting solution is degassed and then the casting solution is poured evenly onto the non-woven fabric laid on the glass plate. A film scraper with a certain gap (150μm) is used to scrape the film at a constant speed. The scraped film is then immediately transferred into a deionized water coagulation bath at 25℃ for phase inversion to form a membrane. After 20 minutes, the phase inversion is completed. The non-woven fabric is removed to obtain a loose nanofiltration membrane with non-woven fabric as the support layer and lignin-based polyurethane membrane as the separation layer, which is denoted as M3. (4) Soak the lignin-based polyurethane loose nanofiltration membrane M3 in deionized water for 12 hours to remove residual solvent in the membrane pores.

[0025] Example 4 This embodiment provides a method for preparing a lignin-based polyurethane porous nanofiltration membrane, including the following steps: (1) Alkali lignin with an ash content of 1.2% and a total hydroxyl content of 3.4 mmol / g was placed in a vacuum oven and dried at 60°C for 24 h to remove moisture; the dried lignin was mixed with dimethyl sulfoxide and stirred at 40°C for 1 h to fully dissolve the lignin and obtain a lignin solution with a lignin content of 15 wt%. (2) Hexamethylene diisocyanate was slowly added dropwise to the lignin solution at a rate of 5s / drop through a constant pressure dropping funnel, and the mass ratio of lignin solid content to the added hexamethylene diisocyanate was 0.9:1. Then dibutyltin dilaurate was added dropwise, and the reaction was carried out for 2 hours. The amount of dibutyltin dilaurate added was 1wt% of the lignin solid content. During this process, lignin and hexamethylene diisocyanate underwent an esterification reaction, that is, the reaction of cyano group and hydroxyl group, to obtain a casting solution containing lignin-based polyurethane. (3) After the esterification reaction is completed, the casting solution is degassed and then the casting solution is poured evenly onto the non-woven fabric laid on the glass plate. A film scraper with a certain gap (150μm) is used to scrape the film at a constant speed. The scraped film is then immediately transferred into a deionized water coagulation bath at 25℃ for phase inversion to form a membrane. After 20 minutes, the phase inversion is completed. The non-woven fabric is removed to obtain a loose nanofiltration membrane with non-woven fabric as the support layer and lignin-based polyurethane membrane as the separation layer, which is denoted as M4. (4) Soak the lignin-based polyurethane loose nanofiltration membrane M4 in deionized water for 12 hours to remove residual solvent from the membrane pores.

[0026] Example 5 This embodiment provides a method for preparing a lignin-based polyurethane porous nanofiltration membrane, including the following steps: (1) Alkali lignin with 1.2% ash content and 3.4 mmol / g total hydroxyl content was placed in a vacuum oven and dried at 60°C for 24 h to remove moisture; the dried lignin was mixed with dimethyl sulfoxide and stirred at 40°C for 1 h to fully dissolve the lignin and obtain a lignin solution with a lignin content of 14 wt%. (2) Hexamethylene diisocyanate was slowly added dropwise to the lignin solution at a rate of 5s / drop through a constant pressure dropping funnel, and the mass ratio of lignin solid content to the added hexamethylene diisocyanate was 0.8:1. Then dibutyltin dilaurate was added dropwise, and the reaction was carried out for 2 hours. The amount of dibutyltin dilaurate added was 1wt% of the lignin solid content. During this process, lignin and hexamethylene diisocyanate underwent an esterification reaction, that is, the reaction of cyano group and hydroxyl group, to obtain a casting solution containing lignin-based polyurethane. (3) After the esterification reaction is completed, the casting solution is degassed and then the casting solution is poured evenly onto the non-woven fabric laid on the glass plate. A film scraper with a certain gap (150μm) is used to scrape the film at a constant speed. The scraped film is then immediately transferred into a deionized water coagulation bath at 25℃ for phase inversion to form a membrane. After 20 minutes, the phase inversion is completed. The non-woven fabric is removed to obtain a loose nanofiltration membrane with non-woven fabric as the support layer and lignin-based polyurethane membrane as the separation layer, which is denoted as M5. (4) Soak the lignin-based polyurethane loose nanofiltration membrane M5 in deionized water for 12 hours to remove residual solvent in the membrane pores.

[0027] Reference Figure 1 In Examples 1-5 above, lignin undergoes an esterification reaction with hexamethylene diisocyanate, i.e., a reaction between cyano and hydroxyl groups, to obtain lignin-based polyurethane.

[0028] Before conducting tests on the lignin-based polyurethane loose nanofiltration membranes M1-M5 prepared in Examples 1-5, all membranes were stored in deionized water, with the deionized water being replaced periodically (every 24 hours). Before testing, the membranes were removed, and for infrared spectroscopy and X-ray electron spectroscopy, the membranes needed to be dried.

[0029] The lignin-based polyurethane loose nanofiltration membranes and lignin SWKL prepared in Examples 2-4 above were subjected to attenuated total reflectance-Fourier transform infrared spectroscopy and X-ray electron spectroscopy, respectively. The test results are as follows: Figure 2 As shown. Figure 2 As shown in (a), the N content in lignin is very low, and a new nitrogen peak appears in M4, indicating that HDI was added in excess and the reaction produced primary amine and CO2. Figure 2 As shown in (b), M2~M4 are at 1720cm -1 A new absorption peak appears at 2270 cm⁻¹, indicating a successful reaction due to the CO stretching vibration in the carbamate bond. Additionally, M4 shows an absorption peak at 2270 cm⁻¹. -1 The appearance of a new absorption peak indicates the tensile vibration of free -NCO in the raw material hexamethylene diisocyanate, proving that too much HDI was added. The above test results demonstrate the successful reaction between lignin and hexamethylene diisocyanate.

[0030] The lignin-based polyurethane porous nanofiltration membranes prepared in Examples 2-4 above were characterized by SEM testing. The test results are as follows: Figure 3 As shown (same scale). From Figure 3 As can be seen, the cross-sections of M2 to M4 have a typical asymmetric finger-shaped hole structure, with long and uniformly distributed pores.

[0031] The lignin-based polyurethane porous nanofiltration membranes prepared in Examples 1-5 above were subjected to single salt retention tests. At a pressure of 3.0 bar, the retention performance of different lignin-based polyurethane porous nanofiltration membranes for single monovalent and divalent salts (sodium chloride and sodium sulfate) at a concentration of 2000 ppm was tested. The results are as follows: Figure 4 As shown. From Figure 4 It can be seen that the retention rates of NaCl for M1~M5 are 0.26%, 0.31%, 2.64%, 0.44%, and 0.19%, respectively, with the retention rate increasing first and then decreasing. The retention rates of Na2SO4 are 6.36%, 12.89%, 19.56%, 15.28%, and 10.39%, respectively, with the retention rate increasing first and then decreasing.

[0032] The lignin-based polyurethane porous nanofiltration membranes prepared in Examples 1-5 above were subjected to single dye retention tests. At a pressure of 3.0 bar, the retention performance of different lignin-based polyurethane porous nanofiltration membranes for a single dye at a concentration of 50 ppm was tested: methylene blue MB (Mw=799.8), Congo red CR (Mw=696.66), and primrose oil Pr (Mw=475.54). The pH of the dye solutions was neutral in all cases. The results are as follows: Figure 5 As shown. From Figure 5 The results show that the retention rates of M1 to M5 for methylene blue were 84.59%, 94.88%, 98.73%, 93.62%, and 83.29%, respectively; the retention rates for Congo red were 77.35%, 92.78%, 97.36%, 90.01%, and 74.29%, respectively; and the retention rates for primrosein were 76.74%, 91.27%, 93.41%, 89.66%, and 73.62%, respectively. All of these results show a trend of initial increase followed by decrease in retention rates.

[0033] Regarding the above Figure 4 and Figure 5 Analysis of the test results showed that the lignin-based polyurethane loose nanofiltration membrane could achieve a dye rejection rate of over 90%, while the rejection rate for monovalent salts was only 2.64% and the rejection rate for divalent salts was only 19.56%. The rejection rate of dyes by the lignin-based polyurethane loose nanofiltration membrane was much higher than that for monovalent and divalent salts.

[0034] Furthermore, the membranes prepared in Examples 1-5 exhibit different retention capacities for different dyes, which may be related to factors such as steric hindrance (pore size sieving) and electrostatic repulsion. The membrane M3 prepared in Example 3 was used to test the retention rates of three anionic dyes—Pr (475 Da), CR (697 Da), and MB (800 Da)—and neutral polyethylene glycol (PEG) solutions with different molecular weights (PEG400, PEG600, PEG800). The test results were fitted to obtain the trend of retention rate as a function of molecular weight, as shown below. Figure 6 As shown. From Figure 6It can be observed that the retention rates of membrane M3 for Pr (93.4%), CR (97.4%), and MB (99.1%) are significantly higher than those for neutral polyethylene glycol molecules of the same molecular weight (475, 697, and 800 Da), at 17.3%, 22.0%, and 23.1%, respectively. This indicates that, in addition to pore size sieving, the electrostatic interaction between the negatively charged membrane and anionic dyes plays an important role in achieving high dye retention rates. To investigate the contributions of pore size sieving and electrostatic repulsion to the retention rate, the ratio of the retention rate of PEG with the same molecular weight to the retention rate of the corresponding dye was calculated. For example, the retention rate of PEG with a molecular weight of 475 Da (close to Pr) was 17.3%, while the retention rate of Pr was 93.4%. The contribution of the membrane to the pore size sieving of dye Pr was calculated to be 18.5% (PEG retention rate / Pr retention rate), and the contribution of electrostatic repulsion was 81.5% (100 - contribution rate of pore size sieving). Similarly, the contribution of the membrane to the pore size sieving of dye CR was 22.6%, and the contribution of electrostatic repulsion was 77.4%; the contribution of the membrane to the pore size sieving of dye MB was 25.2%, and the contribution of electrostatic repulsion was 75.8%. The calculated contribution rates are summarized as follows: Figure 7 As shown. From Figure 7 It can be observed that the contribution rate of electrostatic repulsion is much greater than that of pore size sieving, and the contribution rate of pore size sieving increases with the increase of dye molecular weight, rising from 18.5% to 25.2%. In addition, the above test and analysis results also indicate that the retention of different dyes in a single dye solution can be attributed to the combined effect of pore size sieving and electrostatic repulsion.

[0035] Based on the above analysis of the factors affecting the dye rejection rate of lignin-based polyurethane loose nanofiltration membranes, we will continue to investigate the above... Figure 4 and Figure 5 Analysis of the test results showed that when the mass ratio of lignin to hexamethylene diisocyanate decreased, the retention rates for both single dyes and single salts first increased and then decreased. Since the pore sizes of the membranes prepared in Examples 1-5 were not significantly different ( Figure 3 Since the pore sizes of membranes M2, M3, and M4 are basically the same, this variation may be related to the electronegativity of the membrane. To characterize the electronegativity of the membrane, the Zate potentials of membranes M2, M3, and M4 prepared in Examples 2-4 were measured at different pH values ​​(test solution: 0.075 g·L⁻¹). -1 The KCl solution was prepared using 0.1 mol·L⁻¹. -1 HCl solution and 0.1 mol·L -1 pH adjustment with NaOH solution, such as Figure 8 As shown. From Figure 8It can be seen that when the pH is greater than 4.3, the membrane electronegativity first increases and then decreases with the increase of hexamethylene diisocyanate (HDI) dosage, thus affecting the retention rates of salts and dyes. The main reason for the change in membrane electronegativity is that when a small amount of HDI reacts, the lignin molecular chains change from coiled to extended and fixed in the three-dimensional network, allowing the negatively charged groups inside to be fully exposed on the membrane surface and in the pores. When the amount of HDI increases to an excess, there will be unreacted HDI. This part of HDI reacts with water during the phase transformation process, first forming unstable carbamic acid, and then generating polyurea long chains. The polyurea long chains cover the membrane surface, thereby weakening the electronegativity of the membrane surface. At the same time, the -NH2 on the undecomposed carbamic acid will be protonated to become -NH3. ＋ A positive charge can also lead to a decrease in the electronegativity of the membrane.

[0036] In addition, the retention performance of the lignin-based polyurethane loose nanofiltration membrane M3 in Example 3 on the binary composite fouling system (NaCl-CR composite fouling system and NaCl-MB composite fouling system) composed of salt and dye was tested under a pressure of 3.0 bar. The test results are as follows. Figure 9 As shown. From Figure 9 It can be seen that when the dye concentration remains constant at 50 ppm, as the salt concentration increases from 1 g / L to 40 g / L, the rejection of salt and dye by M3 decreases, but the rejection of dye remains high. Furthermore, as the salt concentration increases, the membrane's separation selectivity for salt and dye (dye rejection rate / salt rejection rate) increases. Figure 9 It can also be observed that flux decreases with increasing salt concentration. This is because as salt concentration increases, the osmotic pressure of the mixed solution rises significantly. With a constant operating pressure, the effective driving force for water permeation equals the operating pressure minus the osmotic pressure. Therefore, the increase in osmotic pressure directly leads to a decrease in the effective driving force, thus reducing water flux. Furthermore, high salt concentrations result in more pronounced concentration polarization, leading to extremely high local osmotic pressure within the membrane boundary layer, further reducing flux. In addition, higher salt concentrations make the membrane more susceptible to fouling, further contributing to flux reduction.

[0037] It should be noted that any parts not mentioned in this invention can be achieved by using or referencing existing technologies.

[0038] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for preparing a lignin-based polyurethane porous nanofiltration membrane, characterized in that, Including the following steps: (1) The lignin is dried, and the dried lignin is mixed with dimethyl sulfoxide and stirred to fully dissolve the lignin to obtain a lignin solution; (2) Hexamethylene diisocyanate is slowly added dropwise to the lignin solution obtained in step (1), and then dibutyltin dilaurate is added dropwise. The lignin and hexamethylene diisocyanate undergo an esterification reaction to obtain a casting solution containing lignin-based polyurethane. (3) The casting solution in step (2) is degassed, and then the casting solution is poured evenly onto the non-woven fabric. The casting solution is scraped into a membrane using a film scraper. Then it is transferred to a deionized water coagulation bath for phase inversion to form a membrane. After the phase inversion is completed, it is taken out to obtain a loose nanofiltration membrane with non-woven fabric as the support layer and lignin-based polyurethane membrane as the separation layer. In step (1), the lignin is alkali lignin with an ash content of 1.0-1.5% and a total hydroxyl content of 3-4 mmol / g. In step (2), the mass ratio of lignin solids content to hexamethylene diisocyanate is 0.7~1.2:

1.

2. The method for preparing a lignin-based polyurethane porous nanofiltration membrane according to claim 1, characterized in that, The lignin solution in step (1) has a lignin mass fraction of 14-18%.

3. The method for preparing a lignin-based polyurethane porous nanofiltration membrane according to claim 1, characterized in that, In step (2), the dropping rate of hexamethylene diisocyanate is 3~8 s / drop.

4. The method for preparing a lignin-based polyurethane porous nanofiltration membrane according to claim 1, characterized in that, In step (2), the mass ratio of the amount of dibutyltin dilaurate added to the lignin solid content is 0.005~0.2:

1.

5. The method for preparing a lignin-based polyurethane porous nanofiltration membrane according to claim 1, characterized in that, The esterification reaction time of lignin and hexamethylene diisocyanate in step (2) is 1-4 hours.

6. The method for preparing a lignin-based polyurethane porous nanofiltration membrane according to claim 1, characterized in that, In step (3), the temperature of the deionized water coagulation bath is 15~25℃ and the phase transformation time is 10~25min.

7. A lignin-based polyurethane loose nanofiltration membrane, prepared by the method according to any one of claims 1-6, wherein the lignin-based polyurethane loose nanofiltration membrane is electronegative.

8. The application of the lignin-based polyurethane porous nanofiltration membrane as described in claim 7 in the separation of solutions containing anionic dyes.

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