Biomass nanofiltration membrane and preparation method thereof

Abstract

The invention relates to the technical field of nanofiltration membranes, in particular to a biomass nanofiltration membrane and a preparation method thereof. The nanofiltration membrane comprises a matrix membrane and an L-x selection layer, the L-x selection layer is formed by coordination of lignin and metal ions. The preparation method comprises the following steps: (1) preparing a metal ion solution; (2) coating the matrix membrane with a metal ion solution, and standing; (3) washing with ultrapure water to remove excess solution; (4) dissolving the enzymatic hydrolysis lignin in an ethanol solution with the mass concentration of 70%, stirring, and performing ultrasonic oscillation to prepare an L-EtOH solution; (5) coating the surface of the substrate obtained in the step (3) with an L-EtOH solution, and standing; and (6) placing in ultrapure water, and carrying out anti-solvent precipitation to obtain the biomass nanofiltration membrane. The stable lignin-metal complex selection layer is utilized to improve the mechanical strength of the nanofiltration membrane and improve the membrane pore defect formed in the lignin self-assembly process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nanofiltration membrane, and particularly relates to a biomass nanofiltration membrane and a preparation method thereof. BACKGROUND

[0002] Water shortage and water pollution are serious challenges to global sustainable development today. Membrane separation technology, especially nanofiltration (NF), has shown great potential in water treatment and resource recovery due to its high efficiency and energy saving. However, traditional petroleum-based polymeric nanofiltration membranes have high energy consumption and large carbon emissions in the production process, and are not easy to degrade after being discarded. Therefore, developing new green and renewable membrane materials is the key to realizing the sustainable development of membrane technology.

[0003] Lignin, as the second most abundant aromatic macromolecule polymer in nature, is mainly derived from the huge by-products of papermaking and biorefining industries, with an annual production of about 800 million tons of lignin by-products. Most of the lignin is directly incinerated as low-value waste, not only causing resource waste, but also causing environmental pollution. Lignin is a natural polymer with amorphous structure, which is synthesized by three phenylpropane units (syringyl S, guaiacyl G and p-hydroxyphenyl H) through free radical coupling reaction. Existing studies have shown that lignin structure is rich in active groups such as phenolic hydroxyl and carboxyl groups, which can form functional complexes through metal coordination. For example, a cross-linked lignin adsorbent is prepared by freeze-drying and low-temperature annealing method, which confirms that the high adsorption efficiency of Cu is due to the complexation of phenolic hydroxyl groups in lignin; a hydrophobic fabric is constructed by in-situ precipitation of lignin-metal chelation, which realizes oil-water separation efficiency of more than 99.9%. By LbL self-assembly and glutaraldehyde (GA) cross-linking strategy, papermaking waste lignin sulfonate is converted into high-performance nanofiltration membrane material, which realizes high permeate flux (39.6 L / (m²·h)) while maintaining high rejection rate (>91%), providing a new paradigm for the high-value utilization of lignin sulfonate and the development of green membrane technology. These technologies provide a feasible path for the high-value utilization of lignin.

[0004] However, the above existing technologies still have obvious limitations: (1) lignin is used to prepare adsorbents or hydrophobic fabrics, which does not provide a feasible solution for the utilization of lignin in the field of nanofiltration membranes; (2) the lignin self-assembly layer is formed by using a cross-linking agent to fix the self-assembly, and the membrane preparation process is complex; (3) the existing research does not fully utilize the coordination mechanism of metal and lignin to strengthen the research on the binding strength of lignin and membrane surface; therefore, the present application proposes a theory of using metal and lignin coordination to form a complex selection layer, and develops a new type of nanofiltration membrane based on lignin-metal coordination, which realizes resource recycling while having high separation performance, excellent mechanical stability and environmental compatibility.

[0005] Document 1: Chen F, SHAHABADI S I S, ZHOU D, et al. Facile preparation of cross-linked lignin for efficient adsorption of dyes and heavy metal ions [J]. Reactive and Functional Polymers, 2019, 143: 104336. DOI: 10.1016 / j. reactfunctpolym. 2019.104336

[0006] Document 2: Liu X, Chen X, Huiyang Bian Shuzhen Ni Zongquan Li Na Liu Menghua Qin Fengshan Zhang. Highly hydrophobic cotton fabric by in-situ co-deposition of lignin / metal particles for oil / water separation [J]. Industrial Crops and Products, 2023, 204 (Pt. B): 117393-1-117393-12. DOI: 10.1016 / j.indcrop.2023.117393.

[0007] Document 3: Liu W, Geng X, Li S, et al. Preparation of lignosulfonate-based nanofiltration membranes with improved water desalination performance [J]. Engineering in Life Sciences, 2021, 21(6): 417-28. DOI: 10.1002 / elsc.202000102 SUMMARY

[0008] In view of the deficiencies of the existing method, the present application provides a preparation method of biomass nanofiltration membrane. The coordination structure formed by lignin and metal constitutes a stable lignin-metal complex selection layer, which not only can stably improve the mechanical strength of the nanofiltration membrane, but also can perfect the membrane pore defects formed in the self-assembly process of lignin.

[0009] Firstly, metal ions are adsorbed on the membrane surface through electrostatic interaction force, and then a lignin-metal complex selective layer is formed through the metal coordination ability of lignin to prepare a biomass nanofiltration membrane, and the specific steps are as follows: (1) prepare a metal ion solution; (2) coat the metal ion solution on the substrate membrane and stand; (3) rinse with ultrapure water to remove excess solution; (4) dissolve enzymatic lignin in a 70% mass concentration ethanol solution, stir and ultrasonic oscillation, and then prepare an L-EtOH solution; (5) coat the L-EtOH solution on the surface of the substrate obtained in step (3) and stand; (6) place in ultrapure water for anti-solvent precipitation to obtain a biomass nanofiltration membrane.

[0010] Take 3mL L-EtOH solution and pour it on the membrane surface to react with metal ions for 2min to prepare a lignin-metal nanofiltration membrane.

[0011] Further, the metal ions in step (1) include zinc ions, copper ions, iron ions and zirconium ions.

[0012] Further, the concentration of the metal ion solution in step (1) is 0.1%-2%.

[0013] Further, the substrate membrane in step (2) is a polyacrylonitrile (PAN) membrane.

[0014] Further, the coating amount of the metal ion solution in step (2) is 0.3-0.5 mL / cm 2 .

[0015] Further, the standing time in step (2) is 1min-5min.

[0016] Further, the concentration of the L-EtOH solution in step (4) is 60%-80%.

[0017] Further, the coating amount of the L-EtOH solution in step (5) is 0.3-0.5 mL / cm 2 .

[0018] The beneficial effects of the present application are: The application utilizes the strong metal chelation of lignin to make L-x selective layer on the surface of the base film by forming coordination compounds of lignin and metal ions, so as to improve the mechanical tensile strength of the nanofiltration membrane and expand the high-value utilization way of lignin. The lignin is used as an organic ligand to coordinate with different metals to form a long-term stable nanofiltration membrane with dense defect-free and excellent performance. The selective layer structure of the L-x / NF membrane can be regulated by adjusting the ratio of the coordination metal and lignin: (1) Lignin, as a natural strong electron donor, coordinates with metal ions (such as Fe 3+ and Zr 4+ ) with strong potential energy to form a defect-free selective layer structure, and has a higher rejection rate for multivalent salt Na2SO4.

[0019] (2) The selective layer structure of the lignin-metal coordination nanofiltration membrane can be regulated by the concentration of the metal ions. With the increase of the concentration of the metal ions, larger coordination bodies are formed. By regulating the ratio of the metal ions and lignin, the ideal selective layer of the nanofiltration membrane is prepared through the synergistic self-assembly of lignin-metal coordination. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 SEM characterization of PAN base film, L / NF, L-x / NF (x is Zn, Cu, Fe and Zr respectively); Figure 2 EDS spectrum of L-x / NF (x is Zn, Cu, Fe and Zr respectively); Figure 3 (a) Infrared spectrum of PAN base film, L-x / NF (x is Zn, Cu, Fe and Zr respectively), (b) Water contact angle of PAN base film, L / NF, L-x / NF (x is Zn, Cu, Fe and Zr respectively), (a) L-x / NF (x is Zn, Cu, Fe and Zr respectively) is represented by Zn, Cu, Fe or Zr for each L-x / NF; Figure 4 Filtering performance of L-x / NF (x is Zn, Cu, Fe and Zr respectively) on (a) Na2SO4, (b) MgSO4, (c) MgCl2, (d) NaCl; Figure 5 Na2SO4 rejection performance of L-Fe / NF nanofiltration membranes prepared by different Fe 3+ concentrations; Figure 6 Particle size of lignin coordination compounds with different Fe 3+ concentrations; Figure 7 SEM characterization of L-Fe / NF nanofiltration membranes prepared by different Fe 3+ concentrations; Figure 8 The filtration performance of L-5Fe / NF on metal wastewater at different pH values; Figure 9 The long-term stability test results of L-5Fe / NF; Figure 10 The influence of L-5Fe / NF on the Na2SO4 rejection performance after being immersed in different concentrations of HCl solution (a) and H2SO4 solution (b) for different times; Figure 11 SEM characterization of the membrane surface of L-5Fe / NF after being immersed in different concentrations of HCl solution (0-1 mol / L) for 7 days. DETAILED DESCRIPTION

[0021] The application will be further described below in conjunction with the accompanying drawings and examples.

[0022] The materials involved in the following examples and comparative examples are as follows: polyacrylonitrile (PAN) membranes were purchased from Taian Lanjing Trade Co., Ltd. Enzymatic hydrolysis lignin (EHL) was purchased from Shandong Longli Biological Technology Co., Ltd. The crude lignin was sieved and used to prepare the selective layer of the NF membrane. CuCl2, FeCl3, ZnCl2, and Zr(NO3)4·5H2O were provided by Shanghai Maikelin Biochemical Co., Ltd. (China) as metal ion chelating agents. Na2SO4, MgSO4, NaCl, and MgCl2 were used to test the desalination performance of the NF membrane and were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Ethanol was provided by Sinopharm Chemical Reagent Co., Ltd. and was used as a lignin solvent; deionized (DI) water was produced by a water purifier (EPED-E3-10 TH) and was used as a lignin anti-solvent.

[0023] Example 1

[0024] (1) Enzymatic hydrolysis lignin (purchased from Shandong Longli) was sieved to 100 mesh to remove impurities in the lignin. 10 g of enzymatic hydrolysis lignin was dissolved in a 70% volume concentration ethanol solution, stirred thoroughly, and then ultrasonically oscillated for 30 min to fully dissolve it. The supernatant was taken after standing overnight and was named L-EtOH solution; (2) Metal ion solutions with the same molar amount of heavy metal ions (4 mol / L) were prepared. 4 g of ZnCl2, 4 g of CuCl2, 4.8 g of FeCl3, or 12.7 g of Zr(NO3)4·5H2O powder was dissolved in 1 L of ultrapure water to fully dissolve it. Then 3 mL of the metal ion solution was poured into a 10 cm 2PAN substrate surface 2 min, metal ions are adsorbed on the surface of PAN membrane by electrostatic adsorption, and the excess solution is washed off with ultrapure water and wiped clean. Take 3 mL L-EtOH solution and pour it on the membrane surface and coordinate with metal ions for 2 min. The prepared lignin-metal nanofiltration membrane is placed in ultrapure water for standby. It is named L-x / NF (x is Zn, Cu, Fe and Zr respectively).

[0025] Comparative Example 1

[0026] Prepare NF membrane without adding metal ions as a control, pour 3 mL L-EtOH solution on the membrane surface and coat for 2 min, and directly prepare lignin nanofiltration membrane with ultrapure water as anti-solvent, named L / NF.

[0027] In order to observe the surface morphology of the composite nanofiltration membrane, the structure is characterized by field emission scanning electron microscope, and the influence of different metal coordination on the selective layer structure of nanofiltration membrane is studied. Figure 1 are the surface scanning electron microscope images of the composite nanofiltration membrane prepared by different metal ions and lignin coordination and PAN base membrane.

[0028] The surface pores of PAN base membrane are large and the structure is rough, and there are uniform membrane pores. After coating lignin on the surface of PAN base membrane, a rough surface layer structure is formed. Due to the agglomeration of lignin, larger particles are formed, which stack on the membrane surface to form larger pores. After adding metal ions, lignin forms more coordination structure with metal ions, and the surface pore becomes smaller, forming a dense selective layer on the surface of PAN base membrane. With the increase of ionization potential and charge of the coordination metal ions, the surface of the polymer becomes rougher, indicating that a violent coordination reaction occurs between the metal ions and lignin.

[0029] It is further proved by EDS spectrum that the metal ions are uniformly attached to the membrane surface (as shown in Figure 2 ).

[0030] The prepared L-x / NF (x is Zn, Cu, Fe and Zr respectively) is characterized by ATR-FTIR. As shown in Figure 3 a, the absorption peaks of lignin hydrophobic group aromatic skeleton stretching vibration appear at 1510 cm -1 and 1599 cm -1 , which confirms that lignin is successfully attached to the membrane surface. The broad peak of the stretching vibration of hydroxyl group in lignin is observed at 3400 cm -1 , and the strength of the peak increases with the increase of the potential energy of the metal, which confirms that the increase of the surface charge and ionization potential of the metal will facilitate the coordination of more lignin and form a more dense selective layer structure.

[0031] The contact angle of L-x / NF (x is Zn, Cu, Fe and Zr respectively) is as followsFigure 3 b shows that the water contact angle of the NF membranes modified by different metals remains stable and does not change significantly compared with L / NF, which can be attributed to the fact that the formation of the selective layer of L-x / NF (x represents Zn, Cu, Fe and Zr, respectively) is the result of the synergistic effect of lignin-metal coordination and lignin self-assembly. The hydrophilic groups on the membrane surface are mainly provided by lignin. After the formation of the coordination structure of lignin and metal ions on the surface of the PAN-based membrane, lignin self-assembles on this basis to form a more compact selective layer, thus forming a selective layer structure with an inner dense and outer loose structure, so the water contact angle of the NF membrane does not change significantly.

[0032] In the evaluation of the filtration performance of the membrane, the key performance parameters include the solute permeate flux (J) and the rejection rate (R). The filtration performance of the prepared NF membrane was tested by a laboratory-made cross-flow filtration device. The effective filtration area of the membrane was 9.616 cm 2 The flux and rejection rate of different inorganic salts or metal solutions were tested at 5 bar and 25°C. In order to ensure the stability of the membrane and the device, all the data in this experiment were collected after the system ran for 30 minutes.

[0033] The calculation formula (1) of the permeability J is as follows:

[0034] In the formula, V is the volume of the permeate (L); A is the effective filtration area of the membrane (cm 2 ); t is the filtration time (s); and P is the transmembrane pressure (bar).

[0035] The rejection rate (R) can be calculated by the following equation (2):

[0036] In the formula, A1 is the conductivity value of the filtrate (uS / cm); and A2 is the conductivity value of the original solution (uS / cm). The salt concentration is measured by a conductivity meter (EUTECH COND 6+, Thermo Scientific, Singapore).

[0037] The filtration performance of L / NF and L-x / NF (x represents Zn, Cu, Fe and Zr, respectively) was tested using four inorganic salts. The results are shown in Figure 4 After the addition of the coordination metal, the rejection performance of the L-x / NF membrane to the four inorganic salts is improved. With the increase of the ionization potential (polarity) of the coordination metal (ionization potential of the coordination metal: Zr 4+ > Fe 3+ > Cu 2+ > Zn 2+ > ), the rejection rate of the NF membrane to Na2SO4, MgSO4, CaCl2 and NaCl increases.​ S The rejection rate of the prepared nanofiltration membrane increased from 81.3% to 97.8% (L-Zr / NF), while the flux decreased with the increase of the polarity of the coordinated metal. According to the theory of hard and soft acids and bases, lignin tends to combine with metal ions with strong electric potential energy to form a tighter selective layer structure and a smaller membrane pore size. In addition, the rejection rate of the prepared nanofiltration membrane for the four salts follows the following order: N S > MgS > NaCl > MgC This is mainly attributed to the Donnan effect, the surface of the nanofiltration membrane is negatively charged, resulting in the divalent anion SO4 2- The electrostatic force is higher than that of monovalent anion Cl - In addition, due to Cl - It has a relatively smaller Stokes radius and smaller valence state, so both the Donnan effect and the pore size screening effect have a relatively small effect on Cl - The retention is low. At the same time, due to the 2+ Than Na + With more positive charges, it is easier to adsorb with the negative charges on the membrane surface, resulting in a decrease in the retention performance of the NF membrane.

[0038] Example 2

[0039] (1) Same as Example 1; (2) Prepare different concentrations of iron ion solutions (1g / L, 3g / L, 5g / L, 7g / L, 10g / L, 15g / L, 20g / L), take the required amount of FeCl3 powder and dissolve it in 1L ultrapure water, stirring thoroughly to dissolve it completely. Then take 3mL of iron ion solution and pour it on a 10cm 2 The PAN substrate was treated for 2 minutes. Iron ions were electrostatically adsorbed onto the PAN membrane surface. Excess solution was rinsed with ultrapure water and the membrane was wiped clean. 3 mL of L-EtOH solution was poured onto the membrane surface to react with the metal ions for 2 minutes. The prepared lignin-metal nanofiltration membrane was placed in ultrapure water until ready for use. This membrane was named L-yFe / NF (y represents the concentration of the iron ion solution, 1, 3, 5, 7, 10, 15, or 20).

[0040] Figure 5 Shows different concentrations of FeC The effect of nanofiltration membrane prepared from solution on N S The filtration performance of L-yFe / NF membrane S The retention rate of Fe 3+ The concentration increases first and then decreases. The peak concentration was 7 g / L. When the metal ion concentration was low, the coordination reaction occurred between the coordination metal and the excess lignin molecules, and a large number of lignin molecules that did not undergo coordination self-assembled to form larger nanoparticles, which stacked on the membrane surface to form larger pores. As the ratio of coordination metal to lignin increased, more metal ions and lignin underwent coordination, filling the pores between the lignin and forming a more dense selection layer.

[0041] As shown in Figure 6 , as the ratio of coordination metal to lignin increased, the particle size of the coordination compound formed increased, leading to the formation of larger pore structures by stacking, and a downward trend in the retention performance. Therefore, the selection layer structure and pore size of the NF membrane can be regulated by adjusting the ratio of coordination metal to lignin.

[0042] The L-Fe / NF membrane was tested for its retention performance for heavy metal ions in industrial production, and heavy metal solutions with different pH values were prepared to simulate heavy metal wastewater in industrial production. As shown in Figure 8 , the prepared L-5 Fe / NF nanofiltration membrane had a relatively stable filtration effect on Ni, Mn, Fe and Cu in different pH environments, and as the solution pH increased, the lignin depolymerized and the retention rate decreased slightly. Among them, the retention effect on nickel ions was the best, with a retention rate of more than 90% under different acid and alkali conditions.

[0043] The L-5 Fe / NF membrane was placed in a cross-flow device and tested for long-term stability using a 1000 ppm Na2SO4 solution for 120 h, and the results are shown in Figure 9 . After 5 days of testing, the L-Fe / NF membrane still maintained a retention rate of more than 97.5% for Na2SO4, indicating that the prepared composite nanofiltration membrane can be operated stably for a long time.

[0044] The L-5 Fe / NF was soaked in different concentrations of acidic solution for a period of time, and the effect of soaking time on membrane performance in different acid concentration ranges was investigated by testing the Na2SO4 retention rate. Figure 10 The filtration performance after soaking in different concentrations of HCl solution and H2SO4 solution for 1-7 days is shown in Figure 10 , and the L-5 Fe / NF membrane can maintain stable Na2SO4 retention rate in less than 1 M acid solution environment, and has the same trend in HCl and H2SO4, indicating that the membrane has excellent acid resistance. The surface morphology of the membrane after soaking in 0 M, 0.1 M, 0.5 M and 1 M hydrochloric acid solution was observed by SEM (as shown in Figure 11 ), and as the soaking time increased, the L-5 Fe / NF membrane flux had a slight downward trend, which was due to the +The presence of the lignin macromolecules and the hydrogen bonds between the lignin macromolecules and the lignin macromolecules themselves makes the selective layer of the nanofiltration membrane immersed in the acid solution more dense. With further increase of the concentration of the acid solution, the cleavage of the ether bonds in the selective layer on the membrane surface leads to defects in the selective layer.

[0045] The above is based on the ideal embodiments of the present application, through the above description, the related staff can make various changes and modifications without deviating from the scope of the technical idea of the present application. The technical scope of the present application is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. A biomass nanofiltration membrane, characterized in that The nanofiltration membrane comprises a matrix membrane and an Lx selection layer; the Lx selection layer is formed by the coordination of lignin and metal ions.

2. A method for preparing a biomass nanofiltration membrane as claimed in claim 1, characterized in that: The steps include: (1) Prepare metal ion solution; (2) Applying the metal ion solution onto the substrate membrane and allowing it to stand; (3) Rinse with ultrapure water to remove excess solution; (4) Dissolve the enzymatically hydrolyzed lignin in a 70% ethanol solution, stir, and ultrasonically vibrate to prepare an L-EtOH solution; (5) applying the L-EtOH solution to the surface of the substrate obtained in step (3) and allowing it to stand; (6) Place in ultrapure water for anti-solvent precipitation to obtain a biomass nanofiltration membrane.

3. The method for preparing a biomass nanofiltration membrane as claimed in claim 2, wherein: The concentration of the metal ion solution in step (1) is 0.1%-2%.

4. The method for preparing a biomass nanofiltration membrane as claimed in claim 2, wherein: The metal ions in step (1) include zinc ions, copper ions, iron ions, and zirconium ions.

5. The method for preparing a biomass nanofiltration membrane as claimed in claim 2, wherein: The coating amount of the metal ion solution in step (2) is 0.3-0.5 mL / cm 2 .

6. The method for preparing a biomass nanofiltration membrane as claimed in claim 2, wherein: The standing time in steps (2) and (5) is 1-5 minutes.

7. The method for preparing a biomass nanofiltration membrane as claimed in claim 2, wherein: The concentration of the L-EtOH solution in step (4) is 50%-80%.

8. The method for preparing a biomass nanofiltration membrane as claimed in claim 2, wherein: The coating amount of L-EtOH solution in step (5) is 0.3-0.5 mL / cm 2 .

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