Polyurethane loose nanofiltration membrane for efficiently separating salt from dye and preparation method of polyurethane loose nanofiltration membrane

Abstract

The invention provides a polyurethane loose nanofiltration membrane for efficient separation of salt and dye and a preparation method of the polyurethane loose nanofiltration membrane, and relates to the technical field of loose nanofiltration membranes. According to the polyurethane loose nanofiltration membrane for efficient separation of the salt and the dye, xylitol and toluene-2, 4-diisocyanate (TDI) are used as water phase monomers and oil phase monomers respectively, and the polyurethane loose nanofiltration membrane is obtained on a PVDF microfiltration membrane through an interfacial polymerization (IP) strategy and used for separation of the salt and the dye. Influences of different oil phase monomer concentrations on the physicochemical properties of polyurethane layers formed in the IP process are discussed. The structural form and chemical properties of the membrane are characterized, the interception of the prepared polyurethane membrane on single salt and single dye and the interception selectivity of a salt-dye composite system are evaluated respectively, and the feasibility and potential of the polyurethane loose nanofiltration membrane in the aspect of high-salt printing and dyeing wastewater resource utilization are verified.

Description

Technical Field

[0001] The invention relates to the technical field of loose nanofiltration membranes, in particular to a polyurethane loose nanofiltration membrane for efficient separation of salt and dye and a preparation method thereof. Background Art

[0002] Membrane separation technology has shown great application potential in solving water shortage and pollution problems aggravated by rapid population growth and global climate change due to its advantages of low energy consumption, high efficiency, simple process and environmental friendliness. Among them, polyamide nanofiltration membranes based on thin film composite (TFC) technology have become excellent candidates for drinking water production, seawater desalination and wastewater reuse. However, traditional TFC nanofiltration membranes are still limited by the trade-off effect between solvent permeability and solute selectivity. The ubiquitous trade-off effect between permeability and selectivity is one of the main obstacles to further improving its performance and application areas. Current strategies to improve water flux are usually at the expense of solute exclusion, which hinders the application of membrane separation technology in efficient water purification and separation.

[0003] Loose nanofiltration membrane is an important new type of separation membrane in the development of nanofiltration membrane technology in recent years. Because of its low required operating pressure, high flux, larger pore size and higher molecular weight cutoff, it provides ideas and opportunities to break the "trade-off effect" that has always existed in traditional nanofiltration membranes. Compared with traditional nanofiltration membranes, loose nanofiltration membranes can maintain high retention characteristics for organic matter such as organic dye molecules, while achieving high permeability to monovalent and divalent salt ions, thereby achieving the separation and reuse of organic matter and inorganic salts in high-salt organic wastewater. The separation mechanism of loose nanofiltration membranes is mainly steric hindrance effect and charge effect (Donnan effect). Among them, the retention of uncharged dye molecules is mainly achieved by the steric hindrance of the membrane itself, while for charged dye molecules, the retention of dye molecules of different charge types can be achieved by adjusting the charge properties of the membrane surface. In general, the advantage of loose nanofiltration membranes is that they can achieve the removal of low molecular weight organic matter while allowing high permeability of organic salts. In view of the above excellent characteristics, loose nanofiltration membranes have gradually become an important medium for separating and recovering salt and dyes from high-concentration salt dye wastewater.

[0004] It is well known that IP strategy is the main way to obtain nanofiltration membranes. A large number of studies have shown that during interfacial polymerization, the physicochemical properties of the monomers affect the crosslinking degree, pore size, thickness, and hydrophilicity of the target film layer. It is worth noting that a nanofiltration membrane with a loose structure can be achieved by selecting a suitable active monomer and simply adjusting its IP concentration.

[0005] Traditional polyamide nanofiltration membranes are usually prepared by using polyamines with high nucleophilicity as water-phase monomers and polyacyl chlorides through IP reaction. Due to the high degree of cross-linking and narrow pore size, these nanofiltration membranes have a dense structure and are difficult to loosen. Under the action of particle size screening and electrostatic repulsion, dyes and salts will be filtered and removed at the same time, making it difficult to separate dye / salt mixtures. In comparison, because polyols have lower activity, when used as water-phase monomers to prepare polyester membranes, it is difficult for water / oil monomers to fully cross-link during the IP process, resulting in a loose structure that can achieve the separation of salt and dye in printing and dyeing wastewater. However, it is still challenging to prepare nanofiltration membranes with a wide range of adjustable cross-linking degrees and apply them to sewage and wastewater separation and resource utilization. Summary of the invention

[0006] 1. Technical issues to be solved

[0007] In view of the deficiencies in the prior art, the present invention provides a polyurethane loose nanofiltration membrane for efficient separation of salt and dye and a preparation method thereof, which solves the problems raised in the above-mentioned background technology.

[0008] (II) Technical solution

[0009] To achieve the above objectives, the present invention is implemented by the following technical scheme: A method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye, comprising the following steps:

[0010] Step 1: First, cut the PVDF base membrane into appropriate sizes and wash it with ultrapure water to remove the membrane protective agent sodium bisulfite;

[0011] Step 2: Soak the PVDF base membrane in a 4 wt % xylitol aqueous solution for 30 min to allow the aqueous solution to completely penetrate into the pores of the PVDF base membrane, and use a clean paper towel to absorb excess xylitol solution on the surface of the base membrane;

[0012] Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 0.4-1.2wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction;

[0013] Step 4: Take out the soaked membrane and dry it at room temperature for 5 minutes, then transfer it to a 60°C oven and keep it for 10 minutes to fully react and enhance chemical cross-linking;

[0014] Step 5: Take out the membrane, cool it, rinse it with ultrapure water for multiple times to remove unreacted monomers and solvents on its surface, and soak it in ultrapure water until use, thereby obtaining a polyurethane loose nanofiltration membrane.

[0015] Preferably, step three: fix the PVDF base membrane soaked with the xylitol solution in a special polytetrafluoroethylene frame with the active surface facing upward, and slowly pour n-hexane solution containing 0.4 wt % TDI into it to soak the base membrane for 10 minutes for polycondensation reaction.

[0016] Preferably, step three: fix the PVDF base membrane soaked with the xylitol solution with the active surface facing upward in a special polytetrafluoroethylene frame, and slowly pour in a n-hexane solution containing 0.6 wt % TDI to soak the base membrane for 10 minutes for polycondensation reaction.

[0017] Preferably, step three: fix the PVDF base membrane soaked with the xylitol solution in a special polytetrafluoroethylene frame with the active surface facing upward, and slowly pour in a n-hexane solution containing 0.8 wt % TDI to soak the base membrane for 10 minutes for polycondensation reaction.

[0018] Preferably, step three: fix the PVDF base membrane soaked with the xylitol solution with the active surface facing upward in a special polytetrafluoroethylene frame, and slowly pour in a n-hexane solution containing 1.0 wt% TDI to soak the base membrane for 10 minutes for polycondensation reaction.

[0019] Preferably, step three: fix the PVDF base membrane soaked with the xylitol solution in a special polytetrafluoroethylene frame with the active surface facing upward, and slowly pour in a n-hexane solution containing 1.2 wt% TDI to soak the base membrane for 10 minutes for polycondensation reaction.

[0020] The present invention further discloses a polyurethane loose nanofiltration membrane for efficient separation of salt and dye, which is prepared based on the above-mentioned method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye.

[0021] (III) Beneficial effects

[0022] The present invention provides a polyurethane loose nanofiltration membrane for efficient separation of salt and dye and a preparation method thereof, which has the following beneficial effects:

[0023] 1. The thickness of the polyurethane loose nanofiltration membrane increases with the increase of TDI concentration. Due to the high monomer concentration, increased membrane thickness and high cross-linking degree, the water permeability decreases with the increase of TDI concentration.

[0024] 2. When the TDI concentration increases from 0.4wt% to 1.2wt%, the removal rate of the membrane for a single dye is greatly improved, approaching complete removal. When the TDI concentration increases from 0.4wt% to 1.2wt%, the retention rate of all membranes for a single salt is less than 20%, indicating that the prepared series of polyurethane loose nanofiltration membranes have an obvious loose structure, which has significant advantages for the desalination and reuse of high-salinity wastewater.

[0025] 3. The prepared series of polyurethane loose nanofiltration membranes have gradually enhanced separation selectivity for salts and dyes with increasing TDI concentration. When the added TDI reaches 1.0wt% and 1.2wt%, a relatively high salt / dye separation effect (>100%) is achieved overall. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the preparation process of the polyurethane loose nanofiltration membrane in Example 1 of the present invention;

[0027] Figure 2 This is a schematic diagram of the surface morphology of the polyurethane loose nanofiltration membrane in Example 2 of the present invention;

[0028] Figure 3 This is a schematic diagram of pure water flux of a polyurethane loose nanofiltration membrane in Example 2 of the present invention;

[0029] Figure 4 The structural formula and molecular weight diagram of methyl blue, direct blue and direct red dyes in Example 4 of the present invention;

[0030] Figure 5 It is a schematic diagram of the interception efficiency of the polyurethane loose nanofiltration membrane for a single salt and a dye in Example 3 and Example 4 of the present invention;

[0031] Figure 6 Schematic diagram of the selective separation performance of the polyurethane loose nanofiltration membrane for salt-dye composite pollutants in Example 5 of the present invention. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] Embodiment 1:

[0034] like Figure 1 As shown, the embodiment of the present invention provides a method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye, wherein the polyurethane loose nanofiltration membrane is prepared on a PVDF microfiltration base membrane by an interfacial polymerization method, and the method comprises the following steps:

[0035] Step 1: First, cut the PVDF base membrane into appropriate sizes and wash it with ultrapure water to remove the membrane protective agent sodium bisulfite;

[0036] Step 2: Soak the PVDF base membrane in a 4 wt % xylitol aqueous solution for 30 min to allow the aqueous solution to completely penetrate into the pores of the PVDF base membrane, and use a clean paper towel to absorb excess xylitol solution on the surface of the base membrane;

[0037] Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 0.4-1.2wt% TDI (toluene-2,4-diisocyanate) is slowly poured into the base membrane to soak for 10 minutes for polycondensation reaction;

[0038] Step 4: Take out the soaked membrane and dry it at room temperature for 5 minutes, then transfer it to a 60°C oven and keep it for 10 minutes to fully react and enhance chemical cross-linking;

[0039] Step 5: Take out the membrane, cool it, rinse it with ultrapure water for multiple times to remove unreacted monomers and solvents on its surface, and soak it in ultrapure water until use, thus obtaining a polyurethane loose nanofiltration membrane named Xy l / TDI-X (X is the mass concentration of TDI).

[0040] The embodiment of the present invention also provides a polyurethane loose nanofiltration membrane for efficient separation of salt and dye, which is prepared based on the above-mentioned method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye.

[0041] Embodiment 2:

[0042] The water permeability performance of the polyurethane loose nanofiltration membranes with various TDI concentrations in the examples was tested, and the results were as follows: Figure 2 and Figure 3 shown.

[0043] Depend on Figure 3 It can be seen that for different concentrations of TDI monomer, when the concentration of TDI increases from 0.4wt% to 1.2wt%, the water permeability increases from 245.97Lm -2 h -1 bar -1 Reduced to 98.17Lm -2 h -1 bar -1 . SEM results confirmed ( Figure 2 ad) The thickness of the polyester selective membrane increases with the increase of TDI concentration. Due to the high monomer concentration, increased membrane thickness and high cross-linking degree, the water permeability decreases with the increase of TDI concentration.

[0044] Embodiment three:

[0045] Polyurethane loose nanofiltration membrane separation performance test for single inorganic salt. A cross-flow filtration system with a data acquisition balance was used for separation performance test, as follows:

[0046] At a pressure of 1.0 bar, the retention performance of polyurethane loose nanofiltration prepared with TDI of different monomer concentrations for a single monovalent salt and a divalent salt (sodium sulfate, magnesium sulfate, sodium chloride, magnesium chloride) with a concentration of 2000 ppm was tested, as shown in Table 1:

[0047] (1) The prepared polyurethane loose nanofiltration membrane was used to intercept 2000 ppm of sodium sulfate: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the interception trend of the membrane corresponding to sodium sulfate was: 10.22%→11.96%→14.65%→18.08%→18.88%;

[0048] (2) Using the prepared polyurethane loose nanofiltration membrane to intercept 2000 ppm of magnesium sulfate: The results show that when the TDI concentration increases from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the interception trend of the membrane corresponding to magnesium sulfate is: 5.43%→7.36%→7.88%→8.97%→8.93%;

[0049] (3) The prepared polyurethane loose nanofiltration membrane was used to intercept 2000 ppm of sodium chloride: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the interception trend of the membrane corresponding to magnesium sulfate was: 2.36%→3.19%→3.68%→5.60%→6.28%;

[0050] (4) The prepared polyurethane loose nanofiltration membrane was used to intercept 2000 ppm of magnesium chloride: The results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the retention trend of the membrane corresponding to magnesium sulfate was: 2.19%→3.01%→3.24%→5.23%→6.06%.

[0051] Table 1 Retention performance of polyurethane loose nanofiltration membrane for single salt

[0052]

[0053] Embodiment 4:

[0054] At a pressure of 1.0 bar, the retention performance of polyurethane loose nanofiltration prepared by different monomer concentrations TDI for single dyes with different molecular weights (methyl blue (Mw=799.8), direct blue (Mw=1029.87), direct red (Mw=1373.07)) with a concentration of 200 ppm was tested, as shown in Table 2:

[0055] (1) The prepared polyurethane loose nanofiltration membrane was used to intercept 200 ppm of methyl blue: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the interception trend of methyl blue by the membrane was: 82.24%→91.1%→98.43%→99.19%→99.28%;

[0056] (2) The prepared polyurethane loose nanofiltration membrane was used to intercept 200 ppm of direct blue: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the interception trend of the membrane corresponding to methyl blue was: 87.27%→91.98%→98.54%→99.07%→99.2%;

[0057] (3) The prepared polyurethane loose nanofiltration membrane was used to intercept 200 ppm of direct red: The results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the interception trend of the membrane corresponding to methyl blue was: 96.49%→98.24%→98.52%→99.15%→99.26% Table 2 Retention performance of polyurethane loose nanofiltration membrane for single dyes

[0058]

[0059] In summary, the separation performance of Xy l / TDI polyurethane loose nanofiltration membrane was studied using 200 ppm single dye solution (MB, DB, RB, relevant information is shown in 4a-c) and 2000 ppm single salt solution (NaCl, MgCl2, Na2SO4, MgSO4). Figure 5Tables 1 and 2 above show the retention rates of dye solutions and salts at different TDI concentrations. The results show that when the TDI concentration increases from 0.4wt% to 1.2wt%, the removal rate of the membrane for a single dye is greatly improved, approaching complete removal. Particle size screening and the Donnan Effect determine the retention performance of the membrane. When the TDI concentration is higher, the thicker the selective layer formed, the higher the degree of crosslinking, and the smaller the voids, resulting in a higher retention rate for the dye and reduced permeability. In addition, the slight increase in electronegativity due to the increase in TDI concentration also helps to improve the removal rate of the dye. The retention results for a single salt show that when the TDI concentration increases from 0.4wt% to 1.2wt%, the retention rate of all membranes for a single salt is less than 20%, indicating that the prepared series of polyurethane loose nanofiltration membranes have a significant loose structure, which has significant advantages for the desalination and reuse of high-salt wastewater.

[0060] Embodiment five:

[0061] At a pressure of 1.0 bar, the interception efficiency and selectivity of the polyurethane loose nanofiltration prepared by TDI with different monomer concentrations on the binary composite pollution system composed of salt and dye were tested, as shown in Table 3:

[0062] (1) The prepared polyurethane loose nanofiltration membrane was used to intercept the methyl blue-sodium sulfate composite system: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the selectivity trend of the membrane corresponding to methyl blue / sodium sulfate was: 5.06%→10.41%→54.36%→101.14%→115.49%;

[0063] (2) The prepared polyurethane loose nanofiltration membrane was used to intercept the methyl blue-sodium chloride composite system: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the selectivity trend of the membrane corresponding to methyl blue / sodium sulfate was: 5.50%→10.88%→61.35%→127.85%→130.16%;

[0064] (3) The prepared polyurethane loose nanofiltration membrane was used to intercept the direct blue-sodium sulfate composite system: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the selectivity trend of the membrane corresponding to methyl blue / sodium sulfate was: 7.05%→10.98%→58.46%→88.09%→101.4%;

[0065] (4) The prepared polyurethane loose nanofiltration membrane was used to intercept the direct blue-sodium chloride composite system: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the selectivity trend of the membrane corresponding to methyl blue / sodium sulfate was: 7.67%→12.07%→65.97%→101.51%→117.15%;

[0066] (5) The prepared polyurethane loose nanofiltration membrane was used to intercept the direct red-sodium sulfate composite system: the results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the selectivity trend of the membrane corresponding to methyl blue / sodium sulfate was: 25.58%→50.02%→57.67%→96.38%→109.62%;

[0067] (6) The prepared polyurethane loose nanofiltration membrane was used to intercept the direct red-sodium chloride composite system: The results showed that when the TDI concentration increased from 0.4wt%→0.6wt%→0.8wt%→1.0wt%→1.2wt%, the selectivity trend of the membrane corresponding to methyl blue / sodium sulfate was: 27.82%→55.01%→65.08%→111.09%→126.65%.

[0068] Table 3 Retention performance of polyurethane loose nanofiltration membrane for salt-dye composite system

[0069]

[0070] In summary, in order to evaluate the selectivity of the prepared polyurethane loose nanofiltration membrane to salt and dye, monovalent salt NaCl and divalent salt Na2SO4 (both at a concentration of 2000 ppm) were selected and mixed with dyes methyl blue, direct blue and direct (both at a concentration of 200 ppm) to prepare salt / dye composite systems. The selectivity of the membrane for salt and dye in the salt-dye composite system was tested in the same way. The test results are shown in Figure 2. Figure 6 ac and as shown in Table 3. The results show that the prepared series of polyurethane loose nanofiltration membranes, with the increase of the concentration of added TDI, the separation selectivity of the membrane for salt and dye gradually increases, among which, when the added TDI reaches 1.0wt% and 1.2wt%, a higher separation effect (>100%) is achieved overall.

[0071] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye, characterized in that: The following steps are involved: Step 1: First, cut the PVDF base membrane into appropriate sizes and wash it with ultrapure water to remove the membrane protective agent sodium bisulfite; Step 2: Soak the PVDF base membrane in a 4 wt % xylitol aqueous solution for 30 min to allow the aqueous solution to completely penetrate into the pores of the PVDF base membrane, and use a clean paper towel to absorb excess xylitol solution on the surface of the base membrane; Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 0.4-1.2wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction; Step 4: Take out the soaked membrane and dry it at room temperature for 5 minutes, then transfer it to a 60°C oven and keep it for 10 minutes to fully react and enhance chemical cross-linking; Step 5: Take out the membrane, cool it, rinse it with ultrapure water for multiple times to remove unreacted monomers and solvents on its surface, and soak it in ultrapure water until use, thereby obtaining a polyurethane loose nanofiltration membrane.

2. The method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye according to claim 1, characterized in that: Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 0.4 wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction.

3. The method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye according to claim 1, characterized in that: Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 0.6 wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction.

4. The method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye according to claim 1, characterized in that: Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 0.8 wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction.

5. The method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye according to claim 1, characterized in that: Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 1.0 wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction.

6. The method for preparing a polyurethane loose nanofiltration membrane for efficient separation of salt and dye according to claim 1, characterized in that: Step 3: The PVDF base membrane soaked with the xylitol solution is fixed in a special polytetrafluoroethylene frame with the active surface facing upward, and a n-hexane solution containing 1.2 wt% TDI is slowly poured into it to soak the base membrane for 10 minutes for polycondensation reaction.

7. A polyurethane loose nanofiltration membrane for efficient separation of salt and dye, characterized in that: The polyurethane loose nanofiltration membrane is prepared based on the preparation method for the highly efficient separation of salt and dye as described in any one of claims 1 to 6.

Images