Preparation method of negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid

Abstract

The present invention discloses a preparation method of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. The nanofiltration membrane comprises a polyethersulfone substrate membrane and a polyamide separation layer located on the substrate membrane; piperazine-2-carboxylic acid is dissolved in water, and the pH is adjusted with sodium hydroxide to obtain an aqueous phase, and trimesoyl chloride is dissolved in n-heptane to obtain an organic phase; the aqueous phase is poured on a plastic polyethersulfone substrate membrane fixed, the substrate membrane is taken out and the surface water droplets are removed with a rubber roller, then the organic phase is poured on the substrate membrane, taken out and cured at room temperature, and washed three times with ethanol and water respectively to obtain the nanofiltration membrane. The water flux of the nanofiltration membrane prepared by the present invention reaches 38.5 Lm-2h-1bar-1, and the relatively high negative charge makes the membrane have a rejection rate of more than 98% for sulfuric acid. The nanofiltration membrane prepared by the present invention is used in the resource treatment of saline wastewater in coal chemical industry, pharmaceutical industry, printing and dyeing industry, chlor-alkali industry, etc., and has relatively high water flux and salt resource utilization efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membrane separation, and particularly to a preparation method of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. Background Art

[0002] Industrial wastewater generated by coal chemical industry, pharmaceutical industry, printing and dyeing industry, chlor-alkali industry, etc. usually contains NaCl and Na2SO4. Discharging sewage and landfilling waste salts will both cause environmental damage and require high treatment costs. Therefore, efficiently separating NaCl and Na2SO4 in wastewater and reducing the cost of wastewater treatment are the keys to the resource utilization of wastewater and achieving "zero liquid" discharge.

[0003] Membrane materials such as polyamide (PA), covalent organic framework (COF), polyester (PE), and polyester amide (PEA) have been applied to the preparation of Cl - / SO4 2- ion separation nanofiltration membranes. The preparation of polyamide thin film composite membranes by interfacial polymerization (IP) of piperazine (PIP) and trimesoyl chloride (TMC) on a porous substrate is still the benchmark for current nanofiltration membranes. Starting from the separation mechanism of nanofiltration membranes, steric hindrance effect is the most fundamental factor affecting the rejection rate, especially for neutral solutes and uncharged membranes. However, for Cl - / SO4 2- screening, the ionic radius difference in aqueous solution is only 0.047 nm, and it is difficult to achieve the screening purpose through size exclusion. Therefore, it is necessary to increase the negative charge density of PA to enhance electrostatic repulsion to break through the trade-off effect. The negative charge of the PA nanofiltration membrane is obtained by hydrolysis of a limited number of unreacted acyl chloride groups into carboxyl groups. Limited by the cross-linking degree of PA, the increase in the number of carboxyl groups is very limited. Thus, adding negatively charged monomers and negatively charged nanomaterials during the synthesis of PA has attracted attention, but the improvement of nanofiltration membrane performance is insufficient, and the process of synthesizing nanomaterials is complex and not conducive to industrial application. Therefore, molecular-level design of the PA layer is required to enhance electrostatic repulsion and achieve efficient ion screening. Summary of the Invention

[0004] In view of the above-mentioned prior art, the present invention provides a preparation method of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. By interfacial polymerization of piperazine-2-carboxylic acid and 1,3,5-benzenetricarbonyl chloride, a negatively charged PA nanofiltration membrane is prepared to improve the separation performance of NaCl and Na2SO4.

[0005] To solve the above technical problems, a preparation method of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid provided by the present invention includes the following steps:

[0006] Step 1, pretreatment of the polyethersulfone-based membrane: Wash the polyethersulfone-based membrane three times with ultrapure water, air-dry it at room temperature, and then fix it in a plastic mold. The bottom of the plastic mold has an exposed area A for exposing the polyethersulfone-based membrane.

[0007] Step 2, soaking in the aqueous solution: Dissolve piperazine-2-carboxylic acid in ultrapure water to obtain an aqueous solution of piperazine-2-carboxylic acid with a mass concentration of 0.3 - 0.7%, and adjust the pH of the aqueous solution of piperazine-2-carboxylic acid to 7.5 - 13 with a sodium hydroxide solution to prepare an aqueous solution; Add an appropriate amount of the aqueous solution to the plastic mold in Step 1, and the volume-area ratio of the aqueous solution to the exposed area A is 0.4 mL / cm2. After soaking and reacting for 2 min, take it out, remove the water droplets on the surface of the polyethersulfone-based membrane, and then fix the polyethersulfone-based membrane in the plastic mold again.

[0008] Step 3, reaction to synthesize polyamide: Dissolve 1,3,5-benzenetricarbonyl chloride in n-heptane to prepare an organic phase solution with a mass concentration of 0.1%, take an appropriate amount of the organic phase solution, and quickly add it to the plastic mold fixed with the polyethersulfone-based membrane in Step 2. The volume-area ratio of the organic phase solution to the exposed area A is 0.2 mL / cm 2 , soak and react for 1 - 6 min, then take it out, air-dry it at room temperature, and wash it three times with ethanol and ultrapure water respectively to obtain a polyamide nanofiltration membrane, abbreviated as NPA.

[0009] Furthermore, in the preparation method of the present invention, where:

[0010] In Step 1, the molecular weight cut-off of the polyethersulfone-based membrane is 15 kDa, and the plastic mold has an exposed area of 5×5 cm to synthesize the nanofiltration membrane.

[0011] In Step 2, the molar concentration of the sodium hydroxide solution is 3M, and the pH adjustment range is 9 - 13. Preferably, the mass concentration of the aqueous solution of piperazine-2-carboxylic acid is 0.4 - 0.6%, and the pH adjustment range is 10 - 12.

[0012] In Step 3, the soaking reaction time is preferably 2 - 4 min.

[0013] Store the obtained NPA in ultrapure water.

[0014] Compared with the prior art, the beneficial effects of the present invention are:

[0015] (1) Piperazine-2-carboxylic acid is selected as the aqueous phase monomer, which can react with 1,3,5-benzenetricarbonyl chloride at normal temperature and pressure. Compared with piperazine, piperazine-2-carboxylic acid has a smaller diffusion coefficient and lower solubility in n-heptane, which is beneficial to forming a thinner active layer to shorten the water transmission path.

[0016] (2) Realize the construction of a strongly negatively charged nanofiltration membrane. Interfacial polymerization of piperazine-2-carboxylic acid and 1,3,5-benzenetricarbonyl chloride can form a strongly negatively charged nanofiltration membrane. While the carboxyl group of piperazine-2-carboxylic acid provides a negatively charged property for the nanofiltration membrane, it can also improve the hydrophilicity of the nanofiltration membrane, thus achieving high flux while maintaining the retention of the nanofiltration membrane.

[0017] (3) The materials used are easily available, the preparation process is simple, the preparation process is short, and the operation is relatively convenient, which is conducive to large-scale industrial production. Description of the Drawings

[0018] Figure 1 is a schematic structural diagram of the preparation process of NPA and the prepared NPA according to the present invention;

[0019] Figure 2 is an atomic force microscope photograph of the NPA-3 membrane prepared in Example 3;

[0020] Figure 3 is an atomic force microscope photograph of the PA-1 membrane prepared in the comparative example;

[0021] Figure 4 is a zeta potential diagram of the NPA-3 membrane and the PA-1 membrane prepared in Example 3 and the comparative example;

[0022] Figure 5 is an elemental analysis diagram of the NPA-3 membrane and the PA-1 membrane prepared in Example 3 and the comparative example;

[0023] Figure 6 is a dynamic water contact angle diagram of a polyethersulfone-based membrane, the NPA-3 membrane prepared in Example 3, and the PA-1 membrane prepared in the comparative example. Detailed Description of the Invention

[0024] The design concept of the preparation method of the negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid is that piperazine-2-carboxylic acid is used as the monomer in the aqueous solution, and 1,3,5-benzenetricarbonyl chloride is used as the monomer in the organic solution. Through interfacial polymerization, it is polymerized on the polyethersulfone-based membrane to form a PA nanofiltration membrane. Among them, compared with piperazine as the aqueous ammonia monomer, piperazine-2-carboxylic acid has a carboxylic acid group by itself, and the formed NPA has rich carboxylic acid groups, so that NPA has a strong negative charge, achieving high flux while maintaining high retention of Na2SO4, such as Figure 1As shown in the figure, the nanofiltration membrane prepared by the present invention comprises a polyethersulfone substrate membrane and a polyamide separation layer located on the substrate membrane; the preparation steps mainly include dissolving piperazine-2-carboxylic acid in ultrapure water and adjusting the pH with sodium hydroxide solution to obtain an aqueous phase, and dissolving 1,3,5-benzenetricarbonyl chloride in n-heptane to obtain an organic phase; then soaking the polyethersulfone substrate membrane in the aqueous phase for a period of time and taking it out, removing the water droplets on the membrane surface with a rubber roller, then soaking it in the organic phase for a period of time and taking it out, curing it in the air, and washing it three times with ethanol and ultrapure water respectively to obtain NPA. This method realizes the preparation of a strongly negatively charged PA nanofiltration membrane, while improving the hydrophilicity of the membrane and reducing the thickness of the membrane. The water flux of the nanofiltration membrane prepared by the present invention reaches 19.8 - 49.3Lm -2 h -1 bar -1 , and the higher negative charge makes the retention rate of sulfuric acid by the membrane greater than 94%. Applying the nanofiltration membrane prepared by the present invention to the resource treatment of saline wastewater in coal chemical industry, pharmaceutical industry, printing and dyeing industry, chlor-alkali industry, etc., has a high water flux and salt resource utilization efficiency.

[0025] The following are specific embodiments of the present invention. The specific embodiments are only used to further illustrate the present invention in detail and do not limit the protection scope of the claims of this application.

[0026] The evaluation of the separation performance of the prepared PA nanofiltration membrane is mainly characterized by two characteristic parameters: the salt rejection rate and water flux of the membrane.

[0027] Rejection rate calculation formula: R = (1 - C p / C f ) × 100%, where R represents the rejection rate, C p and C f are the concentrations of salt (ppm) in the permeate and feed liquid respectively. If not otherwise specified, the feed liquid is a 1000 ppm Na2SO4 aqueous solution.

[0028] Water flux (LMH / bar) is defined as: under certain operating pressure conditions, the volume of water passing through the effective membrane area per unit pressure and per unit time. If not otherwise specified, the test pressure of the membrane is 2 bar.

[0029] Example 1

[0030] The preparation of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid is as follows:

[0031] Step 1, pretreatment of the polyethersulfone substrate membrane: Wash the polyethersulfone substrate membrane three times with ultrapure water, dry it at room temperature and fix it in a plastic mold. The bottom of the plastic mold has an exposed area A for exposing the polyethersulfone substrate membrane, and A = 25 cm 2 ;

[0032] Step 2, soaking the aqueous solution: Dissolve piperazine-2-carboxylic acid in ultrapure water to obtain an aqueous solution with a mass concentration of 0.3%, and adjust the pH to 11.5 with sodium hydroxide solution to prepare the aqueous solution. Take 10 mL of the aqueous solution and add it to the mold in Step 1 to soak the polyethersulfone substrate membrane. After 2 minutes, take out the polyethersulfone substrate membrane, remove the water droplets on the surface with a rubber roller, and fix the substrate membrane to the mold again;

[0033] Step 3, reaction synthesis of polyamide: Dissolve 1,3,5-benzenetricarbonyl chloride in n-heptane to obtain an organic phase solution with a mass concentration of 0.1%. Take 5 mL of the organic phase solution and quickly add it to the mold with the substrate membrane fixed again in Step 2. After soaking and reacting for 3 minutes, take it out and air-dry it at room temperature. Wash it three times with ethanol and ultrapure water respectively to obtain the polyamide nanofiltration membrane, denoted as NPA-1.

[0034] After cross-flow filtration testing, the Na2SO4 rejection rate of NPA-1 prepared in Example 1 is 94.0%, and the water flux is 49.3 LMH / bar.

[0035] Example 2

[0036] Preparation of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. The preparation process is basically the same as that in Example 1, except that in Step 2, the mass concentration of the piperazine-2-carboxylic acid is adjusted from 0.3% to 0.4%, and the finally obtained polyamide nanofiltration membrane is denoted as NPA-2.

[0037] After testing, the Na2SO4 rejection rate of NPA-2 prepared in Example 2 is 94.4%, and the water flux is 45.5 LMH / bar.

[0038] Example 3

[0039] Preparation of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. The preparation process is basically the same as that in Example 1, except that in Step 2, the mass concentration of the piperazine-2-carboxylic acid is adjusted from 0.3% to 0.5%, and the finally obtained polyamide nanofiltration membrane is denoted as NPA-3. The active layer thickness of this NPA-3 is 35.5 nm. Figure 2 This is the atomic force microscope photograph of the active layer of NPA-3.

[0040] After testing, the Na2SO4 rejection rate of NPA-3 prepared in Example 3 is 98.4%, and the water flux is 38.5 LMH / bar.

[0041] Example 4

[0042] Preparation of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. The preparation process is basically the same as that in Example 1, except that in Step 2, the mass concentration of the piperazine-2-carboxylic acid is adjusted from 0.3% to 0.6%, and the finally obtained polyamide nanofiltration membrane is denoted as NPA-4.

[0043] After testing, the Na2SO4 rejection rate of NPA-4 prepared in Example 4 was 98.6%, and the water flux was 30.4 LMH / bar.

[0044] Example 5

[0045] Preparation of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid. The preparation process was basically the same as that in Example 1, except that: in Step 2, the mass concentration of piperazine-2-carboxylic acid was adjusted from 0.3% to 0.7%. The finally obtained polyamide nanofiltration membrane was denoted as NPA-5.

[0046] After testing, the Na2SO4 rejection rate of NPA-5 prepared in Example 5 was 98.9%, and the water flux was 19.8 LMH / bar.

[0047] Comparative Example

[0048] Preparation of a polyamide nanofiltration membrane, the steps are as follows:

[0049] Step 1, the pretreatment of the polyethersulfone substrate membrane was the same as that in Example 1;

[0050] Step 2, soak the aqueous solution: Dissolve piperazine in ultrapure water to prepare an aqueous solution with a mass concentration of 0.5%. Take 10 mL of the aqueous solution and add it to the mold in Step 1 to soak the polyethersulfone substrate membrane. After 2 minutes, take out the polyethersulfone substrate membrane, use a rubber roller to remove the water droplets on the surface, and then fix the substrate membrane to the mold again;

[0051] Step 3, reaction synthesis of polyamide: Dissolve 1,3,5-benzenetricarbonyl chloride in n-heptane to prepare an organic phase solution with a mass concentration of 0.1%. Take 5 mL of the organic phase solution, quickly add it to the mold fixed again in Step 2, soak and react for 3 minutes and then take out, air dry at room temperature, and wash three times with ethanol and ultrapure water respectively to obtain the polyamide nanofiltration membrane, denoted as PA-1, and its active layer thickness was 109.9 nm. Figure 3 It is the atomic force microscope photograph of the active layer of the PA-1 membrane.

[0052] After testing, the Na2SO4 rejection rate of PA-1 prepared in the comparative example was 97.6%, and the water flux was 13.1 LMH / bar.

[0053] Figure 4 It is the graph of the zeta potential of PA-1 and NPA-3 varying with pH. When pH varies from 2 to 10, the zeta potential of NPA-3 is always lower than that of PA-1, indicating that NPA-3 has stronger negative charge than PA-1.

[0054] Figure 5Elemental analysis of the active layer of PA-1 and NPA-3. Among them, the oxygen element content of Example NPA-3 is 22.69%, which is higher than the oxygen element content of Comparative Example PA-1, which is 16.57%. It can be seen that the strong negative charge of NPA-3 comes from the carboxyl group of the piperazine-2-carboxylic acid monomer.

[0055] Figure 6 For the water dynamic contact angles of the polyethersulfone-based membrane, PA-1 and NPA-3, it can be seen that the abundant carboxyl groups of piperazine-2-carboxylic acid improve the hydrophilicity of the active layer.

[0056] Table 1 Test data of the examples and comparative examples of the present invention

[0057]

[0058] According to the preparation conditions of all the above examples and comparative examples and Table 1 and Figures 1-6 , it can be concluded that: as the concentration of the piperazine-2-carboxylic acid solution increases, the rejection rate of the nanofiltration membrane increases and the flux decreases. Specifically, combining the comparative example and Example 3, it can be seen that compared with piperazine as the aqueous monomer, piperazine-2-carboxylic acid can provide a strong negatively charged PA nanofiltration membrane. At the same time, due to the hydrophilicity of the carboxyl group, the membrane thickness is reduced. The hydrophilicity of NPA-3 is stronger than that of PA-1. Therefore, compared with PA-1, NPA-3 can still have a high flux while maintaining a high rejection rate.

[0059] In summary, the preparation method of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid provided by the present invention uses easily available materials, has a simple preparation process, a short preparation process, and is relatively convenient to operate. By using piperazine-2-carboxylic acid as the aqueous monomer to participate in interfacial polymerization, a composite nanofiltration membrane with high rejection rate and high flux can be prepared. Due to the introduction of the carboxylic acid group of piperazine-2-carboxylic acid into the polyamide, the negative charge of the membrane is enhanced and the hydrophilicity of the membrane is improved, realizing the simultaneous improvement of the rejection rate and flux of the composite nanofiltration membrane.

[0060] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can make many changes without departing from the purpose of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A preparation method of a negatively charged nanofiltration membrane based on piperazine-2-carboxylic acid, characterized in that, It includes the following steps: Step 1, pretreatment of the polyethersulfone-based membrane: The polyethersulfone-based membrane is washed three times with ultrapure water, dried at room temperature and then fixed in a plastic mold. The bottom of the plastic mold has an exposed area A for exposing the polyethersulfone-based membrane. Step 2, soaking the aqueous solution: Dissolve piperazine-2-carboxylic acid in ultrapure water to obtain an aqueous solution of piperazine-2-carboxylic acid with a mass concentration of 0.3-0.7%, and adjust the pH of the aqueous solution of piperazine-2-carboxylic acid to 7.5-13 with sodium hydroxide solution to prepare an aqueous solution; Add an appropriate amount of the aqueous solution to the plastic mold in Step 1, and the volume-area ratio of the aqueous solution to the exposed area A is 0.4 mL / cm 2 , After soaking and reacting for 2 minutes, remove the water droplets on the surface of the polyethersulfone substrate membrane, and then fix the polyethersulfone substrate membrane to the plastic mold again; Step 3, reaction synthesis of polyamide: Dissolve 1,3,5-benzenetricarbonyl chloride in n-heptane to prepare an organic phase solution with a mass concentration of 0.1%. Take an appropriate amount of the organic phase solution and quickly add it to the plastic mold fixed with the polyethersulfone-based membrane in Step 2. The volume-area ratio of the organic phase solution to the exposed area A is 0.2 mL / cm 2 , take it out after soaking and reacting for 1-6 minutes, air-dry it at room temperature, and wash it three times with ethanol and ultrapure water respectively to obtain a polyamide nanofiltration membrane, simply referred to as NPA.

2. The preparation method according to claim 1, characterized in that, In Step 1, the molecular weight cut-off of the polyethersulfone-based membrane is 15 kDa, and the plastic mold has an exposed area of 5 cm × 5 cm to synthesize a nanofiltration membrane.

3. The preparation method according to claim 1, characterized in that, In Step 2, the molar concentration of the sodium hydroxide solution is 3 M, and the pH adjustment range is 9-13.

4. The preparation method according to claim 3, characterized in that, In Step 2, the mass concentration of the piperazine-2-carboxylic acid aqueous solution is 0.4-0.6%, and the pH adjustment range is 10-12.

5. The preparation method according to claim 1, characterized in that, In Step 3, the soaking reaction time is 2-4 min.

6. The preparation method according to claim 1, characterized in that, The obtained NPA is stored in ultrapure water.

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