Modification method for improving chlorine resistance of polyamide composite semipermeable membrane

Abstract

The invention relates to a modification method for improving chlorine resistance of a polyamide composite semipermeable membrane. The preparation method comprises the following steps: firstly, carrying out membrane surface Fe < 2 + > complexing positioning on the polyamide composite semipermeable membrane by using an aqueous solution containing ferrous sulfate and citric acid, and then carrying out membrane surface in-situ aromatic amine monomer enrichment treatment on the membrane surface subjected to Fe < 2 + > complexing treatment by using an aqueous solution containing aromatic amine monomers; carrying out membrane surface Fenton reaction treatment on the membrane surface subjected to in-situ enrichment of the aromatic amine monomer by using an acidic aqueous solution containing H2O2; and finally, soaking the membrane subjected to Fenton reaction treatment by using a citric acid aqueous solution and washing the membrane with pure water. The aromatic amine monomer is used for assisting the citric acid chelating Fenton reaction, a phenazine structure is generated in situ through the reaction of the aromatic amine monomer and an amino-terminated group, a polyamide composite semipermeable membrane separation layer is reconstructed, and the chlorine resistance is improved. The method has the characteristics of mild conditions, simplicity and feasibility, easiness in industrialization and the like, and the salt rejection rate and the water flux of the obtained chlorine-resistant polyamide composite semipermeable membrane are not lower than those of the existing polyamide composite semipermeable membrane.

Description

Technical Field

[0001] The invention belongs to the technical field of separation membrane preparation, and particularly relates to a modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane. Background Art

[0002] Polyamide composite semipermeable membranes have been widely used in seawater desalination, wastewater treatment and resource utilization, drinking water purification, industrial fluid separation and concentration, and other fields due to their excellent permeability selectivity. In the industrial application of polyamide composite semipermeable membranes, microbial contamination is an important cause of membrane performance degradation and deterioration. Free chlorine-containing disinfectants are usually used to pretreat the feed liquid, and the membrane is cleaned after operation to effectively inhibit the growth of microorganisms and the deposition of pollutants, thereby maintaining the efficient separation performance of the membrane. However, the strong oxidizing property of free chlorine makes it easy for it to react with the amide bond (-CONH-) in the polyamide separation layer, especially attacking the NH group on the amide bond, and then generating irreversible aromatic ring chlorination products through the Orton rearrangement reaction. This process will cause serious damage to the chemical structure of the polyamide separation layer, resulting in a significant decrease in the selectivity of the membrane, and ultimately leading to membrane performance failure. According to relevant literature reports [see Mohammad RM, Arto P, Mehrdad H, et al. Journal of Membrane Science, 2019, 15(584): 300-308], in 2018, the total amount of polyamide composite reverse osmosis membrane elements discarded due to performance degradation exceeded 16,500 tons, and the output showed an increasing trend year by year. The main reason for the performance degradation was the chlorination degradation of the polyamide separation layer. Therefore, improving the chlorine resistance of the polyamide composite semipermeable membrane and extending the membrane service life are relatively preferred solutions to reduce the discard of polyamide composite semipermeable membranes.

[0003] So far, a variety of methods have been proposed to improve the chlorine degradation resistance of polyamide composite semipermeable membranes and extend the life of polyamide composite semipermeable membranes. For example, by optimizing the structure of the monomers during interfacial polymerization, functional groups or steric hindrances with chlorine resistance are introduced, a surface protective layer is formed by cross-linking or end-capping the polyamide separation layer, and functional polymers or nanomaterials (such as graphene, carbon nanotubes, metal organic frameworks, etc.) are introduced into the polyamide separation layer to utilize their chemical stability or physical barrier effect. Therefore, reducing the sensitivity of the polyamide separation layer to free chlorine and introducing chemical sacrificial or chlorine-stabilized units on the membrane surface are a feasible solution to improve the chlorine resistance of polyamide composite semipermeable membranes. However, most of these methods are used to improve the preparation process of polyamide composite semipermeable membranes, and there are problems such as complex process and low modification effect, and they cannot be applied to the in-situ chlorine resistance modification of the polyamide composite semipermeable membrane components actually used.

[0004] Literature research shows that the post-treatment of the polyamide separation layer by oxidation reaction can achieve the regulation of the separation layer structure and composite membrane performance, and has the characteristics of simple operation and significant regulation effect. The inventor's research team has previously disclosed a method of chemically reconstructing the separation layer of a degraded polyamide membrane using a diazotization reaction, and by controlling the conditions, introducing more azo cis structures into the separation layer to achieve the effect of improving the salt rejection performance of the degraded polyamide membrane [see patent ZL 202210317003.2]. There are also related patents that use a diazotization-coupling reaction to introduce phenol / aniline small molecules on the surface of a newly prepared polyamide semipermeable membrane, thereby achieving the effect of improving membrane performance [see Chinese patent application publication number CN110404417A]. These methods involve the regulation, repair and improvement of membrane separation performance, but do not involve the improvement of the chlorine resistance of polyamide composite semipermeable membranes.

[0005] As a classic oxidation reaction, Fenton reaction has gradually attracted attention in the field of membrane modification due to its simple operation, rapid reaction and environmental friendliness. However, the simple combination of traditional Fenton reaction and polyamide semipermeable membrane modification has great limitations. 2+ Reacts quickly with H2O2 to generate a large amount of · OH and OH - , which can easily form iron hydroxide precipitation on the membrane surface and cause membrane pollution. 3+ Difficult to reduce to Fe 2+ , the Fenton reaction is difficult to proceed stably and continuously, affecting the modification effect. In further research, this research team used citric acid (CA) as a co-catalyst, and used the chelation effect of CA and Fe to reduce Fe 3+ With OH - The reaction increases the concentration of soluble iron, promotes the continuation of the Fenton reaction, and the terminal amino group · The phenazine quinone amine is formed under the action of OH, and a phenazine ring protective layer is constructed on the membrane surface by introducing an aniline monomer, thereby improving the chlorine resistance of the polyamide composite semipermeable membrane. This method can be carried out in a room temperature water system and is characterized by simple operation, easy industrialization, and significant results. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane. The surface of the polyamide composite semipermeable membrane is modified by an aromatic amine monomer-assisted citric acid chelating Fenton reaction, thereby effectively reducing the terminal amino groups on the membrane surface and introducing a protective layer containing a phenazine ring structure, thereby achieving the effect of improving the chlorine resistance of the polyamide composite semipermeable membrane. The method has the characteristics of mild conditions, simplicity and ease of implementation, and significant modification effect.

[0007] The present invention provides a modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane. The method comprises the following steps: 2+ Complexation positioning process, in-situ enrichment of aromatic amine monomers on the membrane surface, Fenton reaction modification of the membrane surface and post-treatment methods.

[0008] In order to better understand the present invention, the content of the present invention is further explained below.

[0009] A modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane comprises performing an aromatic amine monomer-assisted citric acid chelation Fenton reaction on the membrane surface of the polyamide composite semipermeable membrane, thereby reducing the number of terminal amino groups on the membrane surface and introducing a protective layer containing a phenazine ring structure, thereby improving the chlorine resistance of the polyamide composite semipermeable membrane. The method comprises the following steps:

[0010] S-1, in a container equipped with a stirring device, a certain amount of FeSO4·7H2O, citric acid monohydrate (CA·H2O) and deionized water are added in a set ratio and stirred to obtain a modification reagent A; the modification reagent A is brought into contact with the surface of a polyamide composite semipermeable membrane at room temperature, and the membrane surface is drained after standing for 5 minutes;

[0011] The mass percentage concentration of ferrous sulfate in the modification reagent A is 0.2-2.0 wt%, and the mass percentage concentration of citric acid is 0.1-1 wt%. Preferably, the mass percentage concentration of ferrous sulfate is 0.5-1.5 wt%, and the mass percentage concentration of citric acid is 0.25-0.75 wt%.

[0012] The separation layer of the polyamide composite semipermeable membrane is a wholly aromatic polyamide, or an aromatic polyamide-urea, or an aromatic-aliphatic mixed polyamide;

[0013] S-2, in-situ enrichment of aromatic amine monomer on the membrane surface: In a container equipped with a stirring device, a certain amount of aromatic amine monomer and deionized water are added according to a set ratio and stirred to obtain a modification reagent B; at room temperature, the modification reagent B is brought into contact with the surface of the polyamide semipermeable membrane treated with the modification reagent A, and the membrane surface is drained after standing for 5 minutes;

[0014] The aromatic amine monomer in the modification reagent B is p-aminobenzoic acid, or p-aminobenzenesulfonic acid, or p-aminobenzenesulfonamide, or m-aminobenzenesulfonic acid, or m-aminobenzoic acid, or m-aminobenzenesulfonamide, or a mixture thereof;

[0015] The mass percentage concentration of the aniline monomer in the modification reagent B is 0.01 to 0.1 wt %; preferably, the mass percentage concentration is 0.02 to 0.06 wt %.

[0016] S-3, membrane surface Fenton reaction: In a container equipped with a stirring device, a certain amount of H2O2, hydrochloric acid, and deionized water are added in a set ratio and stirred to obtain a modification reagent C. Under certain temperature conditions, the modification reagent C is brought into contact with the surface of the polyamide membrane treated with the modification reagent B to react. After a period of time, the membrane surface reagent is drained;

[0017] The mass percentage concentration of H2O2 in the modification reagent C is 0.1-5wt%, and the pH is 2-10; preferably, the mass percentage concentration of H2O2 is 0.5-3wt%, and the pH is 3-7.

[0018] The temperature for the reaction between the modifying reagent C and the membrane surface is 5 to 35° C.; preferably, the temperature is controlled at 15 to 30° C.

[0019] The contact reaction time between the modification reagent C and the membrane surface is 5 to 30 minutes; preferably, the contact time is 10 to 20 minutes.

[0020] S-4, post-treatment: In a container equipped with a stirring device, add a certain amount of citric acid monohydrate (CA·H2O) and deionized water in a set ratio and stir to obtain a modification reagent D. At room temperature, soak the polyamide membrane treated with modification reagent C in modification reagent D for 6 hours. Remove the membrane and rinse it with pure water. Then soak the modified membrane in deionized water or treat the membrane surface with a protective agent and dry it for storage.

[0021] The pH of the modifying reagent D is 2-6; preferably, the pH is 3-5.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) Aromatic amine monomer-assisted citric acid chelation Fenton reaction was carried out on the surface of a polyamide composite semipermeable membrane containing terminal amino groups, and further reaction was performed to achieve the effect of improving the chlorine resistance of the membrane.

[0024] (2) Fe complexation on the membrane surface 2+ , pre-deposited aromatic amine monomers and membrane surface Fenton reaction, to achieve the in-situ reaction of aromatic amine monomers with terminal amino groups to generate phenazine structure, thereby improving chlorine resistance.

[0025] (3) This method has the characteristics of mild conditions and is simple and easy to implement, and the desalination rate and water flux of the obtained chlorine-resistant polyamide composite semipermeable membrane are not lower than those of the existing polyamide composite semipermeable membrane. DETAILED DESCRIPTION

[0026] In order to better understand the present invention, the present invention is further described below in conjunction with specific embodiments.

[0027] The present invention uses salt rejection rate and water flux to evaluate the separation performance of the polyamide composite semipermeable membrane. The salt rejection rate and water flux of the composite membrane are evaluated through a cross-flow permeation test.

[0028] The desalination rate (R) is defined as: under certain operating conditions, the solute concentration of the feed liquid (C f ) and the solute concentration in the permeate (C p ) and then divided by the solute concentration of the feed liquid. The specific calculation formula is as follows:

[0029]

[0030] Water flux (F) is defined as the volume (V) of water that passes through a unit membrane area (A) per unit time (t) under certain operating conditions. Its unit is L / (m 2 h). The specific calculation formula is as follows:

[0031]

[0032] The operating conditions used in the determination of the separation performance of the polyamide reverse osmosis membrane in the present invention are: the inlet liquid is a 500 mg / l sodium chloride aqueous solution, the operating pressure is 0.6 MPa, the operating temperature is 25° C., and the solution pH is 6.8.

[0033] The present invention uses a static immersion process to evaluate the chlorine resistance of polyamide composite semipermeable membranes. The polyamide composite semipermeable membranes are immersed in a 2000 mg / L sodium hypochlorite aqueous solution at a pH of 8.0 and 25°C for 25 hours. The membranes are then removed and rinsed with deionized water. The salt rejection and water flux are then evaluated.

[0034] Comparative Examples 1-3

[0035] Polyamide composite semipermeable membranes with separation layers of aromatic polyamide, aromatic polyamide-urea, and aromatic-aliphatic mixed polyamide were prepared by conventional interfacial polymerization. The separation performance and chlorine resistance of the polyamide composite semipermeable membranes, which were rinsed with deionized water, were evaluated.

[0036] [Table 1]

[0037]

[0038] Obviously, the polyamide composite semipermeable membrane prepared by the conventional interfacial polymerization method, which contains aromatic polyamide, aromatic polyamide-urea, and aromatic-aliphatic mixed polyamide separation layers, after being immersed in a 2000 mg / L sodium hypochlorite aqueous solution at a pH of 8.0 and 25°C for 25 hours, has a significant decrease in membrane desalination rate and a significant increase in flux, and the membrane has poor chlorine resistance.

[0039] Example 1

[0040] A polyamide composite semipermeable membrane with an aromatic polyamide separation layer was prepared by conventional interfacial polymerization. The polyamide composite semipermeable membrane, which had been rinsed with deionized water, was then subjected to the following treatments:

[0041] (1) Membrane surface Fe 2+ Complexation positioning: In a container equipped with a stirring device, a certain amount of FeSO4·7H2O, citric acid monohydrate (CA·H2O), and deionized water were added in a predetermined ratio and stirred to obtain a modification reagent A having a mass percentage concentration of 0.80 wt% ferrous sulfate and 0.50 wt% citric acid. The modification reagent A was brought into contact with the surface of a polyamide composite semipermeable membrane at room temperature, allowed to stand for 5 minutes, and then the membrane surface reagent was drained.

[0042] (2) In-situ enrichment of aromatic amine monomers on the membrane surface: In a container equipped with a stirring device, a certain mass of p-aminobenzoic acid and deionized water are added according to a set ratio, and the mixture is stirred to obtain a modification reagent B with a p-aminobenzoic acid mass percentage concentration of 0.05 wt%; at room temperature, the modification reagent B is brought into contact with the surface of the polyamide composite semipermeable membrane treated with the modification reagent A, and the membrane surface reagent is drained after standing for 5 minutes;

[0043] (3) Membrane surface Fenton reaction: In a container equipped with a stirring device, a certain amount of H2O2, hydrochloric acid and deionized water are added in a set ratio, and stirred and mixed to obtain a modification reagent C with a H2O2 mass percentage concentration of 2.0 wt% and a pH of 5; at 25°C, the modification reagent C is contacted with the surface of the polyamide composite semipermeable membrane treated with the modification reagent B to react, and the membrane surface reagent is drained after 10 minutes;

[0044] (4) Post-treatment: In a container equipped with a stirring device, add a certain amount of monohydrated citric acid (CA·H2O) and deionized water according to a set ratio, and stir and mix them evenly to obtain a modification reagent D with a pH of 4.0; at room temperature, soak the polyamide composite semipermeable membrane treated with modification reagent C with modification reagent D for 6 hours, take it out and rinse it with pure water, and soak the modified membrane in deionized water or treat the membrane surface with a protective agent and dry it for storage.

[0045] The separation performance and chlorine resistance of the modified polyamide composite semipermeable membrane were evaluated.

[0046] Example 2-3

[0047] Except that the separation layer material of the composite semipermeable membrane was changed to the aromatic polyamide-urea and aromatic-aliphatic mixed polyamide described in Table 2, the other processes were the same as those in Example 1.

[0048] [Table 2]

[0049]

[0050] Examples 4-8

[0051] Except that the concentrations of ferrous sulfate and citric acid in the modification reagent A were changed to the concentrations recorded in Table 3, the other processes were the same as those in Example 1.

[0052] [Table 3]

[0053]

[0054]

[0055] Examples 9-10

[0056] Except that the aromatic amine monomer in the modification reagent B was replaced with the monomer described in Table 4, the other modification processes were the same as those in Example 1.

[0057] [Table 4]

[0058]

[0059] Examples 11-12

[0060] Except that the concentration of the aromatic amine monomer in the modification reagent B was changed to the concentration recorded in Table 5, the other processes were the same as those in Example 1.

[0061] [Table 5]

[0062]

[0063]

[0064] Examples 13-14

[0065] Except that the mass concentration of H2O2 in the modification reagent C was changed to the concentration recorded in Table 6, the other processes were the same as those in Example 1.

[0066] [Table 6]

[0067]

[0068] Examples 15-16

[0069] Except that the pH of the modification reagent C was changed to the pH described in Table 7, the other processes were the same as those in Example 1.

[0070] [Table 7]

[0071]

[0072]

[0073] Examples 17-20

[0074] Except that the temperature and time of the contact reaction between the modification reagent C and the membrane surface were changed to the temperature and time recorded in Table 8, the other processes were the same as those in Example 1.

[0075] [Table 8]

[0076]

[0077] Examples 21-22

[0078] Except that the pH of the modification reagent D was changed to the pH described in Table 9, the other processes were the same as those in Example 1.

[0079] [Table 9]

[0080]

[0081] The polyamide composite semipermeable membrane modified by the technology of the present invention is treated by immersing in a 2000 mg / L sodium hypochlorite aqueous solution at a pH of 8.0 and 25° C. for 25 hours. The membrane desalination and flux remain stable, demonstrating excellent chlorine resistance.

[0082] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not possible to enumerate all embodiments here. Any obvious variations or modifications arising from the technical solution of the present invention remain within the scope of protection of the present invention.

[0083] The above are only specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that are not conceived through creative work should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection defined in the claims.

Claims

1. A modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane, characterized in that: An aromatic amine monomer-assisted citric acid chelating Fenton reaction is performed on the surface of a polyamide composite semipermeable membrane to reduce the number of amino groups on the membrane surface while introducing a protective layer containing a phenazine ring structure, thereby improving the chlorine resistance of the polyamide composite semipermeable membrane. The method comprises the following steps: (1) Membrane surface Fe 2+ Complexation positioning: In a container equipped with a stirring device, a certain amount of FeSO4·7H2O, monohydrated citric acid, and deionized water are added in a set ratio and stirred to obtain a modified reagent A. The modified reagent A is brought into contact with the surface of the polyamide composite semipermeable membrane at room temperature, and the membrane surface is drained after the modified reagent A is allowed to stand for 5 minutes. (2) In-situ enrichment of aromatic amine monomers on the membrane surface: In a container equipped with a stirring device, a certain amount of aromatic amine monomers and deionized water are added according to a set ratio, and the modified reagent B is obtained by stirring and mixing. At room temperature, the modified reagent B is brought into contact with the surface of the polyamide composite semipermeable membrane treated with the modified reagent A, and the membrane surface reagent is drained after standing for 5 minutes; (3) Fenton reaction on the membrane surface: In a container equipped with a stirring device, a certain amount of H2O2, hydrochloric acid and deionized water are added in a set ratio, and the mixture is stirred and evenly mixed to obtain a modification reagent C; under certain temperature conditions, the modification reagent C is brought into contact with the surface of the polyamide composite semipermeable membrane treated with the modification reagent B to react, and after a period of time, the membrane surface reagent is drained; (4) Post-treatment: In a container equipped with a stirring device, add a certain amount of monohydrated citric acid and deionized water according to a set ratio, and stir and mix to obtain a modification reagent D; at room temperature, soak the polyamide composite semipermeable membrane treated with the modification reagent C with the modification reagent D for 6 hours, take it out and wash it with pure water, and soak the modified membrane in deionized water or treat the membrane surface with a protective agent and dry it for storage.

2. A method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The separation layer of the polyamide composite semipermeable membrane is wholly aromatic polyamide, or aromatic polyamide-urea, or aromatic-aliphatic mixed polyamide.

3. A method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The mass percentage concentration of ferrous sulfate in the modification reagent A is 0.2-2.0 wt%, and the mass percentage concentration of citric acid is 0.1-1 wt%. Preferably, the mass percentage concentration of ferrous sulfate is 0.5-1.5 wt%, and the mass percentage concentration of citric acid is 0.25-0.75 wt%.

4. A method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The aromatic amine monomer in the modification reagent B is p-aminobenzoic acid, or p-aminobenzenesulfonic acid, or p-aminobenzenesulfonamide, or m-aminobenzenesulfonic acid, or m-aminobenzoic acid, or m-aminobenzenesulfonamide, or a mixture thereof.

5. A modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The mass percentage concentration of the aromatic amine monomer in the modification reagent B is 0.01 to 0.1 wt %; preferably, the mass percentage concentration is 0.02 to 0.06 wt %.

6. The method for chlorine-resistant modification of a polyamide composite semipermeable membrane according to claim 1, wherein: The mass percentage concentration of H2O2 in the modification reagent C is 0.1-5wt%, and the pH is 2-8; preferably, the mass percentage concentration of H2O2 is 0.5-3wt%, and the pH is 3-7.

7. A modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The temperature for the reaction between the modifying reagent C and the membrane surface is 5 to 35° C.; preferably, the temperature is controlled at 15 to 30° C.

8. A modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The contact reaction time between the modification reagent C and the membrane surface is 5 to 30 minutes; preferably, the contact time is 10 to 20 minutes.

9. A modification method for improving the chlorine resistance of a polyamide composite semipermeable membrane according to claim 1, characterized in that: The pH of the modifying reagent D is 2-6; preferably, the pH is 3-5.

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