CN114307649B

CN114307649B - Modification method of polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions

Info

Publication number: CN114307649B
Application number: CN202111351850.2A
Authority: CN
Inventors: 程巍; 张涛; 王丽红
Original assignee: Research Center for Eco Environmental Sciences of CAS
Current assignee: Research Center for Eco Environmental Sciences of CAS
Priority date: 2021-11-16
Filing date: 2021-11-16
Publication date: 2022-10-04
Anticipated expiration: 2041-11-16
Also published as: CN114307649A

Abstract

The invention discloses a modification method of a polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions, belonging to the technical field of membrane separation. The modification method utilizes free radicals generated by activating an oxidant with alkali to treat a functional layer of the polyamide nanofiltration membrane on one hand, introduces the free radicals on a benzene ring structure of the functional layer to initiate benzene ring inter-chain polymerization reaction, and regulates and controls the aperture of the nanofiltration membrane through polymerization degree; and on the other hand, the added aldehyde modifier is treated, the aldehyde is uniformly grafted to the surface of the functional layer of the polyamide nanofiltration membrane, the grafted aldehyde compound is easy to undergo aldehyde condensation reaction under the alkaline condition to generate hydroxyl aldehyde, and the grafting length is controlled by controlling the condensation degree. The introduction of the hydrophilic group enables the surface of the nanofiltration membrane to have better hydrophilicity. The aldehyde condensation reaction improves the alkalinity of the modification liquid, thereby accelerating the modification process. Compared with the traditional modification method, the method has the advantages of easily obtained materials, simple operation, controllable process and obvious advantages.

Description

Modification method of polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions

Technical Field

The invention belongs to the technical field of membrane separation, and particularly relates to a modification method for selectively improving the retention rate of divalent salt and water flux of a polyamide composite nanofiltration membrane.

Background

Among a plurality of water treatment processes, membrane separation refers to the process of separating, purifying and concentrating different components in feed liquid by utilizing the selectivity of a membrane on the molecular level, wherein a nanofiltration technology is a process between reverse osmosis and ultrafiltration, can effectively intercept substances with the molecular weight of 200-1000 Da, and is mainly used for separating inorganic salts and small molecular organic matters. Compared with a reverse osmosis membrane, the nanofiltration membrane has unique ion selectivity and larger membrane flux, has better interception effect on bivalent and high-valence ions, but has weaker interception effect on monovalent ions, and the pressure required by the nanofiltration membrane is much lower at a certain flux, so the nanofiltration membrane has wide application potential in the fields of water softening, multistage desalination treatment and the like.

During nanofiltration, the properties of the membrane, including pore size, surface charge, hydrophilicity and hydrophobicity, roughness, etc., have a significant impact on filtration performance. In order to prepare a nanofiltration membrane with high flux, high rejection rate and more stable structure, people usually modify the nanofiltration membrane by a grafting method. The grafting method adopts a chemical method, high-energy rays and the like to treat the membrane, so that reactive active sites and free radicals are generated on the surface of the membrane, and then a modifier with excellent performance is grafted on the membrane in a covalent bond mode, so that the connection effect is firm, the membrane performance is stable, the modification is carried out on the membrane surface, and the internal structure is not damaged. Different grafting modification methods have different influences on the hydrophilicity and the porosity of the nanofiltration membrane and different effects on improving the permeability, and the separation performance is mainly influenced by the membrane aperture, the hydrophilicity of the membrane material and the surface charge. However, many nanofiltration membrane grafting modification processes are complex and still in the laboratory research stage, and some membrane modifiers are high in cost and are not suitable for large-scale popularization and application. Therefore, the research on the nanofiltration membrane grafting modification process with low cost and simple process is still an important direction for the research efforts of future researchers and engineering technicians.

Disclosure of Invention

In order to solve the problems of the existing nanofiltration membrane in water treatment application, the invention aims to provide a modification method for selectively improving the retention rate of divalent salt and water flux of a polyamide composite nanofiltration membrane, so as to solve the problems of low water flux and poor selectivity of the existing nanofiltration membrane, and the modification method is simple and convenient to operate and easy to regulate and control in structure.

In order to solve the technical problem, the technical scheme is that the modification method of the polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions comprises the following steps: the polyamide composite nanofiltration membrane is immersed in a modification solution for modification, the pH value of the modification solution is 9.5-11.5, the modification solution contains aldehyde free radicals generated by the reaction of an oxidant and aldehyde, the modification solution is taken out after 0.25-2 h, and the residual modification solution is washed away by water.

The modification method of the polyamide composite nanofiltration membrane for selectively intercepting the divalent salt ions is further improved as follows:

preferably, the oxidizing agent is one or a combination of two or more of hydrogen peroxide, potassium persulfate, ammonium persulfate, sodium persulfate, potassium monopersulfate, sodium monopersulfate and ammonium monopersulfate.

Preferably, the oxidizing agent is sodium persulfate.

Preferably, the aldehyde is one or a combination of two or more of formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, 2-methylbutyraldehyde, 3-methylbutyraldehyde, malonaldehyde, succinaldehyde and glutaraldehyde.

Preferably, the aldehyde is acetaldehyde.

Preferably, the concentration of the oxidant is 1-1000 mmol/L.

Preferably, the concentration of the aldehyde is 0.1 to 1mol/L.

Preferably, the pH of the modification solution is adjusted by adding sodium hydroxide solution and/or potassium hydroxide solution.

Preferably, the concentration of the sodium hydroxide solution or the potassium hydroxide solution is 0.05 to 3mol/L.

Compared with the prior art, the invention has the beneficial effects that:

1) The invention relates to a modification method of a high-flux polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions, which has the advantages of cheap selected modification raw materials, simple and easily-controlled modification process and capability of providing possibility for further application and popularization.

2) According to the method, the functional layer of the polyamide nanofiltration membrane is activated simply through free radicals generated by activating the oxidant with the alkali liquor, the free radicals are introduced into a benzene ring structure of the functional layer, so that benzene ring inter-chain polymerization reaction (as shown in figure 1) is initiated, the aperture of the nanofiltration membrane is regulated and controlled through the polymerization degree, and the selectivity of the nanofiltration membrane is improved. Many nanofiltration membrane grafting modification methods reported at present are to graft hydrophilic and amphoteric polymers, which can increase the membrane mass transfer resistance and reduce the water flux of the nanofiltration membrane, but the invention can control the degree of polymerization so that the membrane flux cannot be obviously reduced.

3) The invention further utilizes a mode of activating an oxidant by alkali liquor to carry out free radical activation on the added aldehyde compound, and the aldehyde compound is uniformly grafted to the surface of the functional layer of the polyamide nanofiltration membrane. The grafted aldehyde compound is pulled out of alpha-H under the alkaline condition, the generated enol negative ion or carbanion is easily subjected to electrophilic attack of the carbonyl part of another molecule of aldehyde, namely, aldol condensation reaction is carried out, a series of hydroxy aldehyde is generated, and simultaneously protons are extracted from the modification liquid, so that the alkalinity of the modification liquid is further improved, and the modification process is accelerated. By controlling the condensation degree, the thickness of the modified layer can be controlled, the interception rate is improved, and the mass transfer resistance is controlled. The introduction of a plurality of hydrophilic groups such as hydroxyl, aldehyde group and the like enables the surface of the nanofiltration membrane to have better hydrophilicity, and is beneficial to the improvement of water flux.

4) The high-flux polyamide composite nanofiltration modified membrane selectively intercepted is mainly applied to separation of monovalent salt and multivalent salt, the interception of high-valence salt ions is mainly and obviously improved, and the treatment pressure of a subsequent reverse osmosis process is reduced.

5) It should be noted that the modification method of the invention is not only suitable for the polyamide composite nanofiltration membrane, but also can expect the same chemical reaction process for the polyamide composite reverse osmosis membrane with similar structure, and has similar performance improvement effect for other polyamide membranes.

Drawings

FIG. 1 is a schematic view of an interchain polymerization reaction of a polyamide composite membrane;

FIG. 2 is a schematic view of aldehyde condensation reaction of a polyamide composite membrane.

Detailed Description

The present invention will be further described with reference to the following specific examples, which are intended to be illustrative of the present invention and are not to be construed as limiting the scope of the invention.

Example 1

Selecting a self-made polyamide composite nanofiltration membrane, and immersing the self-made polyamide composite nanofiltration membrane into a modification solution, wherein the modification solution comprises: 100mmol/L sodium persulfate, 1mol/L sodium hydroxide and 0.5mol/L aqueous acetaldehyde solution. And adding a self-made polypiperazine amide composite nanofiltration membrane, modifying for 1h, taking out the membrane, and repeatedly washing with a large amount of deionized water for testing.

The film without the modified solution treatment was a comparative film.

The test conditions were: operating pressure is 6bar, prepressing the modified membrane and the comparison membrane for 2h at 25 ℃ to stabilize the membrane performance, adjusting the operating pressure to 4bar, and measuring pure water flux and salt rejection rate of the modified membrane and the comparison membrane. Preparing 10mmol/L sodium chloride and 5mmol/L calcium chloride and magnesium chloride solutions, and measuring the retention rate of the composite nanofiltration membrane on various electrolyte solutions after 2h filtration before and after modification under the same operation conditions.

Example 2

Selecting a polyamide nanofiltration membrane 1 of Dow chemical company in America, and immersing the polyamide nanofiltration membrane into a modification solution, wherein the modification solution comprises the following components in parts by weight: 100mmol/L sodium persulfate, 100mol/L hydrogen peroxide, 1.5mol/L sodium hydroxide and 0.5mol/L aqueous solution of butanedialdehyde. And (4) taking out the membrane after the modification is carried out for 0.5h, and carrying out testing after repeatedly washing by using a large amount of deionized water.

The film without the modified solution treatment was a comparative film.

The water flux and salt ion rejection were tested as described in example 1.

Example 3

Selecting a polyamide nanofiltration membrane 2 of Dow chemical company in America, and immersing the polyamide nanofiltration membrane into a modification solution, wherein the modification solution comprises the following components: 50mmol/L sodium persulfate, 50mol/L potassium monopersulfate, 0.5mol/L sodium hydroxide and 0.5mol/L isobutyraldehyde aqueous solution. And (4) taking out the membrane after the modification for 2h, and repeatedly washing the membrane with a large amount of deionized water for testing.

The film without the modified solution treatment was a comparative film.

The water flux and salt ion rejection were tested as described in example 1.

See table 1. Table 1 shows the results of the performance tests of the modified films prepared by the modification methods described in examples 1 to 3 and the control film

Table 1 examples 1-3 performance test results for modified and control films

As can be seen from the results in Table 1, different polyimides were first usedAfter the amine composite nanofiltration membrane is modified, the water flux is slightly improved, the improvement rate is between 20 and 50 percent, more obviously, the rejection rate of high-valence salt is obviously improved, and CaCl is treated ₂ And MgCl ₂ The retention rate of the catalyst reaches more than 90 percent. On the whole, the modification method disclosed by the invention is suitable for both self-made and commercial polyamide composite nanofiltration membranes, and has an obvious modification advantage on the interception effect of high-valence salts.

It should be understood by those skilled in the art that the foregoing is only illustrative of several embodiments of the invention, and not of all embodiments. It should be noted that many variations and modifications are possible to those skilled in the art, and all variations and modifications that do not depart from the gist of the invention are intended to be within the scope of the invention as defined in the appended claims.

Claims

1. A modification method of a polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions is characterized by comprising the following steps: the preparation method comprises the steps of immersing a polyamide composite nanofiltration membrane into a modification solution for modification, wherein the polyamide molecular formula of the polyamide composite nanofiltration membrane contains a benzene ring structure, the pH of the modification solution is 9.5-11.5, the modification solution contains an oxidant and aldehyde, the oxidant and the aldehyde can react to generate aldehyde free radicals, and the modified solution is taken out after 0.25-2 h of modification, and then the modified solution is washed away by water.

2. The method for modifying the polyamide composite nanofiltration membrane capable of selectively intercepting divalent salt ions according to claim 1, wherein the oxidant is one or a combination of more than two of hydrogen peroxide, potassium persulfate, ammonium persulfate, sodium persulfate, potassium monopersulfate, sodium monopersulfate and ammonium monopersulfate.

3. The method for modifying a polyamide composite nanofiltration membrane, which selectively retains divalent salt ions, according to claim 2, wherein the oxidant is sodium persulfate.

4. The method for modifying a polyamide composite nanofiltration membrane capable of selectively intercepting divalent salt ions according to claim 1, wherein the aldehyde is one or a combination of two or more of formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, 2-methylbutyraldehyde, 3-methylbutyraldehyde, malonaldehyde, succinaldehyde and glutaraldehyde.

5. The method for modifying a polyamide composite nanofiltration membrane, which selectively retains divalent salt ions, according to claim 4, wherein the aldehyde is acetaldehyde.

6. The method for modifying the polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions according to claim 1 or 2, wherein the concentration of the oxidant is 1 to 1000mmol/L.

7. The method for modifying the polyamide composite nanofiltration membrane for selectively intercepting divalent salt ions according to claim 1 or 4, wherein the concentration of the aldehyde is 0.1-1 mol/L.

8. The method for modifying a polyamide composite nanofiltration membrane, which selectively retains divalent salt ions, according to claim 1, wherein the pH of the modification solution is adjusted by adding sodium hydroxide solution and/or potassium hydroxide solution.

9. The method for modifying the polyamide composite nanofiltration membrane capable of selectively intercepting divalent salt ions according to claim 8, wherein the concentration of the sodium hydroxide solution or the potassium hydroxide solution is 0.05-3 mol/L.