阅读说明：本技术 一种复合纳滤膜及其制备方法 (Composite nanofiltration membrane and preparation method thereof ) 是由 王蒙 王文宠 沈艳君 于 2018-08-31 设计创作，主要内容包括：本发明提供一种复合纳滤膜,所述复合纳滤膜包括基膜和分离层；所述分离层为带双电荷的聚酰胺层。本发明所述的复合纳滤膜是一种双电荷层中空纤维复合纳滤膜,不但能有效提高对二价阳离子和二价阴离子的截留效率,而且具有较高的纯水通量。(The invention provides a composite nanofiltration membrane, which comprises a base membrane and a separation layer; the separation layer is a polyamide layer with double charges. The composite nanofiltration membrane is a double-charge-layer hollow fiber composite nanofiltration membrane, can effectively improve the interception efficiency of divalent cations and divalent anions, and has higher pure water flux.)

1. A composite nanofiltration membrane, which is characterized by comprising a base membrane and a separation layer; the separation layer is a polyamide layer with double charges.

2. The composite nanofiltration membrane of claim 1, wherein the polyamide layer comprises a first desalination layer and a second desalination layer; the first desalting layer is a positively charged desalting layer, and the second desalting layer is a negatively charged desalting layer; or the first desalting layer is a negatively charged desalting layer, and the second desalting layer is a positively charged desalting layer.

3. The composite nanofiltration membrane of claim 2, wherein the polyamide layer has a thickness of 40 to 80nm, and the first desalination layer has a thickness of 10 to 30 nm.

4. The composite nanofiltration membrane of claim 2, wherein the first desalination layer is coated on the surface of the base membrane; the second desalting layer is coated on the first desalting layer.

5. The composite nanofiltration membrane of claim 2, wherein the positively charged desalination layer is formed by interfacial polymerization of a positively charged polyelectrolyte and a polyacyl chloride monomer.

6. The composite nanofiltration membrane according to claim 5, wherein the positively charged polyelectrolyte is selected from at least one of polyallylamine hydrochloride, polydiallyldimethylammonium chloride, polypropyl acryloyloxyethyltrimethyl ammonium chloride, poly-4-vinylpyridine and polyethyleneimine.

7. The composite nanofiltration membrane of claim 2, wherein the negatively charged desalination layer is formed by interfacial polymerization of polyamine monomers and polyacyl chloride monomers.

8. The composite nanofiltration membrane of claim 7, wherein the polyamine monomer is selected from at least one of piperazine, aniline, m-phenylenediamine, o-phenylenediamine, and p-phenylenediamine.

9. The composite nanofiltration membrane according to claim 5 or 7, wherein the poly-acid chloride monomer is at least one selected from terephthaloyl chloride, isophthaloyl chloride and trimesoyl chloride.

10. The composite nanofiltration membrane of claim 1, wherein the base membrane is selected from at least one of a meta-aramid hollow fiber membrane, a para-aramid hollow fiber membrane, a modified meta-aramid hollow fiber membrane, and a modified para-aramid hollow fiber membrane.

11. The composite nanofiltration membrane of claim 1, wherein the molecular weight cut-off of the basement membrane is 5000-50000.

12. The composite nanofiltration membrane of claim 1, wherein the pure water flux of the composite nanofiltration membrane at 0.3MPa is not less than 40L/(m)²·h)。

13. The composite nanofiltration membrane of claim 1, wherein the rejection rate of the composite nanofiltration membrane for dianion salts in 0.2% dianion salt solution is not less than 95% under 0.3 MPa; the desalting rate of the divalent cation salt in 0.2 percent divalent cation salt solution is more than or equal to 90 percent.

14. The preparation method of the composite nanofiltration membrane as claimed in claim 1, comprising the following steps:

(1) mixing the positively charged polyelectrolyte with deionized water to obtain a first aqueous phase solution; mixing polyamine monomer with deionized water to obtain a second aqueous phase solution; mixing a polybasic acyl chloride monomer and an organic solvent to obtain a mixed solution;

(2) providing a base film, immersing the base film into the first aqueous phase solution, taking out and drying;

(3) immersing the base film dried in the step (2) into the mixed solution, taking out and drying to obtain a base film coated by the first desalting layer;

(4) immersing the base film coated by the first desalting layer in the step (3) into a second aqueous phase solution, and then taking out and drying;

(5) and (4) immersing the base membrane coated by the first desalting layer dried in the step (4) into the mixed solution, taking out and drying to obtain the composite nanofiltration membrane.

15. The method of claim 14, wherein the step (1) of mixing the positively charged polyelectrolyte, the additive, and deionized water to obtain a first aqueous solution; and mixing polyamine monomer, an additive and deionized water to obtain a second aqueous phase solution, wherein the additive is at least one selected from sodium hydroxide, potassium hydroxide, triethylamine, sodium carbonate, sodium dodecyl benzene sulfonate, sodium dodecyl sulfate and sodium dodecyl sulfate.

16. The method of claim 15, wherein the additive is present in the first aqueous solution and the second aqueous solution in an amount of 0.03 to 1.0% (w/v) each.

17. The method according to claim 14, wherein the content of the positively-charged polyelectrolyte in the first aqueous solution in the step 1) is 0.1 to 10% (w/v).

18. The method according to claim 14, wherein the content of the polyamine in the second aqueous phase solution of step 1) is 0.1 to 10% (w/v).

19. The method according to claim 14, wherein the organic solvent in step 1) is at least one selected from the group consisting of isoparaffin, n-hexane, toluene and cyclohexane.

20. The method according to claim 14, wherein the content of the polybasic acid chloride in the mixed solution of the step 1) is 0.05 to 1.0% (w/v).

21. The preparation method according to claim 14, wherein the immersion temperature in the step (2) is 20 to 30 ℃; the immersion time is 0.5-15 min; the drying temperature in the step (2) is 10-55 ℃; the drying time is 5-40 min.

22. The preparation method according to claim 14, wherein the immersion temperature in the step (3) is 20 to 30 ℃; the immersion time is 0.1-2.0 min; the drying temperature in the step (3) is 30-100 ℃; the drying time is 2-20 min.

23. The preparation method according to claim 14, wherein the immersion temperature in the step (4) is 20 to 30 ℃; the immersion time is 0.5-15 min; the drying temperature in the step (4) is 10-55 ℃; the drying time is 5-40 min.

24. The preparation method according to claim 14, wherein the immersion temperature in the step (5) is 20 to 30 ℃; the immersion time is 0.1-2.0 min; the drying temperature in the step (5) is 30-100 ℃; the drying time is 2-20 min.

25. The preparation method of claim 14, further comprising immersing the composite nanofiltration membrane in an aqueous solution containing glycerol at step (6), and taking out and drying.

26. The method according to claim 25, wherein the concentration of glycerol in the aqueous solution of glycerol is 10 to 60% (w/v); the immersion time of the step (6) is 1-10 h; and (4) drying for 12-36 h in the step (6).

27. The preparation method of the composite nanofiltration membrane as claimed in claim 1, comprising the following steps:

(1) mixing polyamine monomers with deionized water to obtain a first aqueous phase solution; mixing the positively charged polyelectrolyte with deionized water to obtain a second aqueous phase solution; mixing a polybasic acyl chloride monomer and an organic solvent to obtain a mixed solution;

(2) providing a base film, immersing the base film into the first aqueous phase solution, taking out and drying;

(3) immersing the base film dried in the step (2) into the mixed solution, taking out and drying to obtain a base film coated by the first desalting layer;

(4) immersing the base film coated by the first desalting layer in the step (3) into a second aqueous phase solution, and then taking out and drying;

(5) and (4) immersing the base membrane coated by the first desalting layer dried in the step (4) into the mixed solution, taking out and drying to obtain the composite nanofiltration membrane.

Technical Field

The invention relates to a composite nanofiltration membrane and a preparation method thereof.

Background

Nanofiltration (NF) is a membrane separation technique that is intermediate between Ultrafiltration (UF) and Reverse Osmosis (RO). The nanofiltration membrane has good separation performance on high-valence salt, divalent salt and micromolecular organic matters, has the advantages of low operation pressure, large pure water flux, low cost and the like, and has wide application prospects in the fields of water softening, desalination, wastewater treatment and the like.

The existing composite nanofiltration membrane comprises a positively charged nanofiltration membrane and a negatively charged nanofiltration membrane, and has higher interception efficiency on part of specific divalent cations and divalent anions respectively. In addition, both membranes have good retention of a salt solution containing a high valence ion of the same charge as the central ion, but when present in solution with the central ionWhen the counter ions of high valence of opposite charges of the central ions are used, the interception performance of the nanofiltration membrane on corresponding salt solutions is reduced due to the shielding effect of the counter ions of high valence on the central ions. Such as positively charged nanofiltration membrane vs. Mg²⁺、Ca²⁺The interception rate of the plasma is obviously higher than that of the negatively charged nanofiltration membrane; charged nanofiltration membrane pair SO₄ ^2-The retention rate of the nano-filtration membrane is higher than that of a positively charged nano-filtration membrane.

Patent CN102247771A discloses a preparation method of a nanofiltration membrane with negative charge, which adopts a radiation distribution grafting method to prepare the nanofiltration membrane with negative charge. The nanofiltration membrane prepared by the method only treats high-valence anion salt solution (such as Na)₂SO₄) Shows higher retention rate, has complex preparation process and is difficult to be applied to industrial production.

Patents CN104307391A and CN101934204A disclose methods for preparing amphoteric charged nanofiltration membranes, which all use irradiation methods. Two kinds of amphoteric charged nanofiltration membrane are used for salt solution (such as MgSO)₄、MgCl₂、Na₂SO₄) Has the interception function, but the interception difference of the three salts is larger, the membrane shows weaker double charge and lower water flux.

Disclosure of Invention

The invention provides a composite nanofiltration membrane, which comprises a base membrane and a separation layer. The separation layer is a polyamide layer with double charges. The double-charge-layer hollow fiber composite nanofiltration membrane has high pure water flux and keeps the stability of the desalination rate of divalent cations and divalent anions.

In one embodiment, the polyamide layer includes a first desalting layer and a second desalting layer.

In one embodiment, the first desalination layer is a positively charged desalination layer, and the second desalination layer is a negatively charged desalination layer.

In one embodiment, the first desalination layer is a negatively charged desalination layer, and the second desalination layer is a positively charged desalination layer.

In one embodiment, the polyamide layer has a thickness of 40 to 80 nm. In one embodiment, the polyamide layer has a thickness of 50 to 70 nm.

In one embodiment, the thickness of the first desalting layer is 10 to 30 nm. In one embodiment, the thickness of the first desalting layer is 10 to 25 nm.

In one embodiment, the first desalination layer is coated on the surface of the base membrane, and the second desalination layer is coated on the surface of the first desalination layer.

In one embodiment, the positively charged desalination layer is formed by interfacial polymerization of a positively charged polyelectrolyte and a polyacyl chloride monomer.

In one embodiment, the positively charged polyelectrolyte is selected from at least one of polyallylamine hydrochloride, polydiallyldimethylammonium chloride, polypropyl acryloyloxyethyltrimethyl ammonium chloride, poly-4-vinylpyridine, and polyethyleneimine. In one embodiment, the positively charged polyelectrolyte is selected from polyethyleneimine and/or polydiallyldimethylammonium chloride.

In one embodiment, the negatively charged desalination layer is formed by interfacial polymerization of polyamine monomers and polyacyl chloride monomers.

In one embodiment, the polyamine monomer is at least one selected from the group consisting of piperazine, aniline, m-phenylenediamine, o-phenylenediamine and p-phenylenediamine. In one embodiment, the polyamine monomer is selected from piperazine and/or m-phenylenediamine.

In one embodiment, the poly-acid chloride monomer is at least one selected from the group consisting of terephthaloyl chloride, isophthaloyl chloride, and trimesoyl chloride. As an embodiment, the polybasic acid chloride monomer is selected from trimesoyl chloride and/or m-trimesoyl chloride.

As an embodiment, the base film is selected from at least one of a meta-aramid hollow fiber film, a para-aramid hollow fiber film, a modified meta-aramid hollow fiber film, and a modified para-aramid hollow fiber film.

As an embodiment, the base membrane is selected from a modified meta-aramid hollow fiber membrane and/or a modified para-aramid hollow fiber membrane. Compared with unmodified meta-aramid hollow fiber membranes and para-aramid hollow fiber membranes, the preferred modified meta-aramid hollow fiber membranes and modified para-aramid hollow fiber membranes have good chlorine resistance due to the addition of diaminobenzene fluoride in the preparation process, so that the obtained composite nanofiltration membranes can resist deep oxidation and cleaning. And the raw materials are cheap, the preparation cost is low, the industrialization is easy, and the method has a wide market application prospect.

In one embodiment, the molecular weight cut-off of the base film is 5000 to 50000. In one embodiment, the base film has a molecular weight cut-off of 30000 to 50000. In the invention, if the molecular weight cut-off of the base membrane is too small, namely the aperture of the base membrane is too small, the pure water flux of the obtained composite nanofiltration membrane is too small; if the trapped molecular weight of the base membrane is too large, namely the aperture of the base membrane is too large, the supporting force on the positively charged desalting layer and the negatively charged desalting layer is weakened, and the desalting performance of the composite nanofiltration membrane is unstable. The molecular weight cut-off of the base membrane optimized in the invention can enable the obtained composite nanofiltration membrane to achieve the technical effect of the invention.

In one embodiment, the pure water flux of the composite nanofiltration membrane is more than or equal to 40L/(m) under 0.3MPa²H). The pure water flux of the composite nanofiltration membrane is far greater than that of a common nanofiltration membrane.

In one embodiment, the composite nanofiltration membrane has a rejection rate of 95% or more for divalent anion salt in 0.2% (by mass) divalent anion salt solution and 90% or more for divalent cation salt in 0.2% (by mass) divalent cation salt solution under 0.3 MPa.

In one embodiment, the salt of a divalent anion is at least one of a sulfate, a carbonate, and an oxalate. In one embodiment, the divalent anion salt is at least one selected from magnesium sulfate, sodium carbonate, potassium carbonate, sodium oxalate, and potassium oxalate.

In one embodiment, the divalent cation salt is a magnesium salt and/or a calcium salt. In one embodiment, the divalent cation salt is selected from at least one of magnesium sulfate, magnesium chloride, calcium chloride, and calcium nitrate.

The invention provides a method for preparing the composite nanofiltration membrane, which comprises the following steps:

(1) mixing the positively charged polyelectrolyte with deionized water to obtain a first aqueous phase solution; mixing polyamine monomer with deionized water to obtain a second aqueous phase solution; mixing a polybasic acyl chloride monomer and an organic solvent to obtain a mixed solution;

(2) providing a base film, immersing the base film into the first aqueous phase solution, taking out and drying;

(3) immersing the base film dried in the step (2) into the mixed solution, taking out and drying to obtain a base film coated by the first desalting layer;

(4) immersing the base film coated by the first desalting layer in the step (3) into a second aqueous phase solution, and then taking out and drying;

(5) and (4) immersing the base membrane coated by the first desalting layer dried in the step (4) into the mixed solution, taking out and drying to obtain the composite nanofiltration membrane.

In the invention, the solvent in the mixed solution containing the polyacyl chloride is an organic solution, namely the mixed solution containing the polyacyl chloride is an oil phase, and the aqueous solution of the positively charged polyelectrolyte and the aqueous solution containing the polyamine monomer are water phases. The invention controls the reaction sequence by step-by-step reaction, if the step 2) and the step 3) are changed or the step 4) and the step 5) are changed, namely the basement membrane is firstly immersed into the oil phase and then immersed into the water phase, because the oil phase solvent in the invention is very volatile, if the basement membrane is firstly put into the oil phase, the organic solvent in the oil phase can be completely volatilized in a short time, namely only polybasic acyl chloride exists on the surface of the basement membrane, so that the oil phase and the water phase reaction interface is difficult to form, and the desalting layer can not be obtained. Even if the solvent in the oil phase is not completely volatilized, the desalted layer obtained by the interfacial reaction between the residual oil phase and the water phase on the surface of the basement membrane cannot be uniformly distributed on the surface of the basement membrane. Therefore, the method ensures that the obtained composite nanofiltration membrane has higher pure water flux and stable desalination rate by controlling the sequence of the reaction.

In one embodiment, the step (1) includes mixing a positively charged polyelectrolyte, an additive and deionized water to obtain a first aqueous solution; and mixing polyamine monomer, additive and deionized water to obtain a second aqueous phase solution. According to the invention, by adding the additive, the reaction speed of the positively charged polyelectrolyte and the polybasic acyl chloride to form the first desalting layer on the surface of the base membrane through interfacial polymerization is controlled, and the reaction speed of the polyamine monomer and the polybasic acyl chloride to form the second desalting layer on the surface of the base membrane through interfacial polymerization is also controlled, so that the obtained first desalting layer and the obtained second desalting layer are kept relatively loose, and the pure water flux of the obtained composite nanofiltration membrane is obviously improved.

In one embodiment, the additive is at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, triethylamine, sodium carbonate, sodium dodecylbenzenesulfonate, sodium dodecylsulfate and sodium dodecylsulfate. In one embodiment, the additive is sodium lauryl sulfate and/or triethylamine.

In one embodiment, the additive is contained in the first aqueous solution and the second aqueous solution in an amount of 0.03 to 1.0% (w/v). In one embodiment, the additive is contained in the first aqueous solution and the second aqueous solution in an amount of 0.10 to 0.40% (w/v). When the content of the additive in the first aqueous phase solution and the second aqueous phase solution is too large, the reaction rate of the positively charged polyelectrolyte and the polybasic acyl chloride on the surface of the basement membrane and the reaction rate of the polyamine monomer and the polybasic acyl chloride on the surface of the basement membrane are too high, and the generated first desalting layer and the second desalting layer are too compact, so that the water flux of the obtained composite nanofiltration membrane is greatly reduced; when the content of the additive in the first aqueous phase solution and the second aqueous phase solution is too small, the additive has a weak promoting effect on the reaction of the positively charged polyelectrolyte and the polyacyl chloride on the surface of the base membrane and the reaction of the polyacyl chloride and the polyamine monomer on the surface of the base membrane, so that the reaction rate is too slow, the generated first desalting layer and the second desalting layer are too loose, and although the pure water flux of the composite nanofiltration membrane is improved, the desalting rate of the obtained composite nanofiltration membrane is greatly reduced. Therefore, the amount of the additive optimized by the invention not only can increase the pure water flux of the composite nanofiltration membrane, but also ensures the stability of the desalination rate of the composite nanofiltration membrane.

In one embodiment, the content of the positively charged polyelectrolyte in the first aqueous solution in the step 1) is 0.1-10% (w/v). In one embodiment, the content of the positively charged polyelectrolyte in the first aqueous solution in the step 1) is 0.1-5.0% (w/v). In the invention, when the content of the positively charged polyelectrolyte is too high, the reaction rate of the positively charged polyelectrolyte and the polyacyl chloride on the surface of the base membrane is too high, and the generated first desalting layer is too compact, so that the pure water flux of the composite nanofiltration membrane is greatly reduced; when the content of the positively charged polyelectrolyte is too small, the reaction rate of the positively charged polyelectrolyte and the polyacyl chloride on the surface of the base membrane is too slow, the generated first desalting layer is too loose, and the desalting rate of the obtained composite nanofiltration membrane on divalent cations in a divalent cation salt solution is greatly reduced although the pure water flux of the composite nanofiltration membrane is ensured to be large.

In one embodiment, the content of the polyamine in the second aqueous phase solution in the step 1) is 0.1-10% (w/v). In one embodiment, the content of the polyamine in the second aqueous phase solution in the step 1) is 0.1-5.0% (w/v). In the invention, when the content of the polyamine is too high, the reaction rate of the polyamine monomer and the polybasic acyl chloride on the surface of the base membrane is too high, and the generated second desalting layer is too compact, so that the pure water flux of the composite nanofiltration membrane is greatly reduced; when the content of the polyamine is too small, the reaction rate of the polyamine monomer and the polybasic acyl chloride on the surface of the base membrane is too slow, and the generated second desalting layer is too loose, so that the pure water flux of the composite nanofiltration membrane is ensured to be large. But the salt rejection rate of the obtained composite nanofiltration membrane to the divalent anions in the divalent anion salt solution is greatly reduced.

As an embodiment, the organic solvent in step 1) is at least one selected from isoparaffin, n-hexane, toluene and cyclohexane. As an embodiment, the organic solvent in step 1) is isoparaffin and/or cyclohexane. The isoparaffin is Isopar G, Isopar E, Isopar H, Isopar L and Isopar M (the manufacturer is Exxon Mobil company), and the preferable isoparaffin is tasteless, safe and environment-friendly, and is suitable for industrial application.

In one embodiment, the content of the polyacyl chloride in the mixed solution in the step 1) is 0.05-1.0% (w/v). In one embodiment, the content of the polyacyl chloride in the mixed solution in the step 1) is 0.10-0.30% (w/v). In the invention, when the content of the polyacyl chloride is too large, the reaction rate of the positively charged polyelectrolyte and the polyacyl chloride on the surface of the base membrane and the reaction rate of the polyamine monomer and the polyacyl chloride on the surface of the base membrane are too high, and the generated first desalting layer and the second desalting layer are too compact, so that the water flux of the composite nanofiltration membrane is greatly reduced; when the content of the polyacyl chloride is too small, the reaction rate of the positively charged polyelectrolyte and the polyacyl chloride on the surface of the base membrane and the reaction rate of the polyamine monomer and the polyacyl chloride on the surface of the base membrane are too slow, and the generated first desalting layer and the generated second desalting layer are too loose, so that the pure water flux of the composite nanofiltration membrane is ensured to be very large, but the desalting rate of the obtained composite nanofiltration membrane on divalent cations in a divalent cation salt solution is greatly reduced.

As an embodiment, the method further comprises the step of soaking the base film in pure water before the step (2), cleaning, taking out and drying. In one embodiment, the base film is soaked in pure water for 5-24 hours.

The invention removes partial residual impurities on the base membrane by pretreating the base membrane, reduces the influence of partial impurities on the interfacial polymerization reaction, and further improves the pure water flux and the desalination rate of the prepared composite nanofiltration membrane.

As an embodiment, the immersion temperature in the step (2) is 20-30 ℃; the immersion time is 0.5-15 min; the drying temperature in the step (2) is 10-55 ℃; the drying time is 5-40 min.

As an embodiment, the immersion temperature in the step (2) is 20-25 ℃; the immersion time is 0.5-5.0 min; the drying temperature in the step (2) is 20-35 ℃; the drying time is 10-30 min.

As an embodiment, the immersion temperature in the step (3) is 20-30 ℃; the immersion time is 0.1-2.0 min; the drying temperature in the step (3) is 30-100 ℃; the drying time is 2-20 min.

As an embodiment, the immersion temperature in the step (3) is 20-25 ℃; the immersion time is 0.5-1.0 min; the drying temperature in the step (3) is 40-80 ℃; the drying time is 5-10 min.

As an embodiment, the immersion temperature in the step (4) is 20-30 ℃; the immersion time is 0.5-15 min; the drying temperature in the step (4) is 10-55 ℃; the drying time is 5-40 min.

In one embodiment, the immersion temperature in the step (4) is 20-25 ℃; the immersion time is 0.5-5.0 min; the drying temperature in the step (4) is 20-35 ℃; the drying time is 10-30 min.

In one embodiment, the immersion temperature in the step (5) is 20-30 ℃; the immersion time is 0.1-2.0 min; the drying temperature in the step (5) is 30-100 ℃; the drying time is 2-20 min.

In one embodiment, the immersion temperature in the step (5) is 20-25 ℃; the immersion time is 0.5-1.0 min, and the drying temperature in the step (5) is 40-80 ℃; the drying time is 5-10 min.

In the present invention, the drying in step (2) and the drying in step (4) are both low-temperature drying, which aims to remove excess water droplets from the surface of the base film (or the surface of the first desalting layer). The drying in the step (3) and the drying in the step (5) are high-temperature drying, and the purpose is to promote the interfacial polymerization reaction of the oil phase and the water phase, so that the obtained desalting layer can realize uniform coating.

In one embodiment, the method further comprises the step (6) of immersing the composite nanofiltration membrane into an aqueous solution containing glycerol, and taking out and drying the composite nanofiltration membrane.

According to the invention, the composite nanofiltration membrane is taken out and dried after entering the aqueous solution containing glycerol, so that the obtained composite nanofiltration membrane can remove polymerization products which are not adhered to the surface of the composite nanofiltration membrane, the pores of the composite nanofiltration membrane are prevented from being blocked, the prepared composite nanofiltration membrane can be stored in a dry state, and the storage time is prolonged.

In one embodiment, the concentration of glycerol in the aqueous solution of glycerol is 10 to 60% (w/v). In one embodiment, the concentration of glycerol in the aqueous solution of glycerol is 10-40% (w/v).

In one embodiment, the immersion time in the step (6) is 1 to 10 hours. In one embodiment, the immersion time in the step (6) is 1 to 5 hours.

In one embodiment, the drying time in the step (6) is 12-36 h. In one embodiment, the drying time in the step (6) is 20-24 h.

The invention also provides a method for preparing the composite nanofiltration membrane, which comprises the following steps:

(1) mixing polyamine monomers with deionized water to obtain a first aqueous phase solution; mixing the positively charged polyelectrolyte with deionized water to obtain a second aqueous phase solution; mixing a polybasic acyl chloride monomer and an organic solvent to obtain a mixed solution;

(2) providing a base film, immersing the base film into the first aqueous phase solution, taking out and drying;

(3) immersing the base film dried in the step (2) into the mixed solution, taking out and drying to obtain a base film coated by the first desalting layer;

(4) immersing the base film coated by the first desalting layer in the step (3) into a second aqueous phase solution, and then taking out and drying;

(5) and (4) immersing the base membrane coated by the first desalting layer dried in the step (4) into the mixed solution, taking out and drying to obtain the composite nanofiltration membrane.

In the invention, a first desalting layer is firstly generated on the surface of the base film, and then a second desalting layer is formed on the first desalting layer. When the first desalting layer is a positively charged desalting layer, the second desalting layer is a negatively charged desalting layer; or when the first desalting layer is a negatively charged desalting layer, the second desalting layer is a positively charged desalting layer. The preparation method changes the preparation sequence of the negatively charged desalting layer and the positively charged desalting layer, and can also achieve the technical effect of the invention. Meanwhile, the change of the preparation order does not require the change of the preparation conditions or parameters in each step.

Has the advantages that:

the invention takes the hollow fiber membrane prepared by the modified meta-aramid as the base membrane, and the base membrane and the separation layer have better chlorine resistance, so that the prepared composite nanofiltration membrane can resist deep oxidation and cleaning, thereby having higher pure water flux, and having good desalination rate on divalent cation salts and divalent cation salts in divalent anion salt solutions and divalent cation salt solutions, effectively overcoming the problem of high energy consumption of the existing composite nanofiltration membranes due to higher operation pressure, and the raw materials are cheap, the preparation cost is lower, the industrialization is easy, and the invention has larger market application prospect.

The% (w/v) as used herein refers to% (g/mL), e.g., 1% (g/mL) refers to the addition of 0.01g solute per 1mL solvent during the formulation process.

Detailed Description

The present invention will be described in greater detail with reference to specific embodiments, however, the present invention is not limited to the embodiments described below, and may be replaced by other embodiments in which some elements are replaced by equivalent means.

The method for testing the pure water flux and the desalination rate of the composite nanofiltration membrane is as follows:

prepressing the prepared composite nanofiltration membrane for half an hour by using pure water under 0.3MPa, and respectively testing the pure water flux of the composite nanofiltration membrane by using the pure water and 0.2 percent MgSO₄、0.2％MgCl₂、0.2％Na₂SO₄The electrolyte solution is used for testing the interception performance of the composite nanofiltration membrane, and the recovery rate of pure water is controlled to be 15%.

The calculation formula of the pure water flux of the membrane is shown as (1):

wherein A ═ π DL (A-effective membrane area, m)²(ii) a D-average diameter of membrane filaments, m; l-the effective length of the membrane filaments, m); t-time required for collecting Q volume of produced fluid, h; q-volume of product fluid collected over time t, L.

The retention performance of the membrane is calculated as shown in (2):

wherein, the retention rate of R-membrane, C_f-the conductivity of the stock solution,. mu.S/cm; c_pConductivity of the produced water,. mu.S/cm.

And (3) repeatedly measuring the composite nanofiltration membrane for 3 times, and taking an average value to obtain the rejection rate of the composite nanofiltration membrane.