Double positive layer nanofiltration membrane, preparation method and application thereof

Abstract

The application discloses a double positive layer nanofiltration membrane and a preparation method and application thereof, and belongs to the technical field of nanofiltration membrane preparation. The double positive layer nanofiltration membrane is prepared by the following method: (1) polyether sulfone ultrafiltration membrane is soaked in an aqueous monomer solution; then, the polyether sulfone ultrafiltration membrane is soaked in an oil phase monomer solution to perform an interfacial polymerization reaction; (2) the polyether sulfone ultrafiltration membrane is soaked in a grafting solution for 2-20 min; and (3) the polyether sulfone ultrafiltration membrane is heated at 60-80 DEG C for 2-20 min, and the double positive layer nanofiltration membrane is obtained. The double positive layer nanofiltration membrane is applied to treatment of heavy metal-containing wastewater and softening of ultra-high hardness underground water. The nanofiltration membrane has a double positive layer and a wrinkled texture form, the water permeability is greatly improved, and the heavy metal ion interception performance is increased. The application breaks through the trade-off effect between repulsion and permeability, and has great application value in treatment of heavy metal-containing wastewater, softening of high-hardness underground water and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a double positive layer nanofiltration membrane and a preparation method and application thereof, and belongs to the technical field of nanofiltration membrane preparation. BACKGROUND

[0002] The rapid development of science and technology and economy has brought many conveniences to life, but also accompanied by problems such as water pollution and water resource shortage. Natural biological amplification and biological accumulation of heavy metal ions make their toxicity reach a very high level, which seriously threatens the survival of humans and other organisms. In addition, after entering the human body, heavy metals are easy to combine with other toxins to form more toxic organic matter, which accumulates in certain organs of the human body and may cause chronic poisoning with serious consequences. Therefore, the treatment of wastewater containing heavy metals is particularly important in the construction of green water. Traditional water purification technology methods, such as adsorption, coagulation, sedimentation, ion exchange, biological treatment, etc., have some shortcomings in removing heavy metal ions contained in water. Compared with these methods, membrane water treatment technology has obvious advantages in removing heavy metal ions, especially nanofiltration membrane technology, which has the advantages of low energy consumption, high separation performance and environmental friendliness. The positively charged nanofiltration membrane plays a more important role. Due to the charge characteristics of the positively charged membrane, it can adsorb negatively charged colloidal particles, bacterial endotoxins, etc., and because they have the same positive charge as heavy metal ions, the Donnan exclusion effect is more obvious, and most of the heavy metals are removed, while the trace elements beneficial to the human body are retained.

[0003] Polyethyleneimine is an excellent monomer material for preparing positively charged nanofiltration membranes due to its high charge density and abundant amine groups. Some studies have reported the preparation of positively charged nanofiltration membranes using polyethyleneimine (Desalination, 2019, 449:57-68). Although good rejection performance has been achieved, due to the high reactivity and high charge density of polyethyleneimine, the selective layer of the nanofiltration membrane is too compact, resulting in low water permeability. Due to the trade-off between retention and permeability, the preparation of positively charged nanofiltration membranes with high permeability and retention is still a bottleneck in engineering applications. Therefore, it is necessary to modify these basic nanofiltration membranes to improve the permeability while ensuring the retention performance. Currently, there is no related research report on the performance of salt-regulated coupling surface graft modification of positively charged nanofiltration membranes. SUMMARY

[0004] In view of the above prior art, the present application provides a double positive layer nanofiltration membrane, which can efficiently retain heavy metals. The present application also provides a preparation method of the double positive layer nanofiltration membrane and its application in the treatment of wastewater containing heavy metals.

[0005] The present application is realized by the following technical solutions:

[0006] A double positive layer nanofiltration membrane is prepared by the following method:

[0007] (1) Preparation of separation layer: immerse the polyethersulfone ultrafiltration membrane in the aqueous monomer solution for 1-15 min; pour out the aqueous monomer solution and blow the polyethersulfone ultrafiltration membrane with an air gun until no solution is visible (1-10 min) to completely remove the solution on the surface; immerse the polyethersulfone ultrafiltration membrane in the oil monomer solution for interfacial polymerization reaction, and immerse for 0.5-10 min, and pour out the oil monomer solution.

[0008] The aqueous monomer solution is composed of inorganic salt, polyethyleneimine and water, wherein the concentration of sodium chloride is 0.5-20 wt%, and the concentration of polyethyleneimine is 0.2-2 wt%.

[0009] The oil monomer solution is composed of trimesoyl chloride and organic solvent, wherein the concentration of trimesoyl chloride is 0.05-0.5 wt%.

[0010] (2) Preparation of grafting layer: immerse the polyethersulfone ultrafiltration membrane treated above in the grafting solution for 2-20 min, and take out the polyethersulfone ultrafiltration membrane.

[0011] The grafting solution is composed of sodium dodecyl sulfate, high content amine monomer and water, wherein the concentration of sodium dodecyl sulfate is 0.05-0.2 wt%, and the concentration of high content amine monomer is 0.2-3 wt%.

[0012] (3) Heat treatment: heat the polyethersulfone ultrafiltration membrane treated above at 60-80℃ for 2-20 min to obtain the double positive layer nanofiltration membrane.

[0013] Further, in the step (1), the average pore size of the polyethersulfone ultrafiltration membrane is 20-200 kDa, and specifically can be selected as 50 kDa, 100 kDa or 150 kDa.

[0014] Further, in the step (1), the immersion time in the aqueous monomer solution is 5 min, and the immersion time in the oil monomer solution is 3 min.

[0015] Further, in the step (1), the inorganic salt is selected from any one or two or more of sodium chloride, calcium chloride, magnesium chloride, manganese chloride, copper chloride, cadmium chloride, cobalt chloride, nickel chloride and sodium bicarbonate, and preferably is sodium chloride.

[0016] Further, in the step (1), the concentration of the inorganic salt is 5-15 wt%, and preferably is 10-15 wt%, and specifically can be selected as 10 wt%, 15 wt%; and the concentration of the polyethyleneimine is 0.5 wt%.

[0017] Further, in the step (1), the polyethyleneimine has a molecular weight of 500-100000 Da, and can be selected from 600 Da, 1800 Da, 10000 Da or 70000 Da.

[0018] Further, in the step (1), the organic solvent is selected from any one or more of n-hexane, cyclohexane or isomeric alkane solvents, and preferably n-hexane.

[0019] Further, in the step (1), the concentration of trimesoyl chloride is 0.1 wt%.

[0020] Further, in the step (1), the polyethersulfone ultrafiltration membrane is pretreated by immersing the polyethersulfone ultrafiltration membrane in an isopropanol solution and shaking for 5-20 minutes to remove chemical impurities on the surface of the polyethersulfone ultrafiltration membrane; and the polyethersulfone ultrafiltration membrane is cleaned with ultrapure water for at least three times to remove residual isopropanol on the surface.

[0021] Further, in the step (2), the grafting time is 10-20 min.

[0022] Further, in the step (2), the high-content amine monomer is selected from any one or more of polyethyleneimine, polyallylamine, triaminoguanidine and chitosan, and preferably polyethyleneimine and polyallylamine.

[0023] Further, in the step (2), the concentration of sodium dodecyl sulfate is 0.1 wt%, and the concentration of the high-content amine monomer is 1 wt% polyethyleneimine.

[0024] Further, in the step (3), the heat treatment temperature is 70-80℃, and the time is 10 min.

[0025] The double-positive layer nanofiltration membrane prepared by the above method has high permeability and high heavy metal retention performance, and can be applied in the treatment of heavy metal-containing wastewater and the softening treatment of ultra-high hardness groundwater. In specific applications, the membrane is used in combination with an ultrafiltration membrane.

[0026] Further, the heavy metal in the heavy metal-containing wastewater is any one or more of nickel, manganese, copper, cadmium and lead.

[0027] The present application aims to break the trade-off effect between the repulsion and permeability of traditional positively charged nanofiltration membranes. The present application adopts a strategy of salt-assisted interfacial polymerization coupled with surface grafting of high content amine monomers, using polyether sulfone ultrafiltration membranes as base membranes, introducing salt-assisted interfacial polymerization in aqueous phase to prepare a polyamide layer, and then grafting a high content amine monomer to prepare a positively charged nanofiltration membrane with a double positive layer and a wrinkled texture morphology. The present application adjusts the adsorption and diffusion of monomers by salt to affect the interfacial polymerization, forming a polyamide layer with a wrinkled texture, which is coarser and thinner, achieving higher hydrophilicity and faster local interfacial polymerization. Grafting a high content amine monomer (polyethyleneimine, polyallylamine, etc.) greatly increases the positive charge of the nanofiltration membrane surface, forming a double positive layer structure. When grafting a high content amine monomer, the salt-regulated micro-ridged positive layer acts as a new intermediate layer, regulating monomer diffusion to affect the grafting process. In addition, the residual trimesoyl chloride and unreacted acyl chloride groups in the salt-regulated micro-ridged positive layer are beneficial to the grafting of high content amine monomers, and the double positive layer nanofiltration membrane after grafting exhibits a larger and more intensive surface wrinkled morphology, further increasing the water permeation area, achieving simultaneous improvement of permeability and heavy metal ion rejection performance. In summary, the nanofiltration membrane of the present application has higher positive charge, stronger hydrophilicity, thinner polyamide layer, larger specific surface area and narrower average pore size, greatly improving the water permeability of the nanofiltration membrane, while also having high rejection performance for heavy metal ions, which can efficiently and energy-efficiently purify heavy metal-containing wastewater.

[0028] The present application has the following beneficial effects:

[0029] (1) The present application adopts a strategy of salt-assisted interfacial polymerization coupled with surface grafting of high content amine monomers to prepare a nanofiltration membrane with a double positive layer and a wrinkled texture morphology, which greatly improves the water permeability while increasing the rejection performance for heavy metal ions. It breaks the trade-off effect between the repulsion and permeability of traditional positively charged nanofiltration membranes.

[0030] (2) The double positive layer nanofiltration membrane prepared by the present application has a double repulsion effect on divalent heavy metals. On the one hand, the polyamide layer prepared by salt-assisted interfacial polymerization has size sieving and electrostatic repulsion effect on metal ions. On the other hand, the high content amine monomer grafting layer also has certain electrostatic repulsion and size sieving effect on metal ions. Under the synergistic effect of the two aspects, excellent rejection performance for divalent metal ions is achieved.

[0031] (3) The present application uses inexpensive and readily available salt materials such as sodium chloride, which is beneficial to reducing the preparation cost of nanofiltration membranes and has the effect of energy saving and carbon reduction.

[0032] (4) The application adopts a salt-assisted interfacial polymerization strategy, which helps to form a thinner polyamide layer and is beneficial to flux improvement.

[0033] (5) The application uses salt-assisted interfacial polymerization to increase flux, and uses grafted high-content amine monomers to increase positive charge. The monomer adjustment and nanofiltration membrane surface modification are combined, and the salt-regulated micro-ridged positive layer plays a role as a new intermediate layer, which regulates monomer diffusion to affect the grafting process. The residual trimesoyl chloride and unreacted acyl chloride groups are beneficial to the grafting of high-content amine monomers, and the double-positive layer nanofiltration membrane after grafting shows a larger and more intensive surface wrinkle morphology, further increasing the water permeation area, and simultaneously solving the bottleneck problem of permeability and retention performance.

[0034] (6) The preparation method of the application has mild conditions, low preparation cost, and simple and flexible operation mode.

[0035] (7) The application range of the application is wide, and it shows great application value in treating heavy metal-containing wastewater, high-hardness groundwater and the like. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 : Comparison diagram of permeability of nanofiltration membranes of Example 1, Comparative Example 1 and Comparative Example 2.

[0037] Figure 2 : Comparison diagram of permeability and retention performance of nanofiltration membranes of Example 1, Comparative Example 1 and Comparative Example 2 to manganese chloride. DETAILED DESCRIPTION

[0038] The application will be further described below in conjunction with examples. However, the scope of the application is not limited to the following examples. Those skilled in the art can understand that various changes and modifications can be made to the application without departing from the spirit and scope of the application.

[0039] The instruments, reagents and materials involved in the following examples are conventional instruments, reagents and materials existing in the prior art, and can be obtained through regular commercial channels if not otherwise specified. The experimental methods and detection methods involved in the following examples are conventional experimental methods and detection methods existing in the prior art if not otherwise specified.

[0040] The polyether sulfone ultrafiltration membrane used in the examples of the application is self-made, and the preparation method is a conventional method, and the average pore size is 150 kDa.

[0041] Before preparing the double-positive layer nanofiltration membrane, the polyether sulfone ultrafiltration membrane is pretreated as follows: first, the polyether sulfone ultrafiltration membrane is cut, and the effective area is about 44.2 cm 2; secondly, the polyethersulfone ultrafiltration membrane is soaked in 30wt% isopropanol solution, and shaken for 10 minutes to remove chemical impurities on the surface of the polyethersulfone ultrafiltration membrane; finally, the polyethersulfone ultrafiltration membrane is cleaned with ultrapure water for three times to remove the residual isopropanol on the surface. The polyethersulfone ultrafiltration membrane is fixed in a circular reactor with an effective area of about 44.2cm 2 .

[0042] Example 1 Preparation of a double positive layer nanofiltration membrane

[0043] The steps are as follows:

[0044] (1) Preparation of a separation layer: the polyethersulfone ultrafiltration membrane is soaked in an aqueous monomer solution for 5 minutes; the aqueous monomer solution is poured out, and the polyethersulfone ultrafiltration membrane is blown with an air gun until no solution is visible (about 3 minutes) to completely remove the solution on the surface; the polyethersulfone ultrafiltration membrane is soaked in an oil monomer solution for interfacial polymerization for 3 minutes, and the oil monomer solution is poured out.

[0045] The aqueous monomer solution is composed of sodium chloride, polyethyleneimine (70000 Da) and water, wherein the concentration of sodium chloride is 10wt%, and the concentration of polyethyleneimine is 0.5wt%.

[0046] The oil monomer solution is composed of trimesoyl chloride and n-hexane, wherein the concentration of trimesoyl chloride is 0.1wt%.

[0047] (2) Preparation of a polyethyleneimine grafted layer: the polyethersulfone ultrafiltration membrane treated above is soaked in a grafting solution for 10 minutes, and the polyethersulfone ultrafiltration membrane is taken out.

[0048] The grafting solution is composed of sodium dodecyl sulfate, polyethyleneimine (70000 Da) and water, wherein the concentration of sodium dodecyl sulfate is 0.1wt%, and the concentration of polyethyleneimine is 1wt%.

[0049] (3) Heat treatment: the polyethersulfone ultrafiltration membrane treated above is heat treated at 70℃ for 10 minutes to obtain a double positive layer nanofiltration membrane.

[0050] Example 2 Preparation of a double positive layer nanofiltration membrane

[0051] The difference from Example 1 is that the concentration of sodium chloride in the aqueous monomer solution is 5wt%. The others are the same as Example 1.

[0052] Example 3 Preparation of a double positive layer nanofiltration membrane

[0053] The difference from Example 1 is that the concentration of sodium chloride in the aqueous monomer solution is 15wt%. The others are the same as Example 1.

[0054] Example 4 Preparation of a double positive layer nanofiltration membrane

[0055] The difference from Example 1 is that the grafting time of the grafting solution is different, which is 5 min. The others are the same as Example 1.

[0056] Example 5 Preparation of a double positive layer nanofiltration membrane

[0057] The difference from Example 1 is that the grafting time of the grafting solution is different, which is 20 min. The others are the same as Example 1.

[0058] Example 6 Preparation of a double positive layer nanofiltration membrane

[0059] The difference from Example 1 is that the temperature of the heat treatment is different, which is 60°C. The others are the same as Example 1.

[0060] Example 7 Preparation of a double positive layer nanofiltration membrane

[0061] The difference from Example 1 is that the temperature of the heat treatment is different, which is 80°C. The others are the same as Example 1.

[0062] Example 8 Preparation of a double positive layer nanofiltration membrane

[0063] The difference from Example 1 is that the grafting solution is different, which is composed of sodium dodecyl sulfate, polyallylamine (350000 Da) and water, wherein the concentration of sodium dodecyl sulfate is 0.1 wt%, and the concentration of polyallylamine is 1 wt%. The others are the same as Example 1.

[0064] Example 9 Preparation of a double positive layer nanofiltration membrane

[0065] The difference from Example 1 is that the type of salt in the aqueous monomer solution is different, which is sodium bicarbonate. The others are the same as Example 1.

[0066] Example 10 Preparation of a double positive layer nanofiltration membrane

[0067] The difference from Example 1 is that the type of salt in the aqueous monomer solution is different, which is manganese chloride. The others are the same as Example 1.

[0068] Comparative Example 1

[0069] The difference from Example 1 is that the grafting of step (2) is not performed, and after the separation layer is prepared, the heat treatment is directly performed. The others are the same as Example 1.

[0070] Comparative Example 2

[0071] The difference from Example 1 is that the aqueous monomer solution does not contain sodium chloride, and the aqueous monomer solution is composed of polyethyleneimine (70000 Da) and water, wherein the concentration of polyethyleneimine is 0.5 wt%. The other is the same as Example 1.

[0072] Preparation of conventional positively charged nanofiltration membrane of Comparative Example 3

[0073] The difference from Example 1 is that the aqueous monomer solution does not contain sodium chloride, and the aqueous monomer solution is composed of polyethyleneimine (70000 Da) and water, wherein the concentration of polyethyleneimine is 0.5 wt%. And no grafting of step (2) is performed, after the preparation of the separation layer, directly heat treatment. The other is the same as Example 1.

[0074] Performance test of experimental double positively charged nanofiltration membrane

[0075] The performance of the nanofiltration membrane was evaluated on a dead-end filtration platform. The effective filtration area of the filtration device is 28.7 cm 2 , and the whole experiment was carried out at a temperature of 25℃ and a driving pressure of 4 bar. The permeability of the nanofiltration membrane was evaluated by using ultrapure water, and the rejection performance of the nanofiltration membrane was evaluated by using a 500ppm solution of five heavy metals (NiCl2, MnCl2, CuCl2, CdCl2 and PbCl2).

[0076]

[0077] In the formula, R represents the rejection rate, %. C f and C p represent the concentration of the feed and the permeate solution (μS·cm -1 or mg·L -1 ).

[0078]

[0079] In the formula, J represents the flux, V represents the permeate volume (L), S represents the effective filtration area of the membrane (m 2 ), Δt represents the filtration operation interval time (h), and P represents the operation applied pressure (bar).

[0080] The rejection rate of heavy metals by the nanofiltration membrane was tested by atomic absorption.

[0081] The performance test results of the double positively charged nanofiltration membrane prepared in each example, and the nanofiltration membranes prepared in Comparative Examples 1, 2 and 3 are shown in Table 1. The comparison of the permeation performance of the nanofiltration membranes of Example 1, Comparative Example 1 and Comparative Example 2 is shown in Figure 1 , and the comparison of the permeation performance and rejection performance of the nanofiltration membranes of Example 1, Comparative Example 1 and Comparative Example 2 to manganese chloride is shown in Figure 2 .

[0082] Table 1

[0083]

[0084]

[0085] As can be seen from Table 1, the dual positive layer nanofiltration membrane prepared in Examples 1-10 has high flux and extremely high heavy metal rejection rate, and can achieve efficient rejection of heavy metals. Specifically, compared with Comparative Example 3, the flux of Example 1 is increased by 111.3%, and the rejection performance to manganese chloride is also increased from 85.2% to 95.1%. Compared with Comparative Example 2, the rejection rate of Example 1 to heavy metals is similar, and the flux is increased by 68.0%. Compared with Comparative Example 1, although the flux of Example 1 decreases, the rejection performance to heavy metals is greatly improved.

[0086] The present application adjusts the adsorption and diffusion of monomers by salt to affect the interfacial polymerization reaction, and the combined action of larger specific surface area, thinner polyamide layer and stronger hydrophilicity significantly improves the water flux. At the same time, grafting high content amine monomers (polyethyleneimine, polyallylamine, etc.) greatly increases the positive charge of the nanofiltration membrane surface, forming a dual positive layer structure. When grafting high content amine monomers, the salt-regulated micro-ridged positive layer acts as a new intermediate layer, regulating monomer diffusion to affect the grafting process. In addition, the residual trimesoyl chloride and unreacted acyl chloride groups in the salt-regulated micro-ridged positive layer are beneficial to grafting high content amine monomers, and the dual positive layer nanofiltration membrane after grafting shows larger and more intensive surface wrinkle morphology, further increasing the water permeation area, achieving simultaneous improvement of permeation performance and heavy metal ion rejection performance. In addition, factors such as salt concentration, grafting time, heat treatment temperature and grafted amine monomer type also affect the performance of the dual positive layer, and when the salt concentration is too high (Example 3), when the grafting time is too short (Example 4), when the heat treatment temperature is too low (Example 6), the rejection performance of the dual positive layer nanofiltration membrane to heavy metals will be reduced.

[0087] The above examples are provided to those skilled in the art to fully disclose and describe how to implement and use the claimed embodiments, rather than to limit the scope of the disclosure disclosed herein. Modifications obvious to those skilled in the art will be within the scope of the appended claims.

Claims

1. A method for preparing a positively charged nanofiltration membrane, characterized in that, The method comprises the following steps: (1) preparing a separation layer: soaking a polyethersulfone ultrafiltration membrane in an aqueous monomer solution for 1-15 min; pouring out the aqueous monomer solution; soaking the polyethersulfone ultrafiltration membrane in an oil monomer solution for interfacial polymerization for 0.5-10 min, pouring out the oil monomer solution, and forming a polyamide layer with a wrinkled texture; The aqueous monomer solution is composed of an inorganic salt, polyethyleneimine and water, and the inorganic salt is selected from any one or two or more of sodium chloride, calcium chloride, magnesium chloride, manganese chloride, copper chloride, cadmium chloride, cobalt chloride, nickel chloride and sodium bicarbonate, wherein when the inorganic salt is selected as sodium chloride, the concentration of sodium chloride is 5-15 wt%, and the concentration of polyethyleneimine is 0.2-2 wt%; The oil monomer solution is composed of trimesoyl chloride and an organic solvent, wherein the concentration of trimesoyl chloride is 0.05-0.5 wt%; (2) preparing a grafted layer: soaking the polyethersulfone ultrafiltration membrane treated above in a grafting solution for 2-20 min, and taking out the polyethersulfone ultrafiltration membrane to form a double positive layer structure; The grafting solution is composed of sodium dodecyl sulfate, a high-content amine monomer and water, wherein the concentration of sodium dodecyl sulfate is 0.05-0.2 wt%, and the concentration of the high-content amine monomer is 0.2-3 wt%; the high-content amine monomer is selected from any one or two or more of polyethyleneimine, polyallylamine, triaminoguanidine and chitosan; (3) heat treatment: heat treating the polyethersulfone ultrafiltration membrane treated above at 60-80℃ for 2-20 min to obtain a double positive layer nanofiltration membrane with a double positive layer and a wrinkled texture.

2. The method of claim 1, wherein: In the step (1), the organic solvent is selected from any one or two or more of n-hexane, cyclohexane or isomeric alkane solvents; The average pore size of the polyethersulfone ultrafiltration membrane is 20-200 kDa; The molecular weight of the polyethyleneimine is 500-100000 Da.

3. The method of claim 1, wherein the method further comprises: In the step (1), the polyethersulfone ultrafiltration membrane is pretreated as follows: soaking the polyethersulfone ultrafiltration membrane in an isopropanol solution, and shaking on a shaker for 5-20 min to eliminate chemical impurities remaining on the surface of the polyethersulfone ultrafiltration membrane; and cleaning the polyethersulfone ultrafiltration membrane with ultrapure water for at least three times to remove the residual isopropanol on the surface.

4. The method of claim 1, wherein: In the step (1), the concentration of the inorganic salt is 10-15 wt%, and the concentration of the polyethyleneimine is 0.5 wt%.

5. The method of claim 1, wherein: In the step (2), the grafting time is 10-20 min.

6. The method of claim 1, wherein: In the step (3), the heat treatment temperature is 70-80℃, and the time is 10 min.

7. A double positive layer nanofiltration membrane prepared by the method of any one of claims 1-6.

8. The double positive layer nanofiltration membrane of claim 7 is applied to the treatment of heavy metal-containing wastewater or the softening treatment of ultra-high hardness underground water.

9. Use according to claim 8, characterized in that: The heavy metals in the heavy metal-containing wastewater are any one or two or more of nickel, manganese, copper, cadmium and lead.

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