High-flux nanofiltration composite membrane with introduced bionic middle layer and preparation method of high-flux nanofiltration composite membrane

Abstract

The invention relates to the technical field of nanofiltration composite membranes, in particular to a high-flux nanofiltration composite membrane with a bionic middle layer introduced and a preparation method of the high-flux nanofiltration composite membrane. According to the preparation method, a brick-mud structure of a shell pearl layer is utilized, a graphene oxide flexible two-dimensional material is blended, a sodium alginate-polyvinyl alcohol mediated internal and external penetrating modification strategy is adopted, and a middle layer, with mechanical stability, of ultra-thin coordination cross-linked shell-like hydrogel is constructed through bionic design regulation and control; the intermediate layer is intended to regulate diffusion of amine monomers and form and form a nanofiltration composite membrane with high throughput. The nanofiltration composite membrane with the ultra-thin polyamide functional layer structure and the functional middle layer can keep stable water flux and ion interception, and is good in structure.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration composite membrane technology, specifically a high-flux nanofiltration composite membrane with an introduced biomimetic intermediate layer and its preparation method. Background Technology

[0002] With continuous economic development and the increase in the world's population, freshwater scarcity has become one of the global threats to life and safety. Therefore, developing and using effective and energy-saving water treatment technologies can help reduce wastewater discharge and alleviate the current water crisis.

[0003] Membrane separation technology, which utilizes chemical potential / pressure difference to separate solvents and solutes in a solution through semi-permeable membrane materials, is widely used in water treatment fields such as seawater desalination and ultrapure water treatment due to its advantages such as simple process, low cost, and ability to be automated. Membranes can be classified into four categories based on the pressure-driven pore size or molecular weight cutoff: microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes. Nanofiltration, as a high-pressure membrane technology, is a porous reverse osmosis membrane and is mainly used for the removal of divalent ions. Compared with reverse osmosis membranes, nanofiltration membranes can achieve a larger permeate flux at lower operating pressures. Its main working principle utilizes size sieving and the Donnan effect. Typically, nanofiltration membranes can achieve a retention efficiency of over 95% for divalent ions with the same charge as the membrane surface, while the retention range for monovalent ions is approximately 20%–80%.

[0004] Currently, the main process used for commercially mass-produced nanofiltration membranes is interfacial polymerization, where the polycondensation reaction of two monomers mainly takes place at the interface of two immiscible solutions. Specifically, the base membrane adsorbs piperazine (PIP) as an aqueous monomer through an aqueous solution. Excess water on the surface of the base membrane is removed by tools such as air knives and rubber rollers. Subsequently, the base membrane containing the aqueous monomer is immersed in an oil-phase solution to react with the oil-phase monomer, forming a polyamide layer at the interface between the oil and aqueous phases, which hinders further diffusion and reaction of the two monomers.

[0005] The polyamide layer of nanofiltration membranes has a dense structure, which can effectively retain multivalent ions and pollutants, but the small free volume within the polyamide layer has strong resistance. Although most pollutants can be retained on the surface of the polyamide membrane, the resistance within the polyamide layer is still high, thus limiting the water flux of the membrane. However, due to the limitation of the "trade-off" effect, the decrease in retention rate after increasing the free volume within the separation layer makes it necessary to find a balance between the two rather than breaking the effect of either one in the fabrication of nanofiltration membranes. With the addition of nanomaterials, the free volume between the polyamide separation layers can be effectively increased, thereby increasing the permeation flux of the membrane. However, with the incorporation of nanomaterials into the polyamide separation layer, there is a problem of excessive surface energy and agglomeration, which is very likely to cause defects in the active layer and damage the retention rate. The modification strategy of the intermediate layer can (1) store more amine monomers; (2) control the diffusion of amine monomers; and (3) effectively inhibit the accumulation of polyamide in the pore size of the base membrane, ultimately achieving a significant improvement in permeation performance without sacrificing selectivity. The main approach is to experimentally add an intermediate layer composed of nanomaterials on top of the original support layer.

[0006] Despite numerous literature reports that the introduction of an intermediate layer can significantly increase permeation flux, it still suffers from instability, poor mechanical strength, and other issues. Summary of the Invention

[0007] To address the aforementioned technical problems in the prior art, this invention provides a method for preparing a high-flux nanofiltration composite membrane by incorporating a biomimetic intermediate layer.

[0008] A high-flux nanofiltration composite membrane with a biomimetic intermediate layer contains a shell-like nacre intermediate layer. The shell-like nacre intermediate layer has a "brick-and-mortar" structure, specifically a structure in which hard 2D calcium carbonate sheets and organic matrix are stacked alternately and orderly.

[0009] Furthermore, the intermediate layer of the imitation seashell nacre layer specifically uses two-dimensional graphene oxide nanosheets as the brick part in the "brick-mud" structure, and sodium alginate-polyvinyl alcohol is prepared as a solution as the organic matter and the two are blended together; then, vacuum-assisted filtration is used to deposit it on the top of the base membrane; finally, the membrane is immersed in a calcium ion solution to crosslink and solidify it with sodium alginate, and then dried for later use.

[0010] Seashell nacre is a unique composite material. Its unique "brick-and-mortar" structure is formed by the alternating and orderly stacking of hard 2D calcium carbonate sheets and an organic matrix. Because the organic matrix and calcium carbonate nanosheets are chemically bonded and physically entangled in this structure, they can effectively disperse and withstand external stress after forming an orderly arrangement, thus achieving an excellent balance between hardness and toughness.

[0011] Furthermore, the sodium alginate-polyvinyl alcohol solution is specifically prepared by blending 1 wt% polyvinyl alcohol (PVA) and 0.5 wt% sodium alginate (SA) in a 1:1 ratio.

[0012] Furthermore, the base film is prepared by the following steps: polysulfone powder and N,N-dimethylformamide solvent are mixed, stirred and heated until the polysulfone powder dissolves in N,N-dimethylformamide and forms a transparent casting solution (the viscosity of the casting solution is 1200 mPa·s as measured by a viscometer); subsequently, the casting solution is converted in situ into a white base film on the basis of a non-woven fabric support layer by a non-solvent phase inversion method.

[0013] Furthermore, the polysulfone powder and N,N-dimethylformamide solvent are mixed, specifically, the polysulfone powder and N,N-dimethylformamide (DMF) solvent are mixed at a ratio of 18%.

[0014] Furthermore, the nanofiltration composite membrane also includes a polyamide skin layer. Anhydrous piperazine is dissolved in deionized water as the aqueous phase, and 1,3,5-tribenzoyl chloride is dissolved in n-hexane as the oil phase, which are placed in two separate containers for later use. Subsequently, the prepared base membrane with the intermediate layer is fixed in a polytetrafluoroethylene frame and placed in the container containing the aqueous phase for 30 seconds. Then, the base membrane immersed in the piperazine solution is subjected to air knife and rubber roller to remove excess aqueous phase from the surface. The base membrane is then immersed in the container containing the oil phase for 30 seconds to obtain a polyamide desalination layer, which is then stored at 70°C for 15 minutes.

[0015] Furthermore, the nanofiltration composite membrane is characterized in that the concentration of piperazine in the aqueous phase is 1 wt%.

[0016] Furthermore, the concentration of 1,3,5-tribenzoyl chloride in the oil phase is 1 wt%.

[0017] The method for preparing the nanofiltration composite membrane includes the following steps:

[0018] ① The intermediate layer of the shell-like structure is prepared by using two-dimensional graphene oxide nanosheets (GO) as the brick part in the "brick-mud" structure. The graphene oxide powder is weighed, added to water, and then ultrasonically dispersed using a cell wall disruptor to obtain a GO suspension with a concentration of 2 mg / L.

[0019] ② 1 wt% polyvinyl alcohol (PVA) and 0.5 wt% sodium alginate (SA) are blended in a 1:1 ratio;

[0020] ③ Take 10 ml of the above-mentioned concentration of GO suspension and mix it with the same volume of PVA-SA mixed solution to prepare a suspension;

[0021] ④ The above GO-PVA-SA suspension is vacuum filtered onto a polysulfone-based membrane to form a shell-like hydrogel mixed matrix composite membrane. This membrane is then placed in a solution containing 0.01 wt% calcium ions to allow sodium alginate to crosslink with the calcium ions, ultimately forming a linear PVA molecular interpenetrating network SA-Ca with graphene oxide as the building block. 2+ The nanofiltration intermediate layer structure serves as a shell-like layer for mud.

[0022] Furthermore, a polyamide skin was formed on the prepared base film with an intermediate layer. Anhydrous piperazine was dissolved in deionized water as the aqueous phase, and 1,3,5-tribenzoyl chloride was dissolved in n-hexane as the oil phase. The two solutions were placed in separate containers for later use. The prepared base film with the intermediate layer was then fixed in a polytetrafluoroethylene frame and placed in the container containing the aqueous phase for 30 seconds. The base film immersed in the piperazine solution was then subjected to air knife and rubber roller to remove excess aqueous phase from the surface. The base film was then immersed in the container containing the oil phase for 30 seconds to obtain a polyamide desalination layer, which was then stored at 70°C for 15 minutes.

[0023] Compared with the prior art, the technical effects of this invention are reflected in:

[0024] 1. The nacreous layer of seashells maintains excellent mechanical properties and chemical stability in complex marine environments due to its multi-scale, multi-level "brick-and-mortar" layered structure, composed of alternating two-dimensional layered minerals and one-dimensional flexible macromolecules. This invention utilizes the "brick-and-mortar" structure of nacreous layers. By blending flexible two-dimensional graphene oxide materials and employing an internal-external through-modification strategy mediated by sodium alginate and polyvinyl alcohol, a biomimetic design is used to construct an ultrathin, coordinated, cross-linked, shell-like hydrogel with mechanical stability. This intermediate layer aims to regulate the diffusion of amine monomers and form a high-flux nanofiltration composite membrane.

[0025] 2. The nanofiltration composite membrane with an ultrathin polyamide functional layer structure obtained in this invention has an intermediate layer thickness of 51.7 nm and a polyamide separation skin layer thickness of 35-65 nm. Furthermore, by comparison with membranes with traditional thick polyamide functional layers, it was found that under the following conditions: 2000 ppm Na₂SO₄ aqueous solution, operating at a pressure of 0.40 MPa (concentrate circulation), the water flux decreased from 16.39 Lm⁻¹. -2 .h-1 .bar -1 The horizontal increase reached 37.81Lm. -2 .h -1 .bar -1 The efficiency increased by approximately two times, and the desalination rate only decreased from 99.36% to 99.17%. Subsequently, by changing to other types of salts (NaCl, MgCl2, MgSO4), the permeability and selectivity of the membrane were tested. It was found that in continuous testing, all generated membranes maintained stable water flux and ion rejection, indicating a good structure. Compared to pure polyamide membranes and polyamide membranes with functional interlayers, the removal rates of divalent ions were higher for both polyamide and monovalent salts (Na2SO4 > MgSO4 > MgCl2 > NaCl). The separation factor between monovalent and polyvalent ions was calculated to be 4. Detailed Implementation

[0026] The technical solution of the present invention will be further defined below with reference to specific embodiments, but the scope of protection is not limited to the description made.

[0027] Example 1

[0028] First, prepare a mixed solution of 0.5 wt% sodium alginate and 1 wt% polyvinyl alcohol. Then, weigh GO powder and add it to distilled water. Sonicate the solution for 30 minutes using a cell disruptor, followed by centrifugation to remove any unpeeled fragments, thus obtaining a 100 mg / L GO aqueous solution. Dilute 1.0 ml of the GO aqueous solution to 50 ml to obtain a 2.0 mg / L GO aqueous solution. Subsequently, 10 ml of 2.0 mg / L GO was mixed with the same volume of PVA-SA solutions of different concentrations and filtered onto the base membrane. The base membrane was then immersed in pure water overnight to ensure that any unwashed organic solvents in the base membrane would not affect the interfacial polymerization reaction. The prepared base membrane was then fixed in a polytetrafluoroethylene frame and placed in a 1 wt% PIP solution for 30 seconds. The base membrane immersed in the PIP solution was then subjected to air knife and rubber roller to remove excess aqueous phase from the surface. The base membrane was then immersed in a 0.1 wt% TMC solution for 30 seconds to obtain a polyamide desalination layer, which was then stored at 70°C for 15 minutes.

[0029] Comparative Example 1

[0030] The polyamide thin-film composite film without the intermediate layer was used as Comparative Example 1, as follows:

[0031] The base membrane was immersed in pure water overnight to ensure that any unwashed organic solvents in the base membrane would not affect the interfacial polymerization reaction. The prepared base membrane was then fixed in a polytetrafluoroethylene frame and placed in a 1 wt% PIP solution for 30 seconds. The base membrane immersed in the PIP solution was then subjected to air knife and roller to remove excess aqueous phase from the surface. Next, the base membrane was immersed in a 0.1 wt% TMC solution for 30 seconds to obtain a polyamide desalination layer, which was then stored at 70°C for 15 minutes. The test results in Na2SO4 solution are shown in Table 1 below (concentrate circulation):

[0032] Table 1

[0033]

[0034]

[0035] The test results in NaCl are shown in Table 2 below (concentrate circulation):

[0036] Table 2

[0037]

[0038] The test results in MgCl2 are shown in Table 3 below (concentrate circulation):

[0039] Table 3

[0040]

[0041] The test results in MgSO4 are shown in Table 4 below (concentrate circulation):

[0042] Table 4

[0043]

[0044]

[0045] In continuous testing, all generated membranes maintained stable water flux and ion rejection, indicating a good structure. The removal rates of divalent ions by both the interlayer-free and interlayer-containing polyamide composite membranes were higher than those by monovalent salts (Na₂SO₄ > MgSO₄ > MgCl₂ > NaCl). The separation mechanism of the interlayer-containing membranes is attributed to the coexistence of size repulsion and Donnan repulsion. The NaCl desalination rate of the interlayer-containing composite membrane was slightly higher than that of the non-interlayer-containing composite membrane, which may be attributed to a sieving effect in the mixing dimension of the interlayer.

[0046] Finally, it should be noted that the above embodiments are merely representative examples of the present invention. Obviously, the technical solution of the present invention is not limited to the above embodiments, and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the content disclosed in this invention should be considered within the scope of protection of this invention.

Claims

1. A high-flux nanofiltration composite membrane incorporating a biomimetic intermediate layer, characterized in that, It contains a shell-like nacre intermediate layer, which has a "brick-and-mortar" structure, specifically a structure in which hard 2D calcium carbonate sheets and organic matrix are stacked alternately and orderly.

2. The nanofiltration composite membrane according to claim 1, characterized in that, The intermediate layer of the imitation seashell nacre layer specifically uses two-dimensional graphene oxide nanosheets as the brick part in the "brick-mud" structure, and sodium alginate-polyvinyl alcohol is prepared as a solution as the organic matter and the two are blended together; then, vacuum-assisted filtration is used to deposit it on the top of the base membrane; finally, the membrane is immersed in a calcium ion solution to crosslink and solidify it with sodium alginate, and then dried for later use.

3. The nanofiltration composite membrane according to claim 2, characterized in that, The sodium alginate-polyvinyl alcohol solution is specifically prepared by blending 1 wt% polyvinyl alcohol (PVA) and 0.5 wt% sodium alginate (SA) in a 1:1 ratio.

4. The nanofiltration composite membrane according to claim 1, characterized in that, The base film is prepared by the following steps: polysulfone powder and N,N-dimethylformamide solvent are mixed, stirred and heated until the polysulfone powder dissolves in N,N-dimethylformamide and forms a transparent casting solution; then, the casting solution is converted in situ into a white base film on the basis of a non-woven fabric support layer by a non-solvent phase inversion method.

5. The nanofiltration composite membrane according to claim 4, characterized in that, The polysulfone powder and N,N-dimethylformamide solvent are mixed, specifically, the polysulfone powder and N,N-dimethylformamide solvent are mixed at a ratio of 18%.

6. The nanofiltration composite membrane according to claim 1, characterized in that, It also includes a polyamide skin layer. Anhydrous piperazine is dissolved in deionized water as the aqueous phase, and 1,3,5-tribenzoyl chloride is dissolved in n-hexane as the oil phase. The two solutions are placed in separate containers for later use. The prepared base film with the intermediate layer is then fixed in a polytetrafluoroethylene frame and placed in a container containing the aqueous phase for 30 seconds. The base film immersed in the piperazine solution is then subjected to air knife and rubber roller to remove excess aqueous phase from the surface. The base film is then immersed in a container containing the oil phase for 30 seconds to obtain a polyamide desalination layer, which is then stored at 70°C for 15 minutes.

7. The nanofiltration composite membrane according to claim 6, characterized in that, The concentration of piperazine in the aqueous phase is 1 wt%.

8. The nanofiltration composite membrane according to claim 6, characterized in that, The concentration of 1,3,5-tribenzoyl chloride in the oil phase is 1 wt%.

9. The method for preparing the nanofiltration composite membrane according to claim 1, characterized in that, Includes the following steps: ① The intermediate layer of the shell-like structure is prepared by using two-dimensional graphene oxide nanosheets (GO) as the brick part in the "brick-mud" structure. The graphene oxide powder is weighed, added to water, and then ultrasonically dispersed using a cell wall disruptor to obtain a GO suspension with a concentration of 2 mg / L. ② 1 wt% polyvinyl alcohol (PVA) and 0.5 wt% sodium alginate (SA) are blended in a 1:1 ratio; ③ Take 10 ml of the above-mentioned concentration of GO suspension and mix it with the same volume of PVA-SA mixed solution to prepare a suspension; ④ The above GO-PVA-SA suspension is vacuum filtered onto a polysulfone-based membrane to form a shell-like hydrogel mixed matrix composite membrane. This membrane is then placed in a solution containing 0.01 wt% calcium ions to allow sodium alginate to crosslink with the calcium ions, ultimately forming a linear PVA molecular interpenetrating network SA-Ca with graphene oxide as the building block. 2+ The nanofiltration intermediate layer structure serves as a shell-like layer for mud.

10. The preparation method according to claim 9, characterized in that, A polyamide skin was also formed on the prepared base film with an intermediate layer. Anhydrous piperazine was dissolved in deionized water as the aqueous phase, and 1,3,5-tribenzoyl chloride was dissolved in n-hexane as the oil phase. The two solutions were placed in separate containers for later use. The prepared base film with the intermediate layer was then fixed in a polytetrafluoroethylene frame and placed in a container containing the aqueous phase for 30 seconds. The base film immersed in the piperazine solution was then subjected to air knife and rubber roller to remove excess aqueous phase from the surface. The base film was then immersed in a container containing the oil phase for 30 seconds to obtain a polyamide desalination layer, which was then stored at 70°C for 15 minutes.

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