CN113797759B

CN113797759B - Based on PDA/SiO2Support layer modified polyamide composite nanofiltration membrane and preparation method and application thereof

Info

Publication number: CN113797759B
Application number: CN202111082607.5A
Authority: CN
Inventors: 王涛; 王亚君; 赵珍珍; 郑喜; 张璐瑶
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2021-09-15
Filing date: 2021-09-15
Publication date: 2022-05-06
Anticipated expiration: 2041-09-15
Also published as: CN113797759A

Abstract

The invention discloses a PDA/SiO-based chip₂A high-performance polyamide composite nanofiltration membrane of a supporting layer and a preparation method and application thereof are disclosed, wherein a PMIA substrate is prepared by a phase inversion method, and then a composite nanofiltration membrane with PDA/SiO is prepared₂And finally, preparing the polyamide composite nanofiltration membrane by interfacial polycondensation through a PMIA base membrane of the supporting layer. The NF membrane surface prepared by the invention evolves into a uniform tubular ridge-like accumulation structure from sparse ridges, the PA surface layer is thinned along with the NF membrane surface, the internal pore structure is kept good, and the NF membrane surface has a thickness of 31.37 L.m^‑2·h^‑1·bar^‑1Ultra-high permeability of (2). Meanwhile, the membrane product shows relatively excellent monovalent/divalent salt separation characteristics, has good structural stability and performance stability, has good application prospect in specific water treatment industry, and can ensure a high-efficiency and durable separation process in long-time operation.

Description

Based on PDA/SiO2Support layer modified polyamide composite nanofiltration membrane and preparation method and application thereof

Technical Field

The invention relates to a nanofiltration membrane, in particular to a nanofiltration membrane based on PDA/SiO₂A support layer modified polyamide composite nanofiltration membrane and a preparation method and application thereof; belongs to the technical field of novel membrane materials.

Background

With the shortage of water resources and the increasing pollution of the environment, the ecological environment and the human health are threatened. Therefore, a great deal of research is invested in the water treatment technology, and the green high-efficiency membrane separation technology is the core of the water treatment technology. The polymer membrane plays a great role in water treatment due to the advantages of high efficiency, environmental protection and the like, particularly the nanofiltration membrane (NF) with the molecular weight of 200Da to 2000Da has excellent performance, and has been widely applied to water separation and purification, seawater desalination and sewage treatment.

The nanofiltration membrane mainly comprises a porous supporting layer and a compact surface layer, and the preparation method comprises interfacial polymerization, layer-by-layer self-assembly, surface grafting and physical grafting coating. Among the nanofiltration membranes, the polyamide composite nanofiltration membrane is a membrane product with excellent performance prepared by an interfacial polymerization method, and the preparation principle of the method is based on a microporous matrix, and a polyamide skin layer (PA layer) is formed by the polycondensation reaction of amine and acyl chloride monomers at an oil/water interface. In actual manufacturing processes, the substrate layer and the skin layer can be optimized by selecting different raw materials and process conditions to achieve optimal separation and filtration performance. From a structural and mass transfer point of view, the thickness and density of the polyamide skin layer are crucial to the permeability and selectivity of the TFC composite nanofiltration membrane. Generally, the thickness of the polyamide selective layer formed by the conventional interfacial polymerization process is about 100 to 200nm, which causes a large mass transfer resistance during application. Therefore, the reduction of the thickness of the skin layer of the polyamide and the improvement of the crosslinking degree of the skin layer of the polyamide are key factors for obtaining the high-performance TFC composite nanofiltration membrane.

Nanofiltration technology inevitably faces a trade-off between permeability and selectivity in the operation process, which greatly limits the popularization and application of the technology. The physical and chemical characteristics of the supporting layer also have great influence on the thickness and compactness of the polyamide skin layer through research, and the microporous supporting layer can be optimized by introducing an intermediate layer structure, so that the ultrathin and compact skin layer is obtained, and the comprehensive performance of the membrane is expected to be further optimized.

In view of the above, it is necessary to develop intensive research, design and construct a nanofiltration membrane with excellent permeability and selectivity, and further break through the bottleneck of the trade-off between membrane selectivity and permeability game, so as to effectively reduce the corresponding application cost and prolong the service life of the filtration membrane, and have very important scientific significance and practical value.

Disclosure of Invention

In order to solve the defects of the prior art, one of the purposes of the invention is to provide a PDA/SiO-based material₂The high-performance polyamide composite nanofiltration membrane is used as the supporting layer;

the second purpose is to provide a preparation method of the polyamide composite nanofiltration membrane;

the third purpose is to provide the application direction of the polyamide composite nanofiltration membrane.

In order to achieve the above object, the present invention adopts the following technical solutions:

the invention firstly provides a PDA/SiO-based₂The preparation method of the support layer modified polyamide composite nanofiltration membrane comprises the following steps:

s1, preparing PMIA substrate: adding PMIA fibers into the homogeneous phase solution, heating and stirring to obtain a membrane casting solution, and preparing a PMIA substrate by a phase inversion method;

s2 preparation of a silicon-on-insulator (PDA/SiO)₂PMIA base film of support layer: mixing SiO₂Dissolving in buffer solution, stirring to obtain uniformly dispersed SiO₂A solution; then adding PDA to the uniformly dispersed SiO₂Stirring in the solution to obtain PDA/SiO₂Depositing the mixture on the PMIA substrate prepared in step S1 to obtain the substrate with PDA/SiO₂PMIA base membrane of the supporting layer is placed in deionized water for standby;

s3, preparing the polyamide composite nanofiltration membrane: dissolving piperazine in deionized water to prepare an aqueous solution, and dissolving trimesoyl chloride in Isopar G to prepare an organic phase solution; removal of the silicon dioxide with PDA/SiO from deionized water₂PMIA base film of the support layer, and removing water on the surface thereof; then soaking the PMIA base membrane in an aqueous solution, standing, taking out, and removing the excess aqueous solution on the surface of the membrane; transferring the film into an organic phase solution, and immediately transferring the film into an oven at 80-150 ℃ for treatment for 1-3 min after the film stays; preparing the polyamide composite nanofiltration membrane and storing the polyamide composite nanofiltration membrane in deionized water.

Preferably, the homogeneous solution in the aforementioned step S1 is obtained by: LiCl and PVP were added to DMAc solvent, andstirring was continued at 25 ℃. When inorganic LiCl is added to DMAc solvent, Li⁺And DMAc, Li⁺And the carbonyl group of PMIA, and hydrogen bonds are formed between Cl-and the amino group of PMIA, so that the PMIA polymer can be sufficiently dissolved by using a transparent homogeneous solution formed by DMAc and LiCl as a solvent in the present invention.

Preferably, in step S1, after the PMIA fiber is added to the homogeneous solution, the reaction vessel is placed in a constant temperature oil bath at 70 to 100 ℃ and stirred for 4 to 12 hours at a stirring speed of 300 to 800rpm to obtain a casting solution.

More preferably, in step S1, the PMIA substrate is prepared by a flat plate casting machine, which comprises the following steps: casting the membrane casting solution on a non-woven fabric at the temperature of 25 ℃ in the thickness of 150 mu m, and immediately transferring a fresh membrane into deionized water at the temperature of 30 ℃ for phase conversion after the membrane casting solution stays in the air for 30 seconds; then washing the membrane with deionized water to remove residual solvent and additives; finally, the resulting PMIA substrate was stored in deionized water for further use.

Further preferably, in step S2, the buffer solution is a Tris-HCl buffer solution with pH 8.5, SiO₂After the buffer solution is added, the mixture is firstly rapidly stirred for 3 to 10 minutes at the temperature of between 20 and 40 ℃ and the stirring speed is higher than 600rpm, then the mixture is ultrasonically stirred for 10 to 80 minutes and finally stirred for 0.5 to 2 hours. Firstly, quickly stirring to primarily disperse the added silicon, then ultrasonically stripping the accumulated and agglomerated silicon balls, and then continuously stirring to ensure further dispersion, wherein the stirring method can ensure that the SiO dispersed particularly uniformly is obtained₂And (3) solution.

Still more preferably, in the step S2, the mass ratio of the PDA to the SiO2 is 20: (1-4) adding PDA to the uniformly dispersed SiO₂After the solution was dissolved, it was stirred for 5 minutes in a dark environment at 15-40 ℃ to ensure that PDA and SiO₂Fully reacting and combining under a dark environment to prepare the PDA/SiO₂Mixing the solution, and adding PDA/SiO₂Coating the mixed solution on the surface of the PMIA substrate, and then shaking the container on an oscillator at the rotating speed of 100-80min。

Still more preferably, in the step S3, the PMIA-based film is allowed to stay in the aqueous solution for 2 to 5min and then in the organic phase solution for 20 to 60S.

The invention also claims the PDA/SiO-based material prepared by the method₂And the support layer is modified polyamide composite nanofiltration membrane.

Preferably, the surface of the composite nanofiltration membrane is in a ridge structure or a tubular layer ridge accumulation structure, and the thickness of the polyamide skin layer is 10-20 nm. The polyamide composite nanofiltration membrane is expected to be applied to specific separation of monovalent salt and divalent salt.

The invention has the advantages that:

(1) the invention successfully prepares the product based on PDA/SiO₂The polyamide composite nanofiltration membrane NF modified by the supporting layer is characterized in that under the modification action of the supporting layer, the surface of the prepared NF membrane is changed into a mountain accumulation structure of uniform tubular layers from a sparse ridge structure, the PA surface layer is thinned accordingly, and the internal pore structure is kept well, so that the NF membrane is a novel nanofiltration membrane; the water contact angle of the surface of the modified film product is greatly reduced, the hydrophilicity is enhanced, and the film product prepared by the invention has the water contact angle as high as 31.37 L.m^-2·h^-1·bar^-1Ultra-high permeability of (2).

(2) The rejection rate of the composite nanofiltration membrane prepared by the invention to divalent anions can be basically maintained in the range of 93-97%, and particularly to Na₂SO₄The rejection rate of the sodium chloride (NaCl) is as high as 97.0%, while the rejection rate of the NaCl is about 30%, and the NaCl shows relatively excellent monovalent/divalent salt separation characteristics, so the NaCl has good application prospect in specific water treatment industries.

(3) The NF membrane prepared by the invention shows good structural stability and performance stability in long-term operation. Within 48h, the water permeation flux of the membrane decreased from 31.37 to 31.13 L.m^-2·h^-1·bar^-1The change can be almost ignored in practical application, the retention rate of the salt is also kept stable, and the efficient and durable separation process can be ensured when the separation device is operated for a long time.

Drawings

FIG. 1 is an infrared spectrum of PMIA substrate and PMIA-based substrate obtained in each example of the present invention and comparative example;

FIG. 2 is an infrared spectrum of a polyamide composite nanofiltration membrane (TFC NF) prepared according to various examples and comparative examples of the present invention;

FIG. 3 is a FESEM representation of the modified PMIA base film produced by each example of the present invention (the left panel a is a surface topography and the right panel b is a cross-sectional view);

FIG. 4 is a FESEM representation of polyamide composite nanofiltration membranes (TFC NF) prepared by various embodiments of the present invention (the graph a in the left column is a surface topography graph, and the graph b in the right column is a cross-sectional view);

FIG. 5 is a comparison graph of the water contact angle test results of the surfaces of various film materials;

FIG. 6 is a comparison graph of water contact angle test results of the composite nanofiltration membrane prepared by the invention;

FIG. 7 is a graph showing the results of water permeability and salt rejection of the composite nanofiltration membrane prepared by the present invention;

FIG. 8 is a graph showing the retention rate of the nanofiltration membrane prepared by the present invention for different salts;

fig. 9 is a graph showing the evaluation result of the performance stability of the nanofiltration membrane exemplified by the NF-6 membrane.

Detailed Description

The invention is described in detail below with reference to the figures and the embodiments.

The raw materials used in the present invention are all commercially available unless otherwise specified.

Wherein the polyisophthaloyl metaphenylene diamine (PMIA) is obtained from Nitetaine and advanced materials, Inc. (China), and has a molecular weight of 200000 g/mol^-1Molecular formula is

Example 1

In this example, an unmodified original PMIA substrate was prepared by a non-solvent induced phase inversion (NIPS) method, which includes the following steps:

first, 4.5g LiCl and 1.0g PVP were added to 83.5g DMAc solvent and stirred at 25 ℃ until a homogeneous solution was obtained; then, 11.0g of PMIA fibers were added to the above homogeneous solution and kept in a constant temperature oil bath at 85 ℃ for about 6 hours while stirring at 500rpm to obtain a homogeneous polymer coating solution (casting solution), the casting solution was cooled to 25 ℃ to defoam, and finally a phase inversion method was employed to prepare a PMIA substrate, which was designated as M-0.

Wherein, the specific preparation process of the phase inversion method comprises the following steps: the casting solution was coated on a non-woven fabric at 25 ℃ to a thickness of 150 μm, and after leaving in air for 30 seconds, the nascent film was immediately transferred to deionized water at 30 ℃ for phase inversion. The film was then rinsed with deionized water to remove residual solvent and additives. Finally, the resulting PMIA substrate was stored in deionized water for further use.

Example 2

This example was carried out using the PMIA substrate M-0 obtained in example 1 to prepare a substrate having PDA/SiO₂The PMIA base film of the support layer comprises the following specific processes:

first, 20mg of SiO₂(average particle diameter 20nm) was dissolved in 200mL of Tris-HCl buffer (10mM, pH 8.5), and the mixture was stirred at 25 ℃ for 5 minutes, further stirred with ultrasound for 30 minutes, and then stirred again for 1 hour. Next, 400mg PDA was added to the uniformly dispersed SiO₂In the solution, and then stirred for 5 minutes under a dark condition at 25 ℃ to obtain PDA/SiO₂And (4) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, coating the mixed solution on the surface of a PMIA substrate, shaking the container on an oscillator at the speed of 130r/min for deposition for 20min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-1.

Example 3

This example also uses the PMIA substrate M-0 obtained in example 1 to prepare a substrate having PDA/SiO₂The PMIA-based film of the support layer was prepared substantially as in example 2, with the following details:

first, 20mg of SiO₂(average particle size 30nm) was dissolved in 200mL of Tris-HCl buffer solution (10mM, pH 8.5) at 25 ℃ and 80 ℃Stirring was carried out at 0rpm for 8 minutes, followed by ultrasonic stirring for 40 minutes and then stirring again for 1 hour. Next, 400mg PDA was added to the uniformly dispersed SiO₂In the solution, and then stirred for 5 minutes under a dark condition at 30 ℃ to obtain PDA/SiO₂And (4) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, coating the mixed solution on the surface of a PMIA substrate, shaking the container on an oscillator at the speed of 100r/min for deposition for 40min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-2.

Example 4

This example also used the PMIA substrate M-0 prepared in example 1 to prepare a substrate having PDA/SiO₂The PMIA-based film of the support layer was prepared substantially as in example 2, specifically as follows:

first, 20mg of SiO₂(average particle diameter 20nm) was dissolved in 200mL of Tris-HCl buffer (10mM, pH 8.5), and the mixture was stirred at 25 ℃ for 3 minutes, further stirred with ultrasound for 80 minutes, and then stirred again for 2 hours. Next, 400mg PDA was added to the uniformly dispersed SiO₂In the solution, and then stirred for 5 minutes under a dark condition at 40 ℃ to obtain PDA/SiO₂And (3) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, coating the mixed solution on the surface of a PMIA substrate, shaking the container on an oscillator at the speed of 150r/min for deposition for 60min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-3.

Example 5

first, 20mg of SiO₂(average particle size 25nm) was dissolved in 200mL of Tris-HCl buffer (10mM, pH 8.5), and the mixture was stirred at 25 ℃ for 10 minutes, further stirred with ultrasound for 80 minutes, and then stirred again for 0 to 5 hours. Next, 400mg PDA was added to the uniformly dispersed SiO₂In solution, then dark bars at 25 deg.CStirring for 5 minutes under the condition of stirring to obtain PDA/SiO₂And (4) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, coating the mixed solution on the surface of a PMIA substrate, shaking the container on an oscillator at the speed of 150r/min for deposition for 80min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-4.

Example 6

first, 40mg of SiO₂(average particle diameter: 30nm) was dissolved in 200mL of Tris-HCl buffer (10mM, pH 8.5), and the mixture was stirred at 25 ℃ for 5 minutes, further stirred with ultrasound for 30 minutes, and then stirred again for 1 hour. Next, 400mg PDA was added to the uniformly dispersed SiO₂In the solution, and then stirred for 5 minutes under a dark condition at 25 ℃ to obtain PDA/SiO₂And (4) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, coating the mixed solution on the surface of a PMIA substrate, shaking the container on an oscillator at the speed of 130r/min for deposition for 40min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-5.

Example 7

first, 60mg of SiO₂(average particle diameter: 30nm) was dissolved in 200mL of Tris-HCl buffer (10mM, pH 8.5), and the mixture was stirred at 25 ℃ for 5 minutes, further stirred with ultrasound for 30 minutes, and then stirred again for 1 hour. Next, 400mg PDA was added to the uniformly dispersed SiO₂In the solution, and then stirred for 5 minutes under a dark condition at 25 ℃ to obtain PDA/SiO₂And (4) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, and coating the PMIA baseThe plate surface, the container deposition by shaking on a shaker at 130r/min for 40min, and finally the film is soaked in deionized water for further use, the resulting film product being labeled M-6.

Example 8

first, 80mg of SiO₂(average particle diameter 25nm) was dissolved in 200mL of Tris-HCl buffer (10mM, pH 8.5), and the mixture was stirred at 25 ℃ for 5 minutes, further stirred with ultrasound for 60 minutes, and then stirred again for 2 hours. Next, 400mg PDA was added to the uniformly dispersed SiO₂In the solution, and then stirred for 5 minutes under a dark condition at 15 ℃ to obtain PDA/SiO₂And (4) mixing the solution. The PMIA substrate was fixed in a self-made closed vessel, and then PDA/SiO₂Pouring the mixed solution into a closed container, coating the mixed solution on the surface of a PMIA substrate, shaking the container on an oscillator at the speed of 130r/min for deposition for 40min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-7.

Comparative example 1

Comparative example PMIA base films having only a PDA support layer were prepared from PMIA substrate M-0 prepared in example 1, and the preparation method was the same as in example 2 except that SiO was not added₂The method comprises the following steps:

400mg of PDA was added to deionized water and stirred for 5 minutes to obtain a PDA solution. Fixing the PMIA substrate in a self-made closed container, pouring the PDA solution into the closed container, coating the PDA solution on the surface of the PMIA substrate, shaking the container on an oscillator at the speed of 130r/min for deposition for 20min, and finally soaking the membrane in deionized water for further use, wherein the obtained membrane product is marked as M-0'.

Examples 9 to 16

Examples 9 to 16 were performed by taking the membranes prepared in examples 1 to 8, respectively, and preparing a polyamide composite nanofiltration membrane by using PIP and TMC as reaction monomers and adopting an interfacial polycondensation reaction, wherein the specific reaction process was as follows:

piperazine (PIP) was dissolved in deionized water to prepare an aqueous solution having a concentration of 1.0 wt.%. Trimesoyl chloride (TMC) was dissolved in Isopar G as an organic phase solution at a concentration of 0.1 w/v%. The PDA/SiO prepared in the previous example was removed from the deionized water₂The modified PMIA was based on a film and water was removed from the surface thereof by a rubber roll.

And then, soaking the base film in the prepared aqueous solution, staying for 2-5 minutes, taking out, and removing the excess aqueous solution on the surface of the film by means of a rubber roller. And then transferring the base film into an organic phase solution, reacting for 20-60s, immediately transferring the base film into an oven at 80-150 ℃ for treatment for 1-3 min to promote further polymerization. Finally, the pmiananf membrane was stored in deionized water for use or to be tested.

The differences between examples 9 and 16 are that the process conditions during the preparation process are slightly different, except for the base film used, which is specifically shown in table 1 below:

TABLE 1 tabulation of the process parameters of examples 9-16

For convenience of explanation, the NF membranes prepared from M-0, M-1, M-2, M-3, M-4, M-5, M-6 and M-7 membranes were named NF-0, NF-1, NF-2, NF-3, NF-4, NF-5, NF-6 and NF-7, respectively.

Comparative example 2

The comparative example 2 is to take the membrane product M-0 'obtained in the comparative example 1, and take PIP and TMC as reaction monomers to prepare the polyamide composite nanofiltration membrane by adopting an interfacial polycondensation reaction, the specific reaction process is the same as that of the examples 9 to 16, the description is omitted, and the obtained nanofiltration membrane is named as NF-0'.

Structural characterization and analysis

(1) Fourier transform infrared spectroscopy (FTIR)

Fig. 1 and 2 are infrared spectra of each basement membrane and NF membrane, respectively.

By analysis of the graph, 1650cm in FIG. 2^-1The newly added peak on the left and right corresponds to the-CO-NH-stretching vibration of the amide, which isIndicating that a PA layer was formed on the PMIA substrate after interfacial polymerization. 1080cm which is present in both FIG. 1 and FIG. 2^-1And 804cm^-1The absorption peaks are respectively attributed to the asymmetric and symmetric tensile vibration of Si-O-Si, which shows that SiO₂The base membrane and the NF membrane are successfully introduced, and then the NF-2, the NF-5, the NF-6 and the NF-7 are compared to find that: the absorption peaks at these two positions become increasingly strong as the amount of silica added increases.

At the same time, we observed a position of 3160cm^-1The apparent peak around the left due to-NH of dopamine₂This indicates that PDA was successfully introduced to the membrane surface. And all films were 3300cm^-1An absorption peak occurs due to tensile vibration of the N — H bond.

In addition, the concentration of NF-0 to NF-7 is 3400cm^-1A new peak appeared, which is attributed to the-OH stretching peak, resulting from hydrolysis of unreacted acid chloride after interfacial polymerization. This further indicates that the polyamide separation layer has been successfully polymerized on the PMIA substrate. Furthermore, the occurrence of-OH stretching vibration peak can also improve the hydrophilicity of the surface of the nanofiltration membrane, which is proved in the following water contact angle test of the surface of the membrane.

The above analysis shows that the method of the present invention successfully produces PDA/SiO₂PMIA base membrane modified by a supporting layer; and the base film is adopted, PIP and TMC are taken as reaction monomers, and the polyamide composite nanofiltration membrane is successfully prepared.

(2) The morphology (surface and cross-section) of the composite NF membranes was determined by field emission scanning electron microscopy (FE-SEM).

The morphology of the PMIA substrate, each base film and the TFC NF membrane was observed by FESEM, and the characterization results are shown in FIGS. 3 and 4, respectively (in which the left column shows the surface morphology and the right column shows the cross-sectional morphology).

It was found from an examination of M-0- (a) in FIG. 3 that the PMIA substrate prepared by the present invention had a smooth and flat surface, and the cross-sectional structure of M-0- (b) corresponded to the typical asymmetric structure obtained by the non-solvent induced phase transition method, consisting of finger-like pores, sponge pores and macroporous cell cavity structures.

As can be clearly seen by comparing M-1(a) to M-7(a) with M-0- (a): compared with PMIA substrateThe surface of the modified PMIA basal membrane has a morphological structure of 'particle accumulation'. As can be seen from the sectional SEM image on the right side of fig. 3, the entire polymerization process only affects the surface structure of the membrane, does not affect the internal structure of the pores, and does not cause the blockage of the internal channels. SiO deposition time extension with PDA₂The increase in the amount, PDA mainly accumulated on the outer surface of the membrane pores, and both the particle size and the bulk density gradually increased while the thickness of the intermediate layer increased.

The morphological structure of the TFC NF membrane produced by the various embodiments of the invention is shown in FIG. 4. Compared to fig. 3, the surface of the composite nanofiltration membrane is covered with ridges or tubular ridges, which indicates that: a polyamide skin layer was successfully built up on the support layer by interfacial polymerization. Wherein the NF-0 surface presents a ridge structure of traditional interface polymerization, and the ridge structure is sparsely distributed. With the increase of the deposition time of the PDA (NF-1), the surface of the nanofiltration membrane evolves into a compact tubular chain structure, the folding form is gradually obvious, and the stacking density is obviously increased. Subsequently, continuing to increase the deposition time (NF-2), the rugate structure gradually developed towards a thick worm-like structure. Further, we have found that with SiO₂The fold structure is changed due to the gradual increase of the content, the polyamide layer structure on the surface of the film becomes rough gradually, and simultaneously the ridge pipe becomes thicker in size and larger in volume density.

Observing NF-6- (a) and NF-7- (a) in FIG. 4, the surface of NF-6 and NF-7 has evolved into a mound structure of tubular layers (also called Tuling structure), which has surprisingly appeared, which greatly improves the membrane flux, widens the water transport channel, provides more water transport sites for the membrane material, and further leads to a significant increase in the water flux and better water permeability. Applicants analyzed that this may be due to the enhanced interaction between the support layer and the PIP monomer, which effectively controls the PIP diffusion rate, resulting in a film surface exhibiting an unexpected mound structure of tubular ridges

In order to further study the influence of the support layer on the surface structure of the polyamide membrane, the cross-sectional morphology of the TFC nanofiltration membrane was also characterized. As shown in the right panel b of fig. 4, an ultra-thin barrier layer with a thickness of about 91.50nm was clearly visible on the NF-0 membrane, while the thickness of the polyamide layer decreased significantly with the introduction of the PDA/SiO2 support layer, with a corresponding polyamide layer thickness of only 11.16nm for the NF-6 membrane. The diameter of the tubular structure is more than 100nm, the thickness of the polyamide layer is reduced, and the thickness of the tubular structure is increased, so that the water conveying resistance is greatly reduced, the water permeation flux is improved, and the water flux is further greatly increased.

Performance detection

(1) Hydrophilicity detection (contact Angle)

The surface hydrophilicity of the film was measured by a contact angle meter (OCA15EC, german data physics) using the drop method, and ten measurements were made per sample and the average value was recorded for improved accuracy.

As shown in fig. 5, all of the PMIA substrates and base films exhibited relatively hydrophilic surface properties. The contact angle of the substrate M-0 is 64.2 degrees, PDA/SiO₂The introduction of the supporting layer leads the water contact angles of M-1 to M-7 to be rapidly reduced, and the surface hydrophilicity of the membrane material is obviously improved.

FIG. 6 shows the water contact angle test results of the composite nanofiltration membranes, and the water contact angles of NF-1 to NF-7 are further reduced to 40 to 45 degrees. This indicates that: the composite NF membrane prepared by the invention has a reduced water contact angle value and a more hydrophilic surface. Via PDA/SiO₂The nanofiltration membrane (NF-1 to NF-7) modified by the supporting layer has better hydrophilic performance than NF-0. The enhancement of the hydrophilic ability helps to increase the water flux of the membrane, which will be verified later by the water permeability test results of NF membranes.

(2) Separation Performance of the Membrane (Water flux and rejection)

And (3) measuring the separation performance of the nanofiltration membrane by adopting a cross-flow filtration test device.

In order to ensure stable water flux, the NF membrane is pre-pressed for 1h under the pressure of 1.0MPa, and then the amount of the permeated water is collected at certain intervals. The pure water flux (unit: L.m) was calculated by the following formula^-2·h^-1)：

Wherein J is the amount of permeated water (unit: L), and S is the effective membrane area (unit: m)²) (ii) a t is the time for collecting the permeated water(unit: h).

In addition, the NF membrane pairs Na of the various embodiments are also studied in the invention₂SO₄、MgSO₄NaCl and MgCl₂The separation performance of (3). The solute rejection (R) is calculated by the following equation:

in the formula, C_pAnd C_fPermeate solution and feed solution concentrations, respectively.

FIG. 7 shows the results of water permeability and salt (sodium sulfate) rejection measurements. Compared with the traditional nanofiltration membrane and the comparative example 2(NF-0), the water flux of the TFC-NF membrane prepared on the modified support membrane is obviously improved, and Na is not sacrificed₂SO₄The retention rate of (c).

Further, we also investigated the rejection performance of each NF membrane at 1000ppm for four inorganic salts, NaCl, Na2SO4, MgCl2 and MgSO 4. Referring to fig. 8, the rejection rates of the nanofiltration membranes prepared in the examples for different salts are shown as follows: r (Na)₂SO₄)>R(MgSO₄)>R(MgCl₂)>R (nacl) (from left to right). As can be seen from fig. 8, the rejection rate of the composite nanofiltration membrane of the present invention for divalent anions can be substantially maintained within a range of 93% to 97%, while the rejection rate of the monovalent salt NaCl is substantially about 30%, which shows relatively excellent monovalent/divalent salt separation characteristics, and has good application prospects in specific water treatment industries.

(3) Stability of NF membranes

The performance stability of NF membranes was verified by long-term filtration experiments. Taking NF-6 as an example, deionized water and Na are respectively used₂SO₄And NaCl as a feed solution, membrane permeation and salt rejection were measured every 2h at 6bar for 48 hours, and the results are shown in fig. 9 to evaluate the stability of NF membranes.

As can be seen from FIG. 9, the film showed high and stable Na₂SO₄Retention (-97.0%) and relatively low NaCl retention (-29.72%). Within 48h, the water permeation flux of the membrane decreased from 31.37 to 31.13 L.m^-2·h^-1·bar^-1The variation is almost negligible. This shows that the NF membrane prepared by the invention has excellent performance stability and structural stability, and is resistant to Na₂SO₄And NaCl has high selectivity, and can ensure a high-efficiency and durable separation process during long-time operation.

In conclusion, the polyamide composite nanofiltration membrane NF modified based on the dopamine/SiO 2 support layer is successfully prepared, and the chemical structure of the polyamide composite nanofiltration membrane NF is characterized by Fourier transform infrared spectroscopy (FTIR). The internal structure and surface morphology of the membrane were characterized by Scanning Electron Microscopy (SEM) and the hydrophilicity of the membrane was studied. In addition, practical application performances such as permeability, rejection rate, stability and the like of the membrane are detected by simulating a practical filtration experiment.

The result shows that under the modification action of the PDA/SiO2 supporting layer, the surface of the NF membrane prepared by the invention develops from a sparse ridge-shaped structure to a dense accumulation structure, even magically, a mountain accumulation structure of uniform tubular layers appears, and the PA layer is thinned, so that the water delivery channel is widened, more water delivery sites are provided for the membrane material, the water flux is obviously increased, meanwhile, the water contact angle of the surface of the modified membrane product is greatly reduced, the hydrophilicity is enhanced, and finally the membrane product prepared by the invention has the water delivery rate as high as 31.37 L.m.m^-2·h^-1·bar^-1Ultra-high permeability of (2). Further, the rejection rate of the composite nanofiltration membrane on divalent anions can be basically maintained in a range of 93-97%, and particularly, the rejection rate on Na₂SO₄The rejection rate of the sodium chloride (NaCl) is as high as 97.0%, while the rejection rate of the NaCl is about 30%, and the NaCl shows relatively excellent monovalent/divalent salt separation characteristics, so the NaCl has good application prospect in specific water treatment industries. In addition, the prepared NF membrane shows good structural stability and performance stability in long-term operation.

The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It should be understood by those skilled in the art that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by using equivalent alternatives or equivalent variations fall within the scope of the present invention.

Claims

1. Based on PDA/SiO₂The preparation method of the support layer modified polyamide composite nanofiltration membrane is characterized by comprising the following steps:

s2 preparation of a silicon-on-insulator (PDA/SiO)₂PMIA base film of support layer: mixing SiO₂Dissolving in buffer solution, stirring to obtain uniformly dispersed SiO₂A solution; then adding PDA to the uniformly dispersed SiO₂In the solution, stirring to obtain PDA/SiO₂Depositing the mixture on the PMIA substrate prepared in step S1 to obtain the substrate with PDA/SiO₂PMIA base membrane of the supporting layer is placed in deionized water for standby;

s3, preparing the polyamide composite nanofiltration membrane: dissolving piperazine in deionized water to prepare an aqueous solution, and dissolving trimesoyl chloride in Isopar G to prepare an organic phase solution; removal of the catalyst from deionized water with PDA/SiO₂PMIA base film of the support layer, and water on the surface thereof is removed; then soaking the PMIA base membrane in an aqueous solution, standing, taking out, and removing the excess aqueous solution on the surface of the membrane; transferring the film into an organic phase solution, and immediately transferring the film into an oven at 80-150 ℃ for treatment for 1-3 min after the film stays; preparing the polyamide composite nanofiltration membrane and storing the polyamide composite nanofiltration membrane in deionized water.

2. PDA/SiO-based on claim 1₂The preparation method of the support layer modified polyamide composite nanofiltration membrane is characterized in that the homogeneous solution in the step S1 is obtained by the following method: LiCl and PVP were added to the DMAc solvent and stirring was continued at 25 ℃.

3. PDA/SiO-based on claim 1₂The preparation method of the support layer modified polyamide composite nanofiltration membrane is characterized in that in the step S1, after PMIA fibers are added into the homogeneous solution, the reaction volume is increasedAnd placing the device in a constant-temperature oil bath at 70-100 ℃ and stirring for 4-12 hours at the stirring speed of 300-800 rpm to obtain the casting solution.

4. PDA/SiO-based on claim 1₂The preparation method of the support layer modified polyamide composite nanofiltration membrane is characterized in that in the step S1, a PMIA substrate is prepared by adopting a flat plate casting machine, and the specific process comprises the following steps: casting the membrane casting solution on a non-woven fabric at the temperature of 25 ℃ in the thickness of 150 mu m, and immediately transferring a fresh membrane into deionized water at the temperature of 30 ℃ for phase conversion after the membrane casting solution stays in the air for 30 seconds; then washing the membrane with deionized water to remove residual solvent and additives; finally, the resulting PMIA substrate was stored in deionized water for further use.

5. PDA/SiO-based on claim 1₂The preparation method of the support layer modified polyamide composite nanofiltration membrane is characterized in that in the step S2, the buffer solution is Tris-HCl buffer solution with pH of 8.5 or SiO₂After the buffer solution is added, the mixture is firstly rapidly stirred for 3 to 10 minutes at the temperature of between 20 and 40 ℃, then is ultrasonically stirred for 10 to 80 minutes, and finally is stirred for 0.5 to 2 hours to obtain evenly dispersed SiO₂And (3) solution.

6. PDA/SiO-based on claim 1₂The preparation method of the polyamide composite nanofiltration membrane modified by the supporting layer is characterized in that in the step S2, the SiO is added₂Spherical particles with the particle size of 20-30 nm, PDA and SiO₂The mass ratio of (A) to (B) is 20: (1-4) adding PDA to the uniformly dispersed SiO₂After the solution is added, stirring is carried out for 5 minutes in a dark environment at the temperature of 15-40 ℃ to prepare the PDA/SiO₂Mixing the solution, and adding PDA/SiO₂The mixed solution is coated on the surface of the PMIA substrate, and then the container is shaken on an oscillator at the rotating speed of 100-150rpm for deposition, wherein the deposition time is 20-80 min.

7. PDA/SiO-based on claim 1₂The polyamide composite nanofiltration membrane modified by the supporting layerThe preparation method is characterized in that in the step S3, the retention time of the PMIA basal membrane in the aqueous solution is 2-5min, and then the retention time in the organic phase solution is 20-60S.

8. PDA/SiO-based material prepared by the process of any one of claims 1 to 7₂And the support layer is modified polyamide composite nanofiltration membrane.

9. PDA/SiO-based as defined in claim 8₂The polyamide composite nanofiltration membrane modified by the supporting layer is characterized in that the surface of the composite nanofiltration membrane is of a ridge structure or a tubular layer ridge accumulation structure, and the thickness of the polyamide surface layer is 10-20 nm.

10. PDA/SiO-based substrate as recited in claim 8₂The support layer modified polyamide composite nanofiltration membrane is applied to separation of monovalent salt and divalent salt.