KR102251906B1 - Filtration membrane and method for manufacturing the same - Google Patents

Abstract

A filtration membrane having excellent performance and a manufacturing method thereof are provided. According to the present invention, the filtration membrane comprises: a polymer matrix; and carbon nanotubes (PuCNTs) dispersed in the polymer matrix and having a partially open structure. The manufacturing method of the filtration membrane comprises the steps of: forming a carbon nanotube (PuCNT) having a partially open structure; forming a mixed solution by mixing the PuCNT with a polymer and a first solvent; and casting the mixed solution.

Description

Filtration membrane and its manufacturing method TECHNICAL FIELD [FILTRATION MEMBRANE AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a filtration membrane and a method of manufacturing the same.

Polymer filtration membranes are most widely used in the water treatment industry, but the inherent hydrophobicity of the polymer limits its application in water treatment. In order to impart hydrophilicity to the polymer filtration membrane, a technique of adding hydrophilic nano fillers such as carbon nanomaterials, cellulose nanocrystals, and metal oxides has been proposed. However, these nano fillers are not properly dispersed in the polymer filtration membrane.

In order to solve the above problems, the present invention provides a filtration membrane having excellent performance.

The present invention provides a method of manufacturing the filtration membrane.

Other objects of the present invention will become apparent from the following detailed description and accompanying drawings.

The filtration membrane according to embodiments of the present invention includes a polymer matrix and carbon nanotubes (PuCNT) dispersed in the polymer matrix and having a partially open structure.

The PuCNT may have an oxidizing group. The weight ratio of the PuCNT to the polymer may be 0.01:100 to 0.5:100.

The polymer may include polyethersulfone (PES). The filtration membrane may be an ultrafiltration membrane.

The method of manufacturing a filtration membrane according to embodiments of the present invention includes forming a carbon nanotube (PuCNT) having a partially open structure, mixing the PuCNT with a polymer and a first solvent to form a mixed solution, and And casting the mixed solution.

The forming of the PuCNT may include mixing a carbon nanotube, an oxidizing agent, and a second solvent, and the carbon nanotubes may be oxidized by the oxidizing agent.

The oxidizing agent may include _{KMnO 4.} The oxidizing agent may be added in an amount of 1 to 5 equivalents based on the weight of the carbon nanotubes.

The weight ratio of the PuCNT to the polymer may be 0.01:100 to 0.5:100. The polymer may include polyethersulfone (PES).

The filtration membrane according to the embodiments of the present invention may have excellent performance. For example, the filtration membrane is excellent in water permeability, removal of contaminants, contamination resistance, and driving stability. In addition, energy required for a water treatment process may be reduced by the filtration membrane.

1 shows a comparison between PuCNT and CNT.
2 is an SEM image showing the surface and cross section of PES-M and PuM.
3 shows the water contact angle and surface free energy of PES-M and PuM.
4 shows the ultrafiltration performance of PES-M and PuM.
5 to 7 show antifouling properties of PES-M and PuM.

Hereinafter, the present invention will be described in detail through examples. Objects, features, and advantages of the present invention will be easily understood through the following embodiments. The present invention is not limited to the embodiments described herein, and may be embodied in other forms. The embodiments introduced herein are provided so that the disclosed contents may be thorough and complete, and the spirit of the present invention may be sufficiently transmitted to those of ordinary skill in the art to which the present invention pertains. Therefore, the present invention should not be limited by the following examples.

In the present specification, terms such as first and second are used to describe various elements, but the elements should not be limited by these terms. These terms are only used to distinguish the elements from each other.

Partially unzipped carbon nanotube , PuCNT ) Of manufacturing example

Carbon nanotubes (MWCNT) (500mg _{) were suspended in concentrated H 2} SO ₄ (120 mL) and stirred for 1 hour. _{H 3} PO ₄ (15 mL) was added to the reaction mixture and further mixing was performed for 15 minutes. KMnO ₄ 1500 mg (3 equivalents) was slowly added to the reaction mixture and stirred for 30 minutes to completely dissolve _{KMnO 4.} The reaction mixture was heated to 65° C. and stirred for 2 hours. Then, the heat source was removed and the reaction mixture was cooled to room temperature. To complete the reaction, the reaction mixture _{was poured into ice containing deionized water (300 mL) and H 2} O ₂ (30 mL) and stirred for 1 hour. The reaction mixture was filtered through an AAO filter (pore size about 0.02 μm). The thus obtained solid was suspended in 20% by volume HCl and subjected to ultrasonic treatment for 30 minutes to remove residual impurities. The solid obtained after vacuum filtration through an AAO filter was suspended in deionized water and centrifuged at 5000 rpm for 20 minutes. This process was repeated until the pH of the supernatant reached 6.0. The final precipitate was washed with ethanol and then coagulated with ether, filtered through a nylon filter (pore size of about 0.45 μm), and vacuum dried at 35° C. for 48 hours. Thereby, PuCNT can be produced. The PuCNT was denoted as PuCNT(x). _{x represents the equivalent of KMnO 4} (eg, 3) to the weight of carbon nanotubes (MWCNT) used in this reaction.

1 shows a comparison between PuCNT and CNT. Referring to FIG. 1, PuCNT is a partially open structure having small pores, so that contaminants do not permeate, but only water molecules can be selectively permeated. In addition, through the partially open structure, the contact area with water increases and the water permeability increases, and the contact area with the polymer matrix increases, thereby improving driving stability.

PuCNT Inclusive PES ( polyethersulfone ) Of the filtration membrane (PuM) Manufacturing example

PES-M (pristine PES membrane) and PuM were prepared by the NIPS (nonsolvent-induced phase inversion) method. The composition of the casting solution is shown in Table 1.

[Table 1]

A suspension of the desired concentration of PuCNT(3) was prepared using stirring and sonication. Completely dried PES flakes and PVP powder were added to the PuCNT dispersion. The mixed solution was stirred at 50° C. for 12 hours and sonicated for 1 hour using a degassing mode. The resulting solution was cast on a nonwoven fabric with a 200 μm-thick casting knife at a shear rate of 70 mm/s and placed in a coagulation bath filled with deionized water. Phase inversion was performed at room temperature. After 24 hours, the resulting membrane was washed with deionized water and stored in 1 vol% ethanol aqueous solution at 5°C. The prepared PES nanocomposite membrane was denoted by PuM-x. x represents the weight percent (0.01, 0.05, 0.1, 0.5 and 1.0) of PuCNT (3) relative to the amount of PES in the casting solution. PES nanocomposite membranes in which graphene oxide (GO), carbon oxide nanotubes (oCNT), or a mixture of GO and oCNT (1/1, w/w) were mixed were also prepared in the same manner and were prepared by the same method, respectively, and GO-x, oCNT- Expressed as x or G/Cx.

Ultra Ultrafiltration test

The ultrafiltration performance of the membrane was performed using a dead-end cell with an effective area ^{of 14.65 cm 2.} Before the test, the water flow was stabilized by compressing the membrane for 20 minutes at 2 bar using deionized water in a nitrogen atmosphere. The pure water flux was measured using deionized water as the feed solution. The filtration test was performed at 25° C. with a fixed stirring speed of 450 rpm for 1 hour at 1 bar in a nitrogen atmosphere. Every 10 minutes, the permeated water was weighed using an electronic balance. The water flux, Jw (L/m ² h ¹ , LMH) was calculated by Equation 1.

[Equation 1]

In Equation 1, V is the permeation amount of water (L), A is the effective membrane area (14.65Х10 ^-4 m ² ), and t is the operating time (h).

BSA was used as a model protein to measure the protein removal rate and fouling behavior of the membrane. The BSA removal rate test was performed using the BSA solution as the feed solution for 1 hour. The removal rate of BSA was calculated by Equation 2.

[Equation 2]

In Equation 2, C ₁ and C ₂ are the BSA concentrations of the feed and permeate solutions, respectively. The concentration of BSA was determined by measuring the UV/Vis absorbance of the solution at a wavelength of 280 nm.

protein Fouling Protein fouling test

The fouling resistance of the membrane was determined by a static protein adsorption test and a dynamic membrane fouling test using an aqueous solution of BSA (1000 ppm, PBS, pH 7.4). The static BSA adsorption capacity was determined as follows. A dry membrane having a size of 1.9 cm ² was immersed in 3 mL of BSA solution at 25° C. for 24 hours. Then, the UV/Vis absorbance of the solution was measured at a wavelength of 280 nm to calculate the concentration of BSA. The static BSA adsorption capacity of the membrane was calculated by Equation 3.

[Equation 3]

In Equation 3, C ₁ and C ₂ are the initial and final concentrations of BSA in the solution, respectively, and A is the effective membrane area (1.9 cm ² ). Results were obtained by performing more than five experiments.

A dynamic fouling test was performed using a dead-end cell at 25° C. at a fixed stirring speed of 450 rpm in a nitrogen atmosphere of 1 bar. First, the water flux (J ₁ ) of the membrane was measured for 1 hour using deionized water as a feed solution. Next, the feed solution was changed to a BSA solution (1000 ppm, PBS, pH 7.4), and the water flux (J ₂ ) was measured. After 1 hour, the membrane was washed with deionized water and the pure water flux (J ₃ ) was recorded for 1 hour. The fouling resistance of the membrane was investigated using four types of fouling parameters: flux recovery rate (FRR), total flux reduction rate (DRt), reversible flux reduction rate (DRr), and irreversible flux reduction rate (DRir) using the following equations 4 to 7 Became.

[Equation 4]

[Equation 5]

[Equation 6]

[Equation 7]

All filtration tests were performed three or more times, and the values obtained were averaged.

PuM Characteristic

The surface and cross-sectional morphology of the membrane was observed using SEM analysis. 2A and 2B show surface SEM images of PES-M and PuM-0.05, respectively. The two membranes represent a crack-free surface, indicating that adjacent pores are connected and cracks are formed. Other PES nanocomposite membranes exhibit similar morphologies to those of PES-M and PuM-0.05.

The internal structure of the membrane is shown in Figure 2 (c) to (l). (c) and (d) represent the cross section of PES-M, (e) and (f) represent the cross section of PuM-0.01, (g) and (h) represent the cross section of PuM-0.05, (i ) And (j) represent the cross section of PuM-0.1, and (k) and (l) represent the cross section of PuM-0.5. All membranes exhibit an asymmetric structure with a dense top layer supported by a finger-like structure. All PuMs are thinner than PES-M because the addition of hydrophilic nanofillers accelerates the precipitation of the polymer film. As the content of PuCNT(3) increases from 0 to 0.05% by weight based on the weight of PES, the thickness of the film decreases from 207.0 to 136.9 μm.

When the content of PuCNT(3) increases from 0.05 to 0.5% by weight, the thickness of the film increases from 136.9 to 245.5 μm due to the addition of the hydrophilic nano filler, which helps to expand the film before solidification. As the content of PuCNT (3) increases from 0 to 0.5% by weight, the thickness of the top layer increases from 170 nm to 310 nm, which is due to the trapping of nano-pillars on the surface of the membrane during phase separation. It hinders the diffusion of the solvent into the non-solvent and increases the thickness of the upper layer. This dense and thick top layer can improve the selectivity of the film.

The hydrophilic properties of the membrane were investigated by measuring the water contact angle. In general, a higher contact angle means relatively hydrophobic, and a lower contact angle means greater hydrophilicity, which is an important parameter for the separation performance of the membrane. As shown in FIG. 3, the contact angle of PES-M is 72.5°, and the contact angle of PuM decreases to 56° as the content of PuCNT (3) increases to 0.1% by weight due to the hydrophilicity of PuCNT (3).

Hydrophilic nanopillars migrate to the surface of the film during film formation. The presence of PuCNT(3) on the membrane surface increases the hydrophilicity of the membrane. As the hydrophilicity of the membrane increases, the surface free energy also increases, so that the adhesion of the hydrophobic material may decrease. The surface charge of the membrane is an important factor in the antifouling properties of the membrane.

Zeta potential analysis was performed to detect the surface charge density of the film, and the values are shown in Table 2.

[Table 2]

The addition of PuCNT reduces the surface charge of the film due to oxygen-containing groups such as carboxyl and hydroxyl. The higher the negative charge, the more negatively charged contaminants such as bovine serum albumin (BSA) and humic acid are removed due to electrostatic repulsion, and the antifouling properties are increased.

Ultra Ultrafiltration test

The ultrafiltration performance of the membrane was determined using a laboratory scale dead-end cell by measuring the pure water flux (PWF) and bovine serum albumin (BSA) removal rate of the membrane. As shown in Fig. 4, the PWF of PES-M is 107±2.2LMH. PWF increases to 245.8±4.8LMH when the content of PuCNT is 0.05% by weight. In general, the pure water flux of the membrane is closely related to hydrophilicity. However, when the content of PuCNT (3) is increased to 0.5% by weight, the thickness of the film increases, and the PWF of the film decreases to 148.2±4.9LMH. Despite the decrease in PWF, all PuMs show a larger PWF compared to PES-M. PuM-0.05 with at least a filler content represents the largest PWF. These results indicate that the open structure of PuCNT(3) acts as a water transport channel to efficiently transport water molecules.

Further, the protein removal rate of the membrane was tested using BSA. A BSA aqueous solution (1000 ppm, PBS buffer, pH 7) was prepared and used as a feed solution. As shown in Figure 4, PES-M exhibits a BSA removal rate of 96.6%. With the loading of PuCNT (3), it increases substantially to 99.7%. This result is due to the large surface negative charge of PuM caused by oxygen-containing functional groups, increasing the electrostatic repulsion between the BSA and the membrane surface.

A short-term stability test was performed to investigate the stability of PuCNT(3) in PES by measuring PWF for 6 hours. All PuM showed a smaller flux reduction than PES-M, indicating that PuCNT(3) could act as a reinforcing material by reducing the destruction of the porous structure of the membrane. In addition, there is no PuM indicating leaching of PuCNT during the filtration test (6 hours), indicating that PuCNT (3) is successfully attached to the PES chain.

Antifouling exam( Antifouling test)

The antifouling properties of the membrane are evaluated by a dynamic fouling test using BSA as a model foulant. 5 shows the normalized flux change of the membrane. All membranes show a large flux reduction at the beginning of the BSA filtration step due to the adhesion of BSA molecules on the membrane surface. For example, when the feed solution is replaced with a BSA solution by deionized water, the molar flux of PES-M is reduced by about 46%. PuM has a smaller flux reduction compared to PES-M, indicating low adhesion of BSA molecules on the membrane surface due to the increase in hydrophilicity and negative surface charge. In addition, PuM has a higher flux recovery ratio (FRR) than PES-M.

In order to evaluate the antifouling properties of the membrane in more detail, four antifouling parameters were calculated and shown in FIG. 6. Larger flux recovery values generally indicate better antifouling properties of the membrane. The FRR value of PuM is always greater than the FRR value of PES-M. As the content of PuCNT in PES increases from 0 to 0.5% by weight, the FRR value of PuM increases from 55.0 to 79.8%, indicating that BSA molecules are more easily separated from the surface of PuM.

Flux reduction ratios such as total flux reduction (DRt), reversible flux reduction (DRr) and irreversible flux reduction (DRir) were calculated to evaluate the recycling properties of the membrane in more detail. Membrane fouling is generally classified into reversible fouling and irreversible fouling. Reversible fouling occurs when contaminants adhere loosely to the membrane surface, which can be easily removed with a simple cleaning process. Irreversible fouling occurs when contaminants are firmly adsorbed to the membrane surface. To solve this problem, chemical cleaning is generally performed to shorten the life of the film. Thus, a smaller irreversible flux reduction (DRir) is needed to design recyclable membranes. As shown in FIG. 6, when the content of PuCNT (3) is 0.5% by weight, the irreversible flux reduction (DRir) is the smallest.

The antifouling properties of the membrane can be further explained by the BSA adsorption behavior at the membrane surface. 7 shows the BSA adsorption capacity in a static state. As shown in FIG. 7, PES-M shows the largest amount of adsorbed BSA of ^{about 144.3±5.1 μg/cm 2.} As the content of PuCNT(3) increases from 0.01 to 0.5% by weight, the BSA adsorption capacity ^{decreases from 119.4±0.64 to 83.2±4.8μg/cm 2} , which means that the incorporation of PuCNT(3) decreases the adsorption of BSA on the membrane surface. Indicates that it interferes. These results are similar to those of water contact angle, surface charge measurement and dynamic fouling test. As such, the antifouling properties of PuM can be explained by its hydrophilicity and surface charge.

Partially open carbon nanotubes (PuCNT) are manufactured by oxidizing carbon nanotubes, and may have multiple oxidizing groups. When the amount of oxidizing agent is 3 equivalents of the amount of CNTs, the partially open structure was observed as the optimal amount of oxidizing functional groups. PuM was prepared with various contents of PuCNT (3). When the hydrophilic PuCNT was added to the PES film, the hydrophilicity increased and the surface charge decreased, which is due to the oxidizer of PuCNT. This change in membrane properties can improve the water flux and antifouling properties of the membrane. As the content of PuCNT was increased from 0 to 0.05% by weight, the PWF and BSA removal rates were simultaneously improved from 107.0 to 148.2LMH and from 96.6 to 99.7%, respectively. In addition, PuM-0.05 exhibited the largest PWF compared to other PES nanocomposite membranes containing at least a carbon nanomaterial in the content of the nano-pillar. These results indicate that the partially open structure of the PuCNT improves the free volume of the carbon nanotubes, thereby allowing more water molecules to be absorbed and transported compared to the carbon nanotubes. When the content of PuCNT (3) was 0.5% by weight, the FRR reached 79.8%, which was 45% greater than the content of PES-M. PuM has higher PWF and BSA removal rates compared to PES nanocomposite membranes containing GO, oCNT and oCNT/GO mixtures. This is due to the better dispersion of PuCNT(3) in the PES film. PuCNT(3) can be used as a nanofiller for polymeric nanocomposite membranes in water treatment applications.

So far, specific examples of the present invention have been looked at. Those of ordinary skill in the art to which the present invention pertains will be able to understand that the present invention may be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments should be considered from an illustrative point of view rather than a limiting point of view. The scope of the present invention is shown in the claims rather than the above description, and all differences within the scope equivalent thereto should be construed as being included in the present invention.

Claims (11)

Polymer matrix; And
It is dispersed in the polymer matrix and includes carbon nanotubes (PuCNT) having a partially open structure,
The PuCNT is a filtration membrane, characterized in that having an oxidizing group. delete The method of claim 1,
The weight ratio of the PuCNT to the polymer is a filtration membrane, characterized in that 0.01:100 ~ 0.5:100. The method of claim 1,
The polymer is a filtration membrane, characterized in that containing PES (polyethersulfone). The method of claim 1,
The filtration membrane is a filtration membrane, characterized in that the ultrafiltration membrane. Forming a carbon nanotube (PuCNT) having a partially open structure;
Mixing the PuCNT with a polymer and a first solvent to form a mixed solution; And
Including the step of casting the mixed solution,
The step of forming the PuCNT,
Including the step of mixing the carbon nanotubes, an oxidizing agent, and a second solvent,
The carbon nanotubes are oxidized by the oxidizing agent to have an oxidizing group. delete The method of claim 6,
The method of manufacturing a filtration membrane, characterized in that the oxidizing agent contains KMnO _4. The method of claim 8,
The oxidizing agent is a method of manufacturing a filtration membrane, characterized in that added in the amount of 1 to 5 equivalents based on the weight of the carbon nanotubes. The method of claim 6,
The method of manufacturing a filtration membrane, characterized in that the weight ratio of the PuCNT to the polymer is 0.01:100 to 0.5:100. The method of claim 10,
The method of manufacturing a filtration membrane, characterized in that the polymer comprises PES (polyethersulfone).

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