A negatively charged carboxylated carbon nanotube modified ultrafiltration membrane, its preparation method and application

Abstract

A negatively charged carboxylated carbon nanotube-modified ultrafiltration membrane: Carboxylated carbon nanotubes are placed in an aqueous solution of FeCl3·6H2O and stirred until homogeneous to obtain a mixed solution of carboxylated carbon nanotubes and hydrated iron oxide; KOH solution is added to the mixed solution of carboxylated carbon nanotubes and hydrated iron oxide, the pH is increased to 7, and then treated in a water bath. The precipitate is washed until neutral and no chloride ions are detected; after aspiration filtration, the precipitate is dried to obtain carboxylated carbon nanotube particles loaded with HFO; the HFO-loaded carboxylated carbon nanotube particles are dispersed in a diluent and ultrasonically treated; polymer particles and a pore-forming agent are added to the diluent sequentially, and the mixture is ultrasonically mixed and stirred until homogeneous. After standing to degas and cool, a casting solution is obtained; the casting solution is scraped onto a glass plate and placed in pure water to undergo non-solvent-induced phase separation. After phase inversion, a negatively charged carboxylated carbon nanotube-modified ultrafiltration membrane is obtained. This invention also provides a preparation method and applications.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, specifically relating to a negatively charged carboxylated carbon nanotube modified ultrafiltration membrane.

[0002] The present invention also relates to a method for preparing the above-mentioned negatively charged carboxylated carbon nanotube modified ultrafiltration membrane.

[0003] This invention also relates to the application of the above-mentioned negatively charged carboxylated carbon nanotube modified ultrafiltration membrane in the extraction of humic acid from kitchen waste biogas slurry. Background Technology

[0004] With the rapid pace of urbanization, the amount of food waste is increasing year by year. Food waste contains a large amount of water, carbohydrates, proteins, lipids, and other organic matter, and has a high total solids (TS) and volatile solids (VS) content, possessing significant resource recovery potential. Its primary treatment method is anaerobic digestion. The biogas slurry produced after food waste treatment is a high-concentration organic wastewater generated during the anaerobic digestion of organic waste. It is characterized by a wide variety and high concentration of organic pollutants, a diverse range of inorganic ions, an imbalanced nutrient ratio, poor biodegradability, and significant treatment difficulty. The organic matter in the biogas slurry is mainly recalcitrant humic acid, which is the main reason for its difficulty in treatment. The disposal and operation costs account for 60%-80% of the direct operating costs of food waste disposal.

[0005] Humic acids (HA) are a class of naturally occurring high-molecular-weight organic compounds widely found in nature. They can stimulate crop growth, enhance crop resistance to adverse conditions, and improve product quality, finding wide applications in industry, agriculture, medicine, and environmental protection. Therefore, extracting humic acids from kitchen waste biogas slurry can not only reduce the disposal cost of biogas slurry but also recover valuable humic acid products, achieving the harmlessness and resource utilization of biogas slurry.

[0006] Currently, methods for extracting humic acid mainly include alkali dissolution and acid precipitation, resin adsorption, and ultrafiltration membrane extraction. In recent years, ultrafiltration membrane technology has been widely used in organic matter extraction due to its advantages such as simple process, low energy consumption, no need for additional reagents, reliable operation, compact equipment, and easy automation. However, commonly used ultrafiltration membrane technologies, including nanofiltration (NF) and reverse osmosis (RO), have poor permeability and high operating pressures. Furthermore, ultrafiltration membranes have a relatively large molecular weight cutoff, resulting in low humic acid extraction rates.

[0007] Therefore, developing a composite ultrafiltration membrane with high stability, high membrane flux, and high extraction rate of humic acid is particularly important. This composite ultrafiltration membrane will provide an effective technical means for extracting humic acid from kitchen waste biogas slurry and realizing its resource utilization, and is expected to promote the development of the biogas slurry treatment field and facilitate its industrialization and commercialization. Summary of the Invention

[0008] Based on the problems existing in the prior art, the purpose of this invention is to provide a negatively charged carboxylated carbon nanotube modified ultrafiltration membrane.

[0009] Another object of the present invention is to provide a method for preparing the above-mentioned negatively charged carboxylated carbon nanotube modified ultrafiltration membrane.

[0010] To achieve the above objectives, the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane provided by the present invention is obtained by the following method:

[0011] (1) Carboxylated carbon nanotubes were placed in FeCl3·6H2O aqueous solution, stirred evenly, ultrasonically treated, and then transferred to a water bath to obtain a mixed solution of carboxylated carbon nanotubes and hydrated iron oxide.

[0012] (2) Add KOH solution to the carboxylated carbon nanotube-hydrated iron oxide mixed solution prepared in step (1), stir at a constant rate, increase pH to 7, and then treat with water bath. Wash the resulting precipitate until it is neutral and no chloride ions are detected. After aspiration filtration, the precipitate is dried to obtain carboxylated carbon nanotube particles loaded with hydrated iron oxide (HFO).

[0013] (3) The carboxylated carbon nanotube particles prepared in step (2) are dispersed into the diluent and ultrasonically treated. The polymer particles and pore-forming agent are added to the diluent in sequence, and the mixture is ultrasonically stirred until the solution is uniform. After standing to degas and cool, the casting solution is obtained.

[0014] (4) The casting solution is scraped onto a glass plate and placed in pure water to undergo non-solvent-induced phase separation. After phase transformation, a negatively charged carboxylated carbon nanotube modified ultrafiltration membrane is obtained.

[0015] The method for preparing the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane provided by this invention is as follows:

[0016] (1) Carboxylated carbon nanotubes were placed in FeCl3·6H2O aqueous solution, stirred evenly, ultrasonically treated, and then transferred to a water bath to obtain a mixed solution of carboxylated carbon nanotubes and hydrated iron oxide.

[0017] (2) Add KOH solution to the carboxylated carbon nanotube-hydrated iron oxide mixed solution prepared in step (1), stir at a constant rate, increase pH to 7, and then treat with water bath. Wash the resulting precipitate until neutral and no chloride ions are detected. After aspiration filtration, the precipitate is dried to obtain HFO-loaded carboxylated carbon nanotube particles.

[0018] (3) The carboxylated carbon nanotube particles prepared in step (2) are dispersed into the diluent and ultrasonically treated. The polymer particles and pore-forming agent are added to the diluent in sequence, and the mixture is ultrasonically stirred until the solution is uniform. After standing to degas and cool, the casting solution is obtained.

[0019] (4) The casting solution is scraped onto a glass plate and placed in pure water to undergo non-solvent-induced phase separation. After phase transformation, a negatively charged carboxylated carbon nanotube modified ultrafiltration membrane is obtained.

[0020] The method wherein step (1) is a 60°C water bath treatment.

[0021] The method wherein the carbon nanotubes in step (1) are one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0022] The method wherein the solubility of the FeCl3·6H2O aqueous solution in step (1) is 0.1 mol / L.

[0023] The method wherein step (2) is a 60°C water bath treatment.

[0024] The method wherein the concentration of the KOH solution in step (2) is 1 mol / L.

[0025] The method wherein step (2) involves ultrasonic mixing at 50°C.

[0026] The raw material components in the casting solution are as follows by mass percentage: 20% polymer particles, 0.2-0.5% carboxylated carbon nanotube particles, 2% pore-forming agent, and the remainder is diluent.

[0027] The polymer particles are 14 wt% polysulfone (PSF).

[0028] The diluent is N-methylpyrrolidone (NMP).

[0029] The pore-forming agent is 4 wt% polyvinylpyrrolidone (PVP).

[0030] The negatively charged carboxylated carbon nanotube modified ultrafiltration membrane of the present invention can be used to extract humic acid from kitchen waste biogas slurry.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] (1) This invention synthesizes carboxylated carbon nanotube-HFO nanocomposite material by loading hydrated iron oxide (HFO) onto carboxylated carbon nanotubes and dispersing it into polysulfone membrane material, so that the surface of the ultrafiltration membrane is negatively charged. Humic acid in biogas slurry mainly manifests as negatively charged macromolecular organic matter. By utilizing the electrostatic repulsion between humic acid in biogas slurry and membrane, the retention rate of humic acid is greatly improved, thereby realizing the enrichment, concentration and extraction of humic acid in biogas slurry of kitchen waste, and realizing the resource utilization of biogas slurry.

[0033] (2) The carboxylated carbon nanotubes in the ultrafiltration membrane of the present invention provide a large number of nanochannels, which have strong hydrophilicity and greatly improve the water flux and mechanical strength of the membrane.

[0034] (3) The process of this invention is simple, does not require the addition of reagents, does not produce secondary pollution, is easy to automatically control, can extract humic acid from kitchen waste biogas slurry to produce water-soluble fertilizer, can reduce membrane pollution, extend the service life of the membrane, reduce costs, and has good economic and environmental benefits. Attached Figure Description

[0035] Figure 1 These are photographs of the negatively charged modified ultrafiltration membranes in the embodiments of the present invention;

[0036] Figure 2 These are scanning electron microscope (SEM) images of the surfaces of unloaded and HFO-loaded carbon nanotube particles, where... Figure 2 b is a scanning electron microscope image of the surface of unloaded HFO carbon nanotube particles. Figure 2 d is a scanning electron microscope image of the surface of the HFO-loaded carbon nanotube particles;

[0037] Figure 3 These are scanning electron microscope images of the surface of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3, where a is Example 1, b is Example 2, and c is Example 3.

[0038] Figure 4 These are the Zeta potential diagrams of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3;

[0039] Figure 5 The flux changes of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3 and the polysulfone ultrafiltration membranes of Comparative Examples 1-2 when passing through the concentrated liquid of kitchen waste biogas slurry.

[0040] Figure 6 The water contact angles of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3 and the polysulfone ultrafiltration membranes of Comparative Examples 1-2 are shown.

[0041] Figure 7 The retention rates of humic acid in kitchen waste biogas slurry are compared between the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3 and the polysulfone ultrafiltration membranes of Comparative Examples 1-2. Detailed Implementation

[0042] The negatively charged carboxylated carbon nanotube modified ultrafiltration membrane of this invention can solve the problems of poor permeability to humic acid, high operating pressure, and low retention rate of conventional ultrafiltration membranes, achieving efficient extraction of humic acid from kitchen waste biogas slurry. It is simple, convenient, and suitable for large-scale production. Specifically, the technical solution adopted in this invention is:

[0043] (1) Carboxylated carbon nanotubes are placed in an aqueous solution of FeCl3·6H2O, stirred until homogeneous, ultrasonically treated, and then subjected to a 60°C water bath to obtain a mixed solution of carboxylated carbon nanotubes and hydrated iron oxide. The carbon nanotubes of this invention are one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. In one embodiment, the solubility of the FeCl3·6H2O aqueous solution of this invention is 0.1 mol / L.

[0044] (2) Add the KOH solution to the mixed solution prepared in step (1), stir at a constant rate, increase the pH to 7, and then treat in a 60°C water bath for 3 hours. Wash the resulting precipitate with deionized water until neutral, and no chloride ions are detected. After aspiration filtration, dry the precipitate at 100°C for 3 hours to obtain HFO-loaded carboxylated carbon nanotube particles. In one embodiment, the concentration of the KOH solution in this invention is 1 mol / L.

[0045] (3) The negatively charged carboxylated carbon nanotube particles prepared in step (2) are dispersed into a diluent and ultrasonically treated at 40 kHz for 1 h. Then, the polymer particles and pore-forming agent are added to the diluent in sequence, and the mixture is ultrasonically stirred at 50 °C until the solution is homogeneous. After standing to remove bubbles and cooling, the casting solution is obtained. The raw material components in the casting solution of the present invention are as follows by mass percentage: polymer particles 20%, carboxylated carbon nanotube particles 0.2-0.5%, pore-forming agent 2%, and the remainder is diluent. The polymer particles of the present invention are 14 wt% polysulfone (PSF), the diluent is N-methylpyrrolidone (NMP), and the pore-forming agent is 4 wt% polyvinylpyrrolidone (PVP).

[0046] (4) The casting solution is scraped onto a clean glass plate by a scraper and placed in pure water to undergo non-solvent-induced phase separation. After phase transformation, a negatively charged carboxylated carbon nanotube modified ultrafiltration membrane is obtained.

[0047] The present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0048] Example 1

[0049] (1) Carboxylated carbon nanotubes were placed in 0.1 mol / L FeCl3·6H2O aqueous solution, stirred evenly, and ultrasonically treated for 1 h before being transferred to a 60℃ water bath to obtain a mixed solution of carboxylated carbon nanotubes and hydrated iron oxide.

[0050] (2) Add 1 mol / L KOH solution to the mixed solution prepared in step (1), stir at a constant rate, increase the pH to 7, and then treat in a 60℃ water bath for 3 h. Wash the resulting precipitate with deionized water until neutral, and detect no chloride ions. After aspiration filtration, dry the precipitate at 100℃ for 3 h to obtain HFO-loaded carboxylated carbon nanotube particles.

[0051] (3) 0.3% HFO-loaded carboxylated carbon nanotubes were dispersed into 77.7% N-methylpyrrolidone and sonicated at 40 kHz for 1 h. Then, 20% 14 wt% polysulfone (PSF) and 2% 4 wt% polyvinylpyrrolidone were added sequentially to the N-methylpyrrolidone mixture, and the mixture was stirred continuously at 50 °C for 24 h. The polymer mixture was sonicated again for 30 min and left at room temperature to remove trapped bubbles, thus obtaining the casting solution.

[0052] (4) The casting solution was scraped onto a clean glass plate using a film scraper and placed in pure water to undergo non-solvent-induced phase separation. After phase inversion, a negatively charged carboxylated carbon nanotube modified ultrafiltration membrane was obtained, denoted as S0. The surface scanning electron microscope image of the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane in this embodiment is shown below. Figure 3 a.

[0053] Example 2

[0054] The difference between this embodiment and Embodiment 1 is that the mass of the carboxylated carbon nanotube particles loaded with HFO in step (3) is 0.4% of the mass of the casting solution. Everything else is the same as in Embodiment 1. The resulting negatively charged carboxylated carbon nanotube modified ultrafiltration membrane is denoted as S1. The surface scanning electron microscope image of the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane in this embodiment is shown below. Figure 3 b.

[0055] Example 3

[0056] The difference between this embodiment and Embodiment 1 is that the mass of the carboxylated carbon nanotube particles loaded with HFO in step (3) is 0.5% of the mass of the casting solution. Everything else is the same as in Embodiment 1. The resulting negatively charged carboxylated carbon nanotube modified ultrafiltration membrane is denoted as S2. The surface scanning electron microscope image of the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane in this embodiment is shown below. Figure 3 c.

[0057] Physical photographs of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes prepared in Examples 1-3 of this invention are shown below. Figure 1 .

[0058] The Zeta potential diagrams of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes prepared in Examples 1-3 of this invention are shown below. Figure 4 .

[0059] Comparative Example 1

[0060] The difference between this comparative example and Example 1 is that no carboxylated multi-walled carbon nanotubes are added, and steps (1) and (2) are omitted. Everything else is the same as in Example 1, and a polysulfone ultrafiltration membrane is finally obtained, which is denoted as S3.

[0061] Comparative Example 2

[0062] The difference between this comparative example and Example 1 is that HFO is not loaded onto carboxylated multi-walled carbon nanotubes, and step (2) is omitted. Everything else is the same as in Example 1, and a polysulfone ultrafiltration membrane is finally obtained, which is denoted as S4.

[0063] The ultrafiltration membranes of the above embodiments and comparative examples were subjected to performance testing and characterization:

[0064] The supernatant (HA concentration 1 g / L) from the solid-liquid separation of kitchen waste biogas slurry was used as the contaminant. The antifouling properties of the modified ultrafiltration membrane were investigated using a dynamic fouling test and a membrane performance evaluation instrument. After the membrane was embedded in the instrument, the pressure was adjusted to 0.1 MPa, and filtration was performed for 0.5 h using deionized water. The water flux was recorded every 3 min during this process. Then, the deionized water was replaced with HA solution, and a filtration experiment was performed for 0.5 h at 0.1 MPa pressure, with the flux recorded every 3 min during this process. The ultrafiltration membrane was then removed, and the surface of the membrane was rinsed with deionized water for 10 minutes to remove contaminants. A filtration experiment was then conducted again with deionized water for 30 minutes over 0.5 hours. Flux was recorded every 3 minutes during this process. The pure water flux and HA flux of the membranes in Examples 1-3 and Comparative Examples 1-2 were finally obtained. After the membrane was embedded in the instrument, the pressure was adjusted to 0.1 MPa. Pre-pressurization was performed at room temperature for 10 minutes to stabilize the pressure and flow rate. The contaminant was then passed through for another 10 minutes, and the filtrate was collected. The HA rejection rates of the membranes in Examples 1-3 and Comparative Examples 1-2 were finally obtained. Specific data are shown in Table 1.

[0065] Figure 5 The flux changes of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3 and the polysulfone ultrafiltration membranes of Comparative Examples 1-2 when passing through the concentrated liquid of kitchen waste biogas slurry membrane.

[0066] Figure 6 The water contact angles are those of the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3 and the polysulfone ultrafiltration membranes of Comparative Examples 1-2.

[0067] Figure 7 The retention rates of humic acid in kitchen waste biogas slurry are compared between the negatively charged carboxylated carbon nanotube modified ultrafiltration membranes of Examples 1-3 and the polysulfone ultrafiltration membranes of Comparative Examples 1-2.

[0068] As shown in the accompanying drawings and Table 1, the negatively charged modified ultrafiltration membrane of the present invention exhibits excellent flux and HA rejection rate. Example 3 shows the best results, with a water flux increased by 65.22% and HA rejection rate increased by 16.47% compared to the undoped carboxylated carbon nanotube modified ultrafiltration membrane. Furthermore, it exhibits higher hydrophilicity compared to ultrafiltration membranes without added carbon nanotubes or without modification.

[0069] pass Figure 2 b and Figure 2The comparison shows that loading HFO can reduce the aggregation of carboxylated carbon nanotubes, improve the dispersibility of carboxylated carbon nanotubes, and enhance the dispersion and uniformity of carboxylated carbon nanotube particles in negatively charged modified ultrafiltration membranes.

[0070] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

[0071] Table 1. Pure water flux, HA rejection rate, and hydrophilicity of the ultrafiltration membranes in the examples and comparative examples.

[0072]

Claims

1. A negatively charged carboxylated carbon nanotube modified ultrafiltration membrane, which is prepared by the following method: (1) placing carboxylated carbon nanotubes into an aqueous solution of FeCl3·6H2O, stirring uniformly, and then treating in a water bath after ultrasonic treatment to obtain a carboxylated carbon nanotube-hydrated iron oxide mixed solution; (2) adding an aqueous KOH solution to the carboxylated carbon nanotube-hydrated iron oxide mixed solution prepared in step (1), stirring at a constant rate, and then treating in a water bath after the pH is increased to 7, washing the obtained precipitate to neutral, and detecting no chloride ions; after suction filtration, drying the precipitate to obtain carboxylated carbon nanotube particles loaded with HFO; (3) dispersing the carboxylated carbon nanotube particles prepared in step (2) into a diluent, ultrasonic treatment, and then sequentially adding polymer particles and a pore-forming agent to the diluent, ultrasonic mixing and stirring until the solution is uniform, and then standing, degassing, and cooling to obtain a casting solution; (4) coating the casting solution on a glass plate, and then performing non-solvent induced phase separation in pure water to obtain the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane after phase inversion.

2. A method for preparing the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane of claim 1: (1) placing carboxylated carbon nanotubes into an aqueous solution of FeCl3·6H2O, stirring uniformly, and then treating in a water bath after ultrasonic treatment to obtain a carboxylated carbon nanotube-hydrated iron oxide mixed solution; (2) adding an aqueous KOH solution to the carboxylated carbon nanotube-hydrated iron oxide mixed solution prepared in step (1), stirring at a constant rate, and then treating in a water bath after the pH is increased to 7, washing the obtained precipitate to neutral, and detecting no chloride ions; after suction filtration, drying the precipitate to obtain carboxylated carbon nanotube particles loaded with HFO; (3) dispersing the carboxylated carbon nanotube particles prepared in step (2) into a diluent, ultrasonic treatment, and then sequentially adding polymer particles and a pore-forming agent to the diluent, ultrasonic mixing and stirring until the solution is uniform, and then standing, degassing, and cooling to obtain a casting solution; (4) coating the casting solution on a glass plate, and then performing non-solvent induced phase separation in pure water to obtain the negatively charged carboxylated carbon nanotube modified ultrafiltration membrane after phase inversion.

3. The method of claim 2, wherein, Step (1) is 60℃ water bath treatment.

4. The method of claim 2, wherein, The carbon nanotubes in step (1) are one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

5. The method of claim 2, wherein, The aqueous solution of FeCl3·6H2O in step (1) has a solubility of 0.1 mol / L.

6. The method of claim 2, wherein, Step (2) is 60℃ water bath treatment.

7. The method of claim 2, wherein, The aqueous KOH solution in step (2) has a concentration of 1 mol / L.

8. The method of claim 2, wherein, Step (2) is ultrasonic mixing and stirring at 50℃. The mass percentages of the components in the casting solution are as follows: 20% of polymer particles, 0.2-0.5% of carboxylated carbon nanotube particles, 2% of a pore-forming agent, and the rest of a diluent. The polymer particles are 14wt% polysulfone. The diluent is N-methyl pyrrolidone. The pore-forming agent is 4wt% polyvinylpyrrolidone.

9. The negatively charged carboxylated carbon nanotube modified ultrafiltration membrane of claim 1 is used for extracting humic acid from kitchen garbage biogas liquid.

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