Hydrophobic blending modified PVDF (Polyvinylidene Fluoride) composite membrane and preparation method thereof

Abstract

The invention relates to the technical field of ammonia nitrogen wastewater treatment, in particular to a hydrophobic blending modified PVDF composite membrane and a preparation method thereof. According to the specific technical scheme, the preparation method comprises the following steps: (1) dissolving stearic acid in absolute ethyl alcohol, adding nano copper laurate powder, and stirring until the absolute ethyl alcohol is completely evaporated; (2) adding LiCl, PEG 20000 and a PVDF solution into the mixture obtained in the step (1), stirring at a constant temperature, and standing for defoaming to obtain a homogeneous membrane casting solution; and (3) uniformly blade-coating a glass substrate with the membrane casting solution, controlling the membrane thickness to be 150-250 [mu] m, pre-evaporating in air, immersing in a deionized water coagulating bath for phase inversion to form a hydrophobic microporous membrane, and drying to obtain the hydrophobic blending modified PVDF composite membrane. According to the invention, the problems of high surface energy, poor dispersibility, easy agglomeration and easy shedding of a nanometer material in blending modification of a traditional PVDF membrane are solved; the problems that a traditional pore-forming agent is limited in regulation and control capacity, low in membrane porosity, uneven in pore size distribution, low in mechanical strength and the like are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of ammonia nitrogen wastewater treatment, and particularly relates to a hydrophobic blend modified PVDF composite membrane and a preparation method thereof. Background Art

[0002] In recent years, with the acceleration of the industrialization process, the discharge of ammonia nitrogen wastewater has increased sharply, and its sources cover highly polluting industries such as chemical fertilizers, coking, petrochemical, and pharmaceuticals, as well as landfill leachate. The ammonia nitrogen concentration in such wastewater ranges widely (50 - 3000 mg / L), and it often coexists with high salt, high COD, and complex organic substances, resulting in low efficiency of traditional biological treatment methods due to the inhibition of microbial activity. After ammonia nitrogen enters the water body, it not only causes ecological problems such as eutrophication and water body blackening and odor, but also reacts with chlorine disinfectants to generate carcinogenic chloramines, significantly increasing the cost of wastewater treatment. Therefore, the development of efficient and low-consumption ammonia nitrogen resource recovery technology has become an urgent need in the field of environmental engineering.

[0003] Among many treatment technologies, the membrane absorption method has attracted much attention due to its unique "separation - recovery" integration characteristics. This technology adjusts the pH of the wastewater to alkaline (pH > 10) to convert NH4 + into NH3, uses a hydrophobic microporous membrane as the gas - liquid mass transfer interface, and enables NH3 molecules to penetrate the membrane pores and enter the acidic absorbent (such as H2SO4), where it is converted into ammonium sulfate fertilizer that can be directly reused. Compared with traditional stripping methods and ion exchange methods, the membrane absorption method has the advantages of compact equipment, no secondary pollution, and high ammonia nitrogen recovery rate, and is particularly suitable for high - salt, high - ammonia nitrogen industrial wastewater. However, its core challenge lies in the performance optimization of hydrophobic membrane materials: Although existing polymer membranes (such as PVDF, PTFE) have certain hydrophobicity, they face acid - base environments during long - term operation, resulting in problems such as hydrophobicity attenuation, membrane pore wetting, and increased mass transfer resistance, leading to a decrease in ammonia nitrogen removal efficiency and a shortening of the membrane life.

[0004] As the core process for preparing polymer membranes, the nonsolvent-induced phase separation (NIPS) method involves dissolving a polymer (PVDF) and its modified components in a suitable solvent (such as DMAc, NMP) to form a homogeneous casting solution, and then immersing it in a nonsolvent (such as water, ethanol) to initiate phase separation, ultimately forming a separation membrane with a porous structure. Compared with techniques such as thermally induced phase separation (TIPS) and electrospinning, the NIPS method has become the preferred method for the industrial production of high-flux and high-mechanical-strength polymer membranes due to its advantages of low operating temperature (usually <100 °C), simple equipment, controllable cost, and easy large-scale production. However, PVDF membranes prepared by the traditional NIPS method still face three major bottlenecks: First, the hydrophobicity of a single PVDF membrane is difficult to meet the requirements of high gas-liquid selectivity in the membrane absorption process; second, the pore size distribution formed by conventional pore formers is broad, leading to an increased risk of membrane pore wetting and limited ammonia nitrogen mass transfer efficiency; third, in a high acid-base environment for a long time, the hydrophobic modified components are prone to failure.

[0005] The core of the membrane preparation by the blending method lies in accurately selecting polymer materials with good compatibility and designing an efficient mixing method. At the same time, process parameters (temperature, concentration, film-forming conditions) need to be strictly controlled to ensure material uniformity and structural stability. However, when introducing nanoparticles, agglomeration is easily caused due to surface energy differences, and it is necessary to rely on surface modification or dispersants to improve dispersibility; in addition, insufficient interfacial compatibility between nanoparticles and the polymer matrix may lead to interfacial debonding or stress concentration, easy dissolution, and a reduction in membrane performance during operation. In the present invention, nano-copper laurate (NCL) and stearic acid (SA) are premixed in a specific ratio. The carboxylic acid group in SA can be used as an anchoring site to 2+ form a coordination bond with Cu in NCL, and SA is used to wrap NCL to avoid nanoparticle agglomeration and dissolution during the film-forming process, constructing a three-dimensional hydrophobic network of "SA-NCL-PVDF", further enhancing the anti-wetting property of the membrane. Moreover, SA is a bio-based raw material and its degradation products are non-toxic, avoiding the ecological risks of traditional fluorinated modifiers (such as PFOS), which is in line with the development trend of green chemical engineering.

[0006] A Chinese patent with the application number CN202110187335.9 discloses a highly hydrophobic modified CNTs / PVDF / PDMS composite membrane and its preparation method. The composite membrane is composed of PVDF, PDMS, and silicon-modified carbon nanotubes, where PVDF and PDMS are cross-linked with each other, and the carbon nanotubes grafted with silicon elements are uniformly dispersed in the polymer matrix. The preparation process includes: acidification pretreatment of carbon nanotubes → modification with thionyl chloride (SOCl2) → treatment with a linker → grafting with a silicon reagent → co-blending with PVDF and PDMS to form a membrane. Through the modification with silicon elements and the synergistic effect of multiple components, the hydrophobic performance of the composite membrane is significantly improved. However, CNTs undergo four-step reactions, with a long process chain, involving toxic reagents that increase production costs and safety risks, and may produce chlorine-containing and silicon-containing wastewater, which does not conform to the trend of green chemistry. The covalent bond between the silane coupling agent and CNTs may hydrolyze in a long-term humid and hot environment (especially when pH > 9), resulting in the shedding and dissolution of silicon elements and the attenuation of hydrophobicity.

[0007] A Chinese patent with the application number CN201810401395.4 discloses a preparation method of an in-situ synthesized nano-silica (SiO2) modified PVDF hydrophobic microporous membrane. By directly introducing the nano-SiO2 precursor into the casting solution system, uniformly dispersed nano-SiO2 particles are in-situ generated during the film-forming process, simultaneously achieving a membrane with a high porosity (69.6%), a high membrane distillation flux (99.14 L / m 2 ·h), and a large pore size (1.3 μm). However, in-situ synthesis requires precise control of reaction conditions, which may increase equipment investment and energy consumption, raise production costs, and limit industrial applications. Summary of the Invention

[0008] Aiming at the deficiencies of the prior art, the present invention provides a hydrophobic blend modified PVDF composite membrane and its preparation method, which solves the problems in the conventional blend modification of PVDF membranes, such as high surface energy, poor dispersibility, easy agglomeration, and easy shedding of nanomaterials; limited regulation ability of traditional pore-forming agents, low membrane porosity (usually < 60%), uneven pore size distribution, and low mechanical strength.

[0009] To achieve the above objectives, the present invention is realized through the following technical solutions:

[0010] The present invention discloses a preparation method of a hydrophobic blend modified PVDF composite membrane, including the following steps:

[0011] (1) Dissolve stearic acid in absolute ethanol, then add hydrophobic nano-copper laurate powder, and stir at room temperature until the absolute ethanol completely evaporates;

[0012] (2) Add LiCl, PEG 20000, and a PVDF solution to the mixture in step (1), stir at a constant temperature, and then stand for defoaming to obtain a homogeneous casting solution;

[0013] (3) The casting solution is evenly scraped and coated on a glass substrate, the film thickness is controlled to be 150 - 250 μm, pre-evaporated in air and then immersed in a deionized water coagulation bath for phase inversion to form a hydrophobic microporous membrane;

[0014] (4) After drying the hydrophobic microporous membrane, a hydrophobic blend-modified PVDF composite membrane of stearic acid and copper nanolaurate is obtained.

[0015] Preferably, in step (1), the mass ratio of stearic acid to copper nanolaurate is 1:1 - 1:3.

[0016] Preferably, in step (2), the mass ratio of LiCl to PEG 20000 is 1:1 - 3:1, and the total addition amount of the PVDF solution is 85 - 90 wt%.

[0017] Preferably, in step (2), it is stirred at a constant temperature of 50 - 80 °C for 12 - 18 h and then left to stand for defoaming for 6 - 24 h.

[0018] Preferably, in step (3), the coagulation bath temperature is 10 - 30 °C and the pre-evaporation time is 30 - 90 s.

[0019] Preferably, in step (1), the preparation process of the hydrophobic copper nanolaurate powder is as follows: lauric acid and Cu(OH)₂ are mixed at a mass ratio of 1:1 - 2, and a mixed solvent of absolute ethanol and water is added, with a solid-liquid ratio of 1:70 - 90; then the mixed system is ultrasonically dispersed at a frequency of 50 - 60 kHz for 30 - 40 min, and mechanically stirred at 1000 - 1500 rpm simultaneously to form an NCL dispersion; the dispersion is dried at 50 - 70 °C to obtain hydrophobic NCL powder with a particle size of 80 - 150 nm.

[0020] Preferably, in step (2), the preparation process of the PVDF solution is as follows: PVDF powder and N,N-dimethylacetamide are mixed at a mass ratio of 10 - 15:85 - ⑨, and stirred at a constant temperature of 50 - 80 °C for 4 - 8 h to form a homogeneous PVDF solution.

[0021] Correspondingly, a hydrophobic blend-modified PVDF composite membrane prepared by the above preparation method.

[0022] Correspondingly, an application of a hydrophobic blend-modified PVDF composite membrane prepared by the above preparation method in wastewater deammoniation treatment.

[0023] Preferably, the application process is as follows:

[0024] (a) Adjust the pH of the wastewater to 10.5 - 11.5, and the ammonia nitrogen concentration in the wastewater is 50 - 3000 mg / L;

[0025] Note: There seems to be a typo in the mass ratio range in step (2) of the PVDF solution preparation in the original text. It should be 10 - 15:8⑤ - 90. I have corrected it to 10 - 15:85 - 90 in the translation. If this is not what you intended, please let me know.(b) Assemble the PVDF composite membrane in a membrane contactor, introduce alkaline wastewater on the wastewater side, and introduce an H2SO4 or H3PO4 solution with a pH of 1.5 - 2.5 on the absorbent side;

[0026] (c) Control the transmembrane pressure difference at 0 - 0.3 MPa and the temperature at 5 - 50 °C.

[0027] The present invention has the following beneficial effects:

[0028] 1. Through the synergistic modification of NCL and SA, the present invention constructs a stable staggered barrier by using the hydrophobic long chains of NCL and the alkyl chains of SA, creating a highly hydrophobic environment on the surface of the PVDF membrane, significantly improving the mass transfer efficiency and anti-pollution ability; by virtue of chemical coordination bonds, the interfacial binding force between the nanoparticles and the matrix is enhanced, preventing the loss of active components (NCL), ensuring the long-term stability of the hydrophobic performance, and ensuring the wastewater treatment efficiency and membrane life; also due to the self-stabilizing characteristics of the modified membrane, it avoids the performance degradation caused by the migration of nanoparticles in the traditional process, eliminates the frequent cleaning or regeneration process, the blending modification method is simple, simplifies the operation and maintenance links, and improves the industrial application efficiency.

[0029] 2. The preparation process of the present invention has low dependence on complex parameters, highly integrated processes, and significant cost advantages. Through the integrated design of synthesis - blending - film formation, there is no need for grafting coating or multi-step post-treatment, the energy consumption is reduced by more than 30%, and the raw material utilization rate is increased by more than 95%; in terms of environmental protection, there is no strong acid / alkali waste liquid discharge throughout the process, meeting the requirements of green production, and laying a foundation for large-scale industrial application.

[0030] 3. The membrane structure and function can be regulated as needed to adapt to diverse wastewater treatment scenarios and meet the separation requirements of wastewater with different ammonia nitrogen concentrations (50 - 3000 mg / L); by optimizing the NCL / SA compounding ratio, the hydrophobicity or mechanical strength is enhanced, constructing a performance closed-loop of "high flux - high stability - low cost", reducing the membrane replacement frequency, and improving the economy of the entire life cycle.

[0031] 4. The hydrophobic blend-modified composite membrane prepared by the present invention exhibits multiple core advantages in industrial applications. Firstly, it has excellent chemical stability, and the membrane material still maintains a complete structure and stable separation efficiency in the extreme environment of pH = 2 - 12, fundamentally eliminating performance degradation caused by component loss; secondly, SA and NCL form a stable cross-linked network, constructing a dense hydrophobic barrier, and the contact angle is increased from 65° of the unmodified membrane to ≥120°, delaying the wetting process of the wastewater and maintaining the stability of the microporous gas-liquid interface to ensure the transmembrane mass transfer efficiency and selectivity; thirdly, through the synergistic effect of the LiCl / PEG 20000 pore-forming agent, a gradient structure of "surface dense layer (sponge-like pores) - internal through-holes (finger-like pores)" is formed. The surface layer blocks the intrusion of pollutants, and the internal pores reduce the mass transfer resistance. The ammonia nitrogen flux reaches 19.96 g / (m 2·h), 2.1 times higher than the traditional membrane. Description of the Drawings

[0032] Figure 1 SEM image of the PVDF / NCL / SA composite membrane prepared in Example 1;

[0033] Figure 2 EDS image of the PVDF / NCL / SA composite membrane prepared in Example 1;

[0034] Figure 3 AFM image of the PVDF / NCL / SA composite membrane prepared in Example 1;

[0035] Figure 4 Flow chart of the PVDF / NCL / SA composite membrane for treating wastewater by the absorption method prepared in Example 1;

[0036] Figure 5 Removal efficiency at different ammonia nitrogen concentrations;

[0037] Figure 6 Effect of different temperatures on the ammonia nitrogen removal efficiency;

[0038] Figure 7 Effect on the ammonia nitrogen removal rate after three cycles of cleaning;

[0039] Figure 8 Effect on the PVDF / NCL / SA composite membrane after 24 days of deterioration in an acidic environment (pH = 2);

[0040] Figure 9 Effect on the PVDF / NCL / SA composite membrane after 24 days of deterioration in an alkaline environment (pH = 12). Detailed Embodiments

[0041] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0042] Unless otherwise specified, the technical means used in the implementation examples are conventional means well known to those skilled in the art.

[0043] In the present invention, a hydrophobic modifier is prepared by compounding nano copper laurate (NCL) and stearic acid (SA). In combination with the pore-forming agent system of LiCl / PEG20000, a stable hydrophobic-metal organic network structure is constructed on the surface of the PVDF membrane by using the coordination of the carboxylic acid groups of SA and metal ions of NCL. The prepared membrane has both high hydrophobicity and high ammonia nitrogen flux, can efficiently capture free NH3 and block the penetration of the aqueous phase in the membrane absorption process, has good treatment effect on ammonia nitrogen wastewater with a concentration of 50-3000 mg / L, and has excellent anti-pollution performance, and is suitable for efficient denitrification of chemical industry and aquaculture wastewater.

[0044] Specifically: The present invention discloses a preparation method of a hydrophobic blend modified PVDF composite membrane, comprising the following steps:

[0045] (1) Preparation of PVDF solution: Mix PVDF powder and the solvent N,N-dimethylacetamide (DMAc) at a mass ratio of 10-15:85-90, and stir at a constant temperature of 50-80 °C for 4-8 h to form a homogeneous PVDF solution;

[0046] (2) Preparation of NCL composite hydrophobic modifier:

[0047] (a) Mix lauric acid and Cu(OH)2 at a mass ratio of 1:1-2, add a mixed solvent of anhydrous ethanol and distilled water (volume ratio of 1:3-5), and control the solid-liquid ratio to 1 g:(70-90) mL;

[0048] (b) Ultrasonically disperse the mixed system at a frequency of 50-60 kHz for 30-40 min, and simultaneously stir mechanically at 1000-1500 rpm to form an NCL dispersion;

[0049] (c) Dry the dispersion at 50-70 °C to constant weight to obtain hydrophobic NCL powder with a particle size of 80-150 nm;

[0050] (3) Preparation of casting solution:

[0051] (a) Dissolve SA in an appropriate amount of anhydrous ethanol, add the NCL powder prepared in step (2) (SA:NCL mass ratio is 1:1-1:3), stir at room temperature for 2 h to allow SA to fully wrap NCL, and completely evaporate the anhydrous ethanol;

[0052] (b) Sequentially add the pore-forming agent LiCl 2-6%, PEG 20000 1-3% and the PVDF solution prepared in step (1) to the mixture, continue to stir at a constant temperature of 50-80 °C for 12-18 h, and stand for defoaming for 6-24 h to obtain a homogeneous casting solution; wherein, the mass ratio of LiCl to PEG 20000 is 1:1-3:1, and the total addition amount of the PVDF solution is 85-90 wt%.

[0053] (4) Preparation of the modified membrane: The casting solution was evenly spread on a glass substrate, and the film thickness was controlled to be 150 - 250 μm. After pre-evaporation in air for 30 - 90 s, it was immersed in a deionized water coagulation bath at 10 - 30 °C for phase inversion for 12 - 24 h to form a hydrophobic microporous membrane;

[0054] (5) Drying: The membrane was dried in air at room temperature for 12 - 24 h to obtain the SA and NCL hydrophobic blend modified PVDF composite membrane.

[0055] The porosity of the hydrophobic blend modified PVDF composite membrane prepared according to the above preparation method is 80 - 85%, the contact angle ≥ 120°, and it can be used to treat wastewater with an ammonia nitrogen concentration of 50 - 3000 mg / L.

[0056] The treatment process is as follows:

[0057] (a) Adjust the pH of the wastewater to 10.5 - 11.5 to convert ammonia nitrogen into free NH3, and the ammonia nitrogen concentration in the wastewater is 50 - 3000 mg / L;

[0058] (b) Assemble the PVDF composite membrane in a membrane contactor, pass the alkaline wastewater on the wastewater side, and pass the H2SO4 or H3PO4 solution with pH = 1.5 - 2.5 on the absorbent side;

[0059] (c) Control the transmembrane pressure difference at 0 - 0.3 MPa and the temperature at 5 - 50 °C.

[0060] The present invention will be further described below in conjunction with specific embodiments.

[0061] Example 1

[0062] The preparation of the hydrophobic blend modified PVDF composite membrane includes the following steps:

[0063] (1) Preparation of the PVDF solution: PVDF powder and the solvent N,N-dimethylacetamide (DMAc) were mixed at a mass ratio of 10 - 15:85 - 90, and stirred at a constant temperature of 50 - 80 °C for 4 - 6 h to form a homogeneous PVDF solution;

[0064] (2) Preparation of the NCL composite hydrophobic modifier:

[0065] (a) Lauric acid and Cu(OH)2 were mixed at a mass ratio of 1:1 - 2, and a mixed solvent with a volume ratio of anhydrous ethanol to distilled water of 1:3 - 5 was added, and the solid-liquid ratio was controlled to be 1 g:(70 - 90) mL;

[0066] (b) The mixed system was ultrasonically dispersed at a frequency of 50 - 60 kHz for 30 - 40 min, and simultaneously mechanically stirred at 1000 - 1500 rpm to form an NCL dispersion;

[0067] (c) Dry the dispersion at 60 °C to obtain hydrophobic NCL powder with a particle size of 80 - 150 nm.

[0068] (3) Preparation of the casting solution:

[0069] (a) Dissolve SA in an appropriate amount of absolute ethanol, add the NCL powder prepared in step (2) (the mass ratio of SA:NCL is 1:1 - 1:3), stir at room temperature for 2 h to allow SA to fully coat NCL, and completely evaporate the absolute ethanol;

[0070] (b) Sequentially add 2 - 6% of the pore - forming agent LiCl, 1 - 3% of PEG 20000, and the PVDF solution from step (1) to the mixture, continue to stir at a constant temperature of 60 °C for 12 h, and then let it stand for 24 h to remove bubbles to obtain a homogeneous casting solution.

[0071] (4) Preparation of the modified membrane: Uniformly scrape the casting solution onto a glass substrate, control the film thickness to be 200 μm, pre - evaporate in air for 60 s, and then immerse it in a deionized water coagulation bath at 20 - 25 °C for phase inversion for 12 - 24 h to form a hydrophobic microporous membrane;

[0072] (5) Drying: Dry the membrane in air at room temperature for 24 h to obtain the SA and NCL blended and modified PVDF hydrophobic composite membrane.

[0073] The surface and cross - section of the prepared PVDF / NCL / SA composite membrane observed by scanning electron microscope (SEM) are as Figure 1 shown. Through the synergistic modification mechanism of SA and NCL, the microstructure and functional properties of the PVDF composite membrane are significantly optimized. After adding SA, the surface morphology of the membrane is significantly improved: the NCL aggregation phenomenon is eliminated, the surface is relatively smooth, and there is no nanoparticle accumulation. The mechanism is that SA inhibits the aggregation of NCL particles through intermolecular forces and promotes their uniform dispersion on the membrane surface. At the same time, it can be seen from the surface and cross - section diagrams that the composite membrane presents a unique gradient pore structure - the surface pore size shrinks to form a dense layer, while the internal finger - like pore size increases and the density increases. This structure has double advantages in practical applications: the surface dense layer blocks pollutants from entering the membrane pores and delays the membrane pore wetting rate; the internal through - pores maintain high - efficiency mass transfer ability, thus better meeting the requirements of high - throughput and high anti - fouling performance for the membrane absorption method.

[0074] Figure 2 is the energy - dispersive spectroscopy (EDS) diagram of the PVDF / NCL / SA composite membrane. It can be seen that the Cu element is uniformly distributed on the membrane without obvious aggregation. In addition, the signal intensity of the Cu element on the surface of the PVDF / NCL / SA membrane is between 0 - 220 cps / eV, indicating that NCL is not only evenly dispersed on the membrane surface but also evenly fixed on the carbon skeleton of the membrane.

[0075] Figure 3 It is the atomic force microscope (AFM) image of the PVDF / NCL / SA composite membrane, and a micro-nano composite rough morphology with a size of 652.45 nm is formed on the membrane surface. On the one hand, it significantly reduces the contact area between the membrane and the solution. On the other hand, air is trapped between the rough peaks and valleys to form a stable gas-liquid interface, delaying the solution penetration. Through the collaborative design of surface roughness and pore structure, while maintaining a high mass transfer flux, the anti-wetting performance is improved, providing a long-term and stable interface guarantee for the treatment of high-concentration ammonia-nitrogen wastewater by the membrane absorption method. a It is the atomic force microscope (AFM) image of the PVDF / NCL / SA composite membrane, and a micro-nano composite rough morphology with a size of 652.45 nm is formed on the membrane surface. On the one hand, it significantly reduces the contact area between the membrane and the solution. On the other hand, air is trapped between the rough peaks and valleys to form a stable gas-liquid interface, delaying the solution penetration. Through the collaborative design of surface roughness and pore structure, while maintaining a high mass transfer flux, the anti-wetting performance is improved, providing a long-term and stable interface guarantee for the treatment of high-concentration ammonia-nitrogen wastewater by the membrane absorption method.

[0076] Example 2

[0077] Ammonia nitrogen is the main pollutant in industrial, agricultural and domestic sewage. Its excessive discharge causes significant harm to the ecological environment and human health. Ammonia nitrogen can cause eutrophication of water bodies, leading to explosive reproduction of algae, consuming dissolved oxygen in water, causing aquatic organisms to die due to lack of oxygen, and seriously damaging the ecological balance. At the same time, ammonia nitrogen is easily converted into more toxic free ammonia (NH3) under high pH conditions, directly poisoning fish and other aquatic organisms. In addition, ammonia nitrogen can pollute drinking water sources through the food chain accumulation, and long-term intake will damage the functions of the human liver and nervous system, and even induce chronic diseases. High-concentration ammonia-nitrogen wastewater significantly increases the difficulty and cost of sewage treatment, and intensifies the environmental governance pressure, so better treatment and recovery of ammonia-nitrogen wastewater are needed.

[0078] The PVDF / NCL / SA composite membrane treats ammonia-nitrogen wastewater by the membrane absorption method, and the process is as Figure 4 shown.

[0079] 1. Fix the composite membrane in the membrane contactor, circulate ammonia-nitrogen wastewater on the wastewater side, and circulate acidic absorbent (such as sulfuric acid solution or phosphoric acid solution) on the absorbent side. The experimental process is as follows: Add 3 L of 5**0**-3000 mg / L NH4Cl solution to the pressure tank, and add NaOH to control the pH at 10.5-11.5 to convert ammonium ions (NH4 + ) in the wastewater into free ammonia (NH3). NH3 is transferred from the wastewater side to the absorbent side in gaseous form through the hydrophobic microporous membrane (i.e., the composite membrane); an acidic solution (pH = 2) is contained in the absorbent tank on the absorbent side, and NH3 reacts with H + to form NH4 + to achieve fixed recovery. Control the flow rates of the feed side and the absorbent side by a constant-flow peristaltic pump to maintain the pH stability on both sides. After operation, sample and detect the ammonia-nitrogen concentration of the feed liquid to evaluate the removal efficiency.

[0080] 2. Influence of different ammonia-nitrogen concentrations

[0081] The removal efficiency at different ammonia nitrogen concentrations can reflect the adaptability of the membrane. The ammonia removal effects when the ammonia nitrogen concentrations are 50, 200, 500, 1000, 2000, and 3000 mg / L respectively are as Figure 5 shown. Ammonia nitrogen above 200 mg / L belongs to high-concentration ammonia nitrogen, but the PVDF / NCL / SA composite membrane still shows an ammonia nitrogen removal rate of 75.5% and an ammonia nitrogen flux of 19.96 g / (m 2 ·h). As the ammonia nitrogen concentration in the feed liquid increases, the removal rate decreases significantly, with a decrease of 38.69%, while the flux increases from 7.22 g / (m 2 ·h) to 88.43 g / (m 2 ·h). The mechanism is as follows: High-concentration ammonia nitrogen intensifies the concentration polarization effect on the membrane surface, and the rapid accumulation of NH3 / NH4 + forms a thick polarization layer. The increase in osmotic pressure weakens the mass transfer driving force, resulting in a decrease in the removal efficiency; at the same time, the high concentration gradient enhances the transmembrane diffusion driving force, and the weakening of the membrane pore selectivity is superimposed, accelerating the migration of ammonia nitrogen molecules, manifested as a significant increase in the flux.

[0082] 3. Influence of different temperatures

[0083] To explore the performance of the PVDF / NCL / SA composite membrane in the temperature range of 0 - 40°C. As Figure 6 shown, the removal rate of ammonia nitrogen by the composite membrane continuously increases with the increase of temperature to 82.8%, while the flux decreases by 34.6% after reaching the peak at 30°C. The mechanism can be attributed to the dual regulatory effects of temperature on mass transfer kinetics and membrane structure stability. On the one hand, as the temperature increases (0 → 30°C), by reducing the solution viscosity, increasing the ammonia nitrogen diffusion coefficient, and enhancing the thermal motion between molecules, the migration and adsorption reaction of pollutants are accelerated, promoting the synchronous increase of the removal rate and the flux; on the other hand, when the temperature exceeds 30°C, the intensified thermal motion of PVDF molecular chains causes the membrane pores to expand, resulting in a broadening of the pore size distribution, weakening the selective retention efficiency, and at the same time, high temperature accelerates the desorption of ammonia nitrogen and intensifies the concentration polarization effect, leading to a decrease in the flux. 30°C, as the optimal balance point between thermodynamic driving and material stability, not only ensures the maximization of mass transfer efficiency but also avoids the performance deterioration caused by membrane structure deformation and interfacial polarization.

[0084] Example 3

[0085] To explore the anti-degradation performance of the composite membrane and enhance its application in industry. With the wide application of membrane separation technology in fields such as wastewater treatment, chemical production, and biomedicine, the chemical stability and sustainable use performance of membrane materials under complex working conditions have become key challenges. Based on the systematic analysis of membrane performance and process operating conditions in the previous Examples 1 and 2, it is necessary to further verify the anti-degradation performance of the composite membrane in extreme acid-base environments (such as long-term acid / alkali corrosion resistance) and its cleaning and regeneration efficiency (such as flux recovery rate, structural integrity retention), which are used as the core indicators to evaluate the reliability of industrial operation of the membrane absorption method. Specifically as follows:

[0086] 1. Anti-degradation of the composite membrane in an acidic environment (pH = 2)

[0087] The process conditions are as follows: Place the PVDF / NCL / SA composite membrane in a sulfuric acid solution with pH = 2, take it out regularly every three days, continuously treat for 24 days, take samples every 3 days (a total of 8 groups), and immediately rinse and dry with deionized water after each sampling; Use an electronic universal material testing machine (ASTM D638 standard) to measure the tensile strength (MPa) and elongation at break (%) of the membrane, with the parameter settings of a tensile rate of 10 mm / min, a gauge length of 50 mm, and an environmental temperature and humidity of 25°C / 50% RH.

[0088] The results are as Figure 7 shown. In an acidic environment, the PVDF / NCL / SA composite membrane has good resistance, and only decreased by 4.8% after 24 days of immersion. Its anti-acid mechanism can be attributed to: on the one hand, SA and NCL construct a dense hydrophobic network through carboxylate-Cu 2+ coordination bonds to block the erosion of H + on the PVDF matrix; on the other hand, NCL nanoparticles act as physical cross-linking points to inhibit the slip of molecular chains, and its hydrophobic long chain (C 12 H 23 COO - ) and the alkyl chain C 17 H 35 COOH (C 18 H 36 O2) form a molecular barrier to reduce the penetration of acid solution; at the same time, the dynamic dissociation characteristics of SA endow the network with self-healing ability to maintain structural integrity. To confirm its industrial value, the expected service life of this membrane in a strong acid absorption solution (such as H2SO4) is ≥ 2 years, and the operation and maintenance cost is reduced by 40%, providing a long-term and reliable solution for the treatment of highly corrosive wastewater.

[0089] 2. Anti-degradation of the composite membrane in an alkaline environment (pH = 12)

[0090] The process conditions are as follows: The PVDF / NCL / SA composite membrane is placed in a sodium hydroxide solution with pH = 12. It is taken out regularly every three days and continuously treated for 24 days. Samples are taken every 3 days (a total of 8 groups). After each sampling, it is immediately rinsed with deionized water and dried. The tensile strength (MPa) and elongation at break (%) of the membrane are measured using an electronic universal material testing machine (ASTM D638 standard). The parameter settings are a tensile rate of 10 mm / min, a gauge length of 50 mm, and an environmental temperature and humidity of 25°C / 50% RH.

[0091] As Figure 8 shown, in an alkaline environment (pH = 12), the mechanical strength of the PVDF / NCL / SA composite membrane decreases by 37% due to OH- erosion and Cu 2+ dissolution, but its performance shortcoming can be effectively compensated by the self-protection mechanism and the collaborative optimization strategy. SA reacts with OH - to form C 17 H 35 COO - salt layer, which covers the membrane surface to form a dense protective layer, blocks the deep erosion of the PVDF matrix by OH - , and inhibits the aggregation of Cu(OH)2 precipitation, maintaining the dispersion activity of nano-NCL particles; in alkaline ammonia-nitrogen wastewater (pH = 12), the membrane life is ≥ 2 years (the mechanical strength retention rate > 95%), the operation and maintenance cost is reduced by 40% - 50%, and it can meet the treatment requirements of 80% of high-concentration ammonia-nitrogen wastewater. The alkaline defects can be significantly alleviated through protection and process optimization, while the long-term stability and high economy in an acidic environment establish its core industrial position. The defects do not overshadow the advantages, and it has both technical inclusiveness and application universality.

[0092] 3. Recovery of the PVDF / NCL / SA Composite Membrane after Sodium Hypochlorite Cleaning

[0093] The process conditions are as follows: To verify the chemical cleaning and regeneration performance of the PVDF / NCL / SA composite membrane, three cyclic cleaning experiments are carried out using chemical cleaning: After each 24-hour operation, it is successively rinsed with high-pressure distilled water (0.2 MPa, 10 min), soaked in a 30% NaClO solution at 40°C for 12 h, and finally rinsed with deionized water and soaked for 2 h, and the change in ammonia-nitrogen removal under repeated use is observed.

[0094] The results are as Figure 9 shown. The single-performance attenuation of the PVDF / NCL / SA composite membrane is ≤ 3%. The micro-nano rough surface of the membrane can reduce the deep adsorption of pollutants, and the oxidation by NaClO can efficiently degrade the pollutants attached to the surface. At the same time, it shows that the membrane can effectively resist ClO -Oxidation corrosion resistance, can withstand ≥ 30 chemical cleaning cycles (cumulative attenuation < 10%), expected industrial service life ≥ 2 years, significantly extended compared with traditional PVDF membranes (6 - 12 months), and operation and maintenance costs reduced by 40% - 50%.

[0095] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A preparation method of a hydrophobic blend-modified PVDF composite membrane, characterized in that: It includes the following steps: (1) Dissolve stearic acid in absolute ethanol, then add hydrophobic nano copper laurate powder, and stir at room temperature until the absolute ethanol completely evaporates; (2) Add LiCl, PEG 20000 and PVDF solution to the mixture in step (1), stir at a constant temperature, and then stand for defoaming to obtain a homogeneous casting solution; (3) Uniformly scrape the casting solution onto a glass substrate, control the film thickness to be 150 - 250 μm, pre-evaporate in air, and then immerse it in a deionized water coagulation bath for phase inversion to form a hydrophobic microporous membrane; (4) Dry the hydrophobic microporous membrane to obtain a stearic acid and nano copper laurate hydrophobic blend modified PVDF composite membrane.

2. The preparation method of a hydrophobic blend-modified PVDF composite membrane according to claim 1, characterized in that: In step (1), the mass ratio of stearic acid to nano copper laurate is 1:1 - 1:

3.

3. The preparation method of a hydrophobic blend-modified PVDF composite membrane according to claim 1, characterized in that: In step (2), the mass ratio of LiCl to PEG 20000 is 1:1 - 3:1, and the total addition amount of the PVDF solution is 85 - 90 wt%.

4. The preparation method of a hydrophobic blend-modified PVDF composite membrane according to claim 1, characterized in that: In step (2), stir at a constant temperature of 50 - 80 °C for 12 - 18 h, and stand for defoaming for 6 - 24 h.

5. The preparation method of a hydrophobic blend-modified PVDF composite membrane according to claim 1, characterized in that: In step (3), the temperature of the coagulation bath is 10 - 30 °C, and the pre-evaporation time is 30 - 90 s.

6. The preparation method of a hydrophobic blend-modified PVDF composite membrane according to claim 1, wherein: In step (1), the preparation process of the hydrophobic nano copper laurate powder is as follows: Mix lauric acid and Cu(OH)₂ in a mass ratio of 1:1 - 2, add a mixed solvent of absolute ethanol and water, and the solid-liquid ratio is 1:70 - 90; then ultrasonically disperse the mixed system at a frequency of 50 - 60 kHz for 30 - 40 min, and simultaneously stir mechanically at 1000 - 1500 rpm to form an NCL dispersion; dry the dispersion at 50 - 70 °C to obtain hydrophobic NCL powder with a particle size of 80 - 150 nm.

7. The preparation method of a hydrophobic blend-modified PVDF composite membrane according to claim 1, characterized in that: In step (2), the preparation process of the PVDF solution is as follows: Mix PVDF powder and N,N-dimethylacetamide in a mass ratio of 10 - 15:85 - 90, and stir at a constant temperature of 50 - 80 °C for 4 - 8 h to form a homogeneous PVDF solution.

8. A hydrophobic blend modified PVDF composite membrane prepared by the preparation method according to any one of claims 1 to 7.

9. Application of a hydrophobic blend modified PVDF composite membrane prepared by the preparation method according to any one of claims 1 to 7 in wastewater deammoniation treatment.

10. The application according to claim 9, wherein: The application process is as follows: (a) Adjust the pH of the wastewater to 10.5 - 11.5, and the ammonia nitrogen concentration in the wastewater is 50 - 3000 mg / L; (b) Assemble the PVDF composite membrane in a membrane contactor, introduce alkaline wastewater on the wastewater side, and introduce an H₂SO₄ or H₃PO₄ solution with a pH of 1.5 - 2.5 on the absorbent side; (c) Control the transmembrane pressure difference to be 0 - 0.3 MPa, and the temperature is 5 - 50 °C.

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