CN110227350B

CN110227350B - Application of attapulgite modified PVDF ultrafiltration membrane in protein solution filtration

Info

Publication number: CN110227350B
Application number: CN201910326283.1A
Authority: CN
Inventors: 薛爱莲; 周守勇; 赵宜江; 李梅生; 邢卫红; 许家兴; 胡磊; 杨大伟; 张艳; 张莉莉; 黄进; 张玉洁; 吴飞跃
Original assignee: Huaiyin Normal University
Current assignee: Huaiyin Normal University
Priority date: 2018-01-08
Filing date: 2018-01-08
Publication date: 2022-01-28
Anticipated expiration: 2038-01-08
Also published as: CN108246110B; CN108246110A; CN110227350A

Abstract

The invention discloses an application of an attapulgite modified PVDF ultrafiltration membrane in protein solution filtration. According to the invention, the attapulgite modified by the N, N-dimethylaminoethyl polymethacrylate is introduced into the polyvinylidene fluoride ultrafiltration membrane, so that a three-dimensional net structure can be formed by utilizing the unique nanofiber structure of the attapulgite and a polyvinylidene fluoride material, and the structure and strength of the pure polyvinylidene fluoride ultrafiltration membrane are effectively improved; and the high hydrophilicity of the attapulgite is utilized to improve the permeability and the hydrophilicity of the membrane, thereby realizing the anti-pollution property and the easy cleaning property.

Description

Application of attapulgite modified PVDF ultrafiltration membrane in protein solution filtration

Technical Field

The invention relates to an attapulgite modified PVDF ultrafiltration membrane, a preparation method and application thereof in protein solution filtration, belonging to the technical field of membrane separation materials.

Background

Polyvinylidene fluoride is a membrane material with excellent performance, has the advantages of good chemical stability, high temperature resistance, oxidation resistance, corrosion resistance, good toughness, high strength and the like, and is widely applied to the fields of food, medicine, sewage treatment and the like. However, the low surface energy of polyvinylidene fluoride makes the membrane surface have stronger hydrophobicity, the adsorption pollution is serious when an oil-water or protein-containing solution system is separated, the flux attenuation speed is high, organic matters are easy to adhere to the membrane surface to cause the blockage of membrane pores and the pollution of a formed membrane, and the performance of the membrane is difficult to fully exert. The development of anti-pollution membranes is the current direction of intense research. The increased hydrophilicity of the membrane can reduce solute entrapment, particularly contact and non-directional binding between proteins, particularly adsorption of biological contaminants. Therefore, it is the direction of continuous research by the current scholars to improve the hydrophilicity of the membrane, reduce the pollution of the membrane and make the membrane have higher and stable permeation flux.

At present, two main methods of modifying the polyvinylidene fluoride membrane include surface modification and blending modification. The surface modification mainly comprises surface coating and surface grafting. The former is simple to operate but the modifier is easy to run off in the using process of the membrane, and the fallen modifying substances can cause pollution to the membrane; the latter requires post-treatment and the modification is not uniform enough and can even block the die holes, impairing the film properties.

The blending modification means that the hydrophilic substance and the polyvinylidene fluoride matrix material are physically blended, the membrane preparation process is simple, complex pretreatment and post-treatment are not needed, the operation is convenient, the efficiency is high, and the method is the most common method for modifying the polymer membrane. The compatibility of the blending material and the polyvinylidene fluoride matrix directly influences the formation of the membrane, and the commonly used blending material comprises an organic copolymer containing a hydrophilic chain segment, inorganic nano particles and the like.

Currently, inorganic nanoparticle TiO for blended materials₂、SiO₂、Al₂O₃And the like are all granular, and the nano-scale inorganic particles are easy to fall off in the phase conversion film forming process, so that the performance of the film is influenced. In contrast, the one-dimensional nanomaterials such as carbon nanotubes have super strong mechanical properties, high aspect ratio and high specific surface area, and the one-dimensional nanomaterials dispersed in the polymeric membrane can effectively improve the stability of the one-dimensional nanomaterials in the membrane material by the spiral winding of the polymeric chain. However, the artificially synthesized one-dimensional nano materials such as carbon nanotubes have high preparation cost, low purity and yield and are difficult to disperse, which greatly limits the large-scale application of the materials in membrane blending modification.

Attapulgite is the main component of attapulgite clay, a nonmetallic clay mineral, and is a typical hydrous magnesium-rich aluminosilicate mineral with a layer chain structure. The attapulgite is a rod-shaped crystal combination, the diameter of a single rod crystal is 20-70 nm, and the length of the single rod crystal is about 0.5-5 mu m. Due to the special one-dimensional nano fibrous structure of the attapulgite and the fact that the surface of the attapulgite is rich in a large number of hydroxyl groups, polymer modification can be further carried out, and therefore the attapulgite can be used as an excellent blending additive of a PVDF membrane material.

Disclosure of Invention

The invention aims to provide an anti-pollution polyvinylidene fluoride ultrafiltration membrane, which utilizes a three-dimensional net structure formed by a unique nanofiber structure of attapulgite and polyvinylidene fluoride to effectively improve the structure and the strength of the polyvinylidene fluoride ultrafiltration membrane, and utilizes the high hydrophilicity of the attapulgite to improve the permeability and the hydrophilicity of the membrane so as to realize anti-pollution and easy cleaning.

In a first aspect of the present invention, there is provided:

the modified attapulgite has the structure shown in the formula (I) on the surface:

（I）；

wherein n is any integer between 1 and 10000.

In a second aspect of the present invention, there is provided:

an attapulgite modified PVDF ultrafiltration membrane is characterized in that the modified attapulgite is also mixed in the PVDF ultrafiltration membrane.

In one embodiment, the weight ratio of the modified attapulgite to the PVDF is 1-10: 20.

In a second aspect of the present invention, there is provided:

a preparation method of an attapulgite modified PVDF ultrafiltration membrane comprises the following steps:

step 1, grafting a coupling agent containing C = C bonds on the surface of attapulgite;

step 2, carrying out polymerization reaction on N, N-dimethylaminoethyl methacrylate and the coupling agent grafted attapulgite obtained in the step 1;

and 3, adding the modified attapulgite obtained in the step 2 into the PVDF membrane casting solution, and preparing the ultrafiltration membrane by a phase inversion method.

In one embodiment, in said step 1, the C = C bond containing coupling agent is the silane coupling agent MPS; the weight to volume ratio of attapulgite to coupling agent containing C = C bonds was 12 g: 5-15 mL, carrying out a grafting reaction under the condition of an organic solvent, wherein the organic solvent is toluene, the reaction temperature is 55-70 ℃, the reaction time is 2-6 h, and after the reaction is finished, washing and drying the product.

In one embodiment, in the step 2, the weight ratio of the N, N-dimethylaminoethyl methacrylate to the coupling agent grafted attapulgite is 0.5-6: 2; the polymerization reaction temperature is 75-85 ℃, the reaction time is 1-5 h, and after the reaction is finished, the product needs to be washed and dried.

In one embodiment, in the step 3, the PVDF casting solution contains PVDF, an organic solvent, and a porogen, and the mass ratio of the modified attapulgite to the PVDF, the organic solvent, and the porogen is: 0.05-0.42: 10-14: 1-5: 30-60 parts of; the organic solvent is triethyl phosphate; the pore-forming agent is PEG.

In one embodiment, in said step 3, the phase inversion method is a film formation by immersion precipitation phase inversion method by scraping the PVDF casting solution onto a flat plate.

In a third aspect of the present invention, there is provided:

the attapulgite modified PVDF ultrafiltration membrane is applied to liquid filtration.

In one embodiment, the liquid comprises a protein.

In a fourth aspect of the present invention, there is provided:

application of the modified attapulgite in the preparation of polymer membranes.

In the application, the modified attapulgite is used for reducing the adsorption of a polymer membrane on protein, reducing the macroporous defect in the polymer, reducing the pore size distribution width, improving the porosity, improving the thermal stability, improving the tensile strength, improving the elongation at break, improving the hydrophilicity, improving the water flux, improving the flux in the process of filtering a protein-containing solution, reducing the membrane pollution index MFI in the filtering process, improving the flux recovery rate after membrane cleaning or reducing the irreversible membrane pollution in the filtering process.

Advantageous effects

1. The anti-pollution polyvinylidene fluoride ultrafiltration membrane not only effectively improves the structure and the strength of the polyvinylidene fluoride ultrafiltration membrane by utilizing the typical nanofiber structure of attapulgite and the three-dimensional net structure formed by the attapulgite and polyvinylidene fluoride, but also effectively improves the permeability and the hydrophilicity of the polyvinylidene fluoride membrane by utilizing the high hydrophilicity of the attapulgite, thereby realizing the anti-pollution easy cleaning property. 2. Compared with the traditional inorganic nano particles, the attapulgite is a typical one-dimensional nano material, has multiple channels inside, high specific surface area, large reserves in China, low cost and no negative influence on the environment, and has obviously better cost performance than the artificially synthesized one-dimensional nano fiber material. 3. The surface of the attapulgite is rich in hydroxyl, which provides convenience for further functional modification of the surface, and compared with unmodified attapulgite, the surface of the attapulgite is grafted with poly N, N-dimethylaminoethyl methacrylate, so that the compatibility between inorganic nano particles and macromolecules is improved, the dispersity of the attapulgite is improved, and the stability of the attapulgite in a film can be improved by winding a macromolecular chain.

Drawings

FIG. 1 is an infrared spectrum of modified attapulgite;

FIG. 2 is an XRD pattern of the modified attapulgite;

FIG. 3 is a SEM image of a material; wherein, the a region is PGS; region b is PGS-MPS; the c region is PGS-g-PDMAEMA;

FIG. 4 shows the result of EDX elemental analysis of the modified attapulgite;

FIG. 5 is a graph comparing the results of adsorption experiments on proteins;

FIG. 6 is an upper surface infrared spectrum of the membrane;

FIG. 7 skin, bottom and cross-sectional views of pure PVDF film and mixed film;

FIG. 8 is a graph of pore size distribution for different membranes;

FIG. 9 is a thermogravimetric plot of pure PVDF ultrafiltration and mixed membranes;

FIG. 10 is a graph comparing contact angles of PGS-g-PDMAEMA films at different levels;

FIG. 11 is a graph comparing initial contact angles after 200 s for films of different modifier content;

FIG. 12 is a graph comparing the dynamic contact angles of PGS-g-PDMAEMA films at different levels;

FIG. 13 is a graph comparing pure water flux of PGS-g-PDMAEMA membranes at different contents;

FIG. 14 is a graph showing a comparison of the adsorption amount of BSA to a membrane;

FIG. 15 is a graph of the permeation flux of membrane filtered BSA as a function of time;

FIG. 16 is a graph comparing rejection and stable permeate flux for membranes;

FIG. 17 is a graph showing a comparison of the amount of BSA adsorbed dynamically by the membrane;

FIG. 18 comparison of membrane fouling indices;

FIG. 19 is a graph comparing the flux recovery of PVDF/P0 with that of PVDF/P4 for different cleaning modes;

FIG. 20 is a graph comparing steady permeate flux and flux recovery for membranes;

FIG. 21 comparative graph of fouling resistance analysis of membranes.

Detailed Description

Attapulgite (ATP) is the main component of Attapulgite clay, a non-metal clay mineral. Bailey recommended its english name palygorskite (pgs) in 1980. Attapulgite belongs to sepiolite family, and is a typical water-containing magnalium silicate mineral with layer chain structure, and the common structural formula is Mg₅(Si₄O₁₀)₂(OH)₂(OH₂)₄·4H₂And O. The attapulgite is a rod-shaped crystal combination, the diameter of a single rod crystal is 20-70 nm, and the length of the single rod crystal is about 0.5-5 mu m. In each 2:1 type chain layer structure layer, the chain layer parallel to the x axis can be formed by turning 180 degrees along the top end of the y axis tetrahedron at certain intervals. The special structural characteristics enable the attapulgite to conform to the characteristics of one-dimensional nano materials, and can form one-dimensional pore canals of 0.38 nm multiplied by 0.63 nm. The attapulgite has specific diameter and pore canals, large specific surface area inside and outside, special surface charge distribution and redundant positive and negative charges, high specific surface area and uneven charge distribution; due to a large amount of negative charges and silicon hydroxyl groups rich on the surface of the attapulgite, the attapulgite can be organically modified by a coupling agent, a polymer and the like.

The grafting modification of poly N, N-dimethylaminoethyl methacrylate (PDMAEMA) on the surface of the attapulgite is divided into two steps, and the flow chart is shown as follows.

Example 1 preparation of modified Attapulgite

Preparation of PGS-MPS

Sequentially adding 12g of dried attapulgite and 200 mL of toluene into a 500 mL four-neck flask, mechanically stirring under the protection of nitrogen and in a 60 ℃ oil bath to fully mix the attapulgite and the toluene, dropwise adding 10 mL of a silane coupling agent MPS after 30 min, and continuing to react for 4 h after the dropwise adding is finished. After the reaction is finished, centrifugally separating the product, washing the product with absolute ethyl alcohol for three times to remove the unreacted silane coupling agent and solvent in the reaction, drying the finally obtained product in vacuum at 60 ℃, and grinding the product after 5 hours to obtain the first-step modified product PGS-MPS.

Preparation of PGS-g-PDMAEMA

2g of PGS-MPS and 80 mL of deionized water are sequentially added into a 250 mL four-neck flask, mechanical stirring is carried out under an oil bath at 80 ℃ after 2 hours of ultrasonic dispersion, 0.06 g of KPS solution is dropwise added into a dropping funnel after 30 minutes, DMAEMA after removal of a polymerization inhibitor is slowly dropwise added after 1 hour, the weight is respectively 0.5 g, 2g, 4 g and 6 g, and the reaction is continuously carried out for 3 hours under the protection of nitrogen. And after centrifugal separation of the product after the reaction is finished, washing the product for three times by using deionized water, removing redundant monomers and initiators in the solution, and finally, grinding the product after freeze drying for 24 hours to obtain the final product PGS-g-PDMAEMA.

Characterization of the modified Attapulgite

(1) Infrared spectrometry (FTIR): mixing the solid powder obtained after reaction with potassium bromide (KBr) solid, grinding to obtain transparent sheet, performing infrared spectroscopic analysis on attapulgite and modified attapulgite (PGS, PGS-MPS, PGS-g-PGS) with AVATAR 360FT-IR spectrometer, scanning accuracy of 4, scanning frequency of 32 times, and scanning range of 4000 cm^-1~500 cm^-1. As shown in FIG. 1, the curves (a), (b) are shown in 1305 cm^-1A new peak appears nearby, corresponding to the C-O stretching vibration peak, and the wavelength is 1708 cm^-1A stretching vibration peak of C = O-O ester bond appears at 2970 cm^-1Nearby occurrence of-CH₂Characteristic peaks of methylene groupsThis shows that the silane coupling agent was successfully grafted on the attapulgite after the first modification step. Observing the curve of (b), (c), the curve of (c) at a wavelength of 1727 cm^-1A distinct characteristic peak appears at the position, which corresponds to the characteristic absorption peak of C = O carbonyl in ester group, and the wavelength is 2722 cm^-1，2822 cm^-1Nearby occurrence of-N (CH)₃)₂A stretching vibration absorption peak of C-H bond in tertiary amino group, and 2970 cm^-1The intensity of the corresponding methylene absorption peak is obviously enhanced. This indicates successful grafting of the monomeric DMAEMA onto attapulgite.

(2) X-ray diffraction spectroscopy (XRD): and (3) carrying out crystal form analysis on the attapulgite by using an X-ray diffractometer, setting the tube current to be 40 mA, setting the tube voltage to be 45 kV, and setting the scanning range to be 5 degrees < 2 theta < 80 degrees. And analyzing whether the crystal form of the attapulgite is changed before and after modification by utilizing X diffraction spectrum. As shown in FIG. 2, the attapulgite PGS, the modified attapulgite PGS-MPS and the PGS-g-PDMAEMA have strong characteristic peaks when the 2 theta is 8.3 degrees, 13.8 degrees, 16.4 degrees, 19.8 degrees, 27.5 degrees and 35.4 degrees, and respectively correspond to a crystal face 110, a crystal face 200, a crystal face 130, a crystal face 040, a crystal face 400 and a crystal face 161 of the attapulgite. Wherein the 110 crystal face corresponding to the most intense characteristic peak is a hydroxyl diffraction peak of the attapulgite, and observation of an XRD curve of the modified attapulgite shows that other characteristic peaks do not appear, which indicates that the crystal structure of the attapulgite is not damaged in the modification process, and the monomer DMAEMA is grafted on the surface of the attapulgite.

(3) Thermogravimetric analysis (TG): performing thermogravimetric analysis by using a thermogravimetric analyzer in a nitrogen atmosphere, wherein the temperature rise speed is set to 10 ℃/min, and the test temperature range is 20-800 ℃. The grafting ratio was calculated using the following formula.

GR represents a graft ratio (%);ΔW ₁the weight loss (%) of the attapulgite (PGS-MPS) modified in the first step when the temperature is raised from 300 ℃ to 700 ℃;ΔW ₂showing modified grafted attapulgite (PGS-g-PDMAEMA)Weight loss during temperature increase from 300 ℃ to 700 ℃;W _pthe weight (%) of PGS-g-PDMAEMA remaining at a temperature of 700 ℃.

(4) Scanning electron microscopy and energy spectroscopy (SEM, EDX): and observing the shape characteristics of the attapulgite by using a cold field emission scanning electron microscope, and analyzing the shape change of the attapulgite before and after modification. Drying attapulgite under an infrared lamp for 10min, vacuumizing for 1min, spraying gold for 90s, and setting the voltage to 10kV in a high vacuum mode. And observing the shapes of PGS, PGS-MPS and PGS-g-PDMAEMA by using a cold field emission scanning electron microscope, and analyzing the composition and content of elements on the surface of the sample by using an energy spectrum. As shown in FIG. 3, the surface of PGS-MPS and PGS-g-PDMAEMA became rugged and more loose than PGS, especially the surface of PGS-g-PDMAEMA which is the product of the last step modification was the roughest, mainly because the surface of attapulgite was grafted with organic matter. From FIG. 4, it can be found that PGS-g-PDMAEMA has 4.98% more N element than PGS-MPS; the content of C is increased from 13.19 percent to 32.59 percent, and the contents of other elements such as O, Mg, Si and the like are reduced in different degrees, which fully indicates that the monomer PDMAEMA is successfully grafted to the surface of the attapulgite.

(5) Adsorption experiment of protein by attapulgite

Drawing a standard curve of the BSA solution: BSA solutions with different mass concentrations are accurately prepared, and the absorbance of different concentrations is measured by a visible spectrophotometer at the maximum absorption wavelength of 278 nm.

Adsorption experiment: weighing 30 mg of attapulgite, pouring the attapulgite into a 250 mL conical flask, preparing 0.1 g/L BSA solution by using a phosphoric acid buffer solution with pH =7.4, transferring 30 mL of the prepared solution into the conical flask by using a pipette, placing the conical flask after sealing into a shaking table of a water bath at 25 ℃, taking out after 300 min, and pouring the mixture in the conical flask into a centrifuge tube. Centrifuging at 4500 r/min for 5 min, collecting supernatant, measuring absorbance at absorption wavelength of 278 nm with visible light photometer, calculating to obtain corresponding mass concentration with standard working curve, and calculating adsorption amount with following formula.

qThe adsorption capacity (mg/g) of the sample after 300 min;Vvolume (L) of BSA solution;mis the mass (g) of the sample;C ₀initial concentration of BSA solution (g/L);C _fthe concentration (g/L) of the BSA solution after adsorption was used.

The adsorption amount of the three samples, namely PGS, PGS-MPS and PGS-g-PDMAEMA, to Bovine Serum Albumin (BSA) after 300 min of adsorption at 25 ℃ is shown in FIG. 5. As can be seen from the figure, the adsorption amount of PGS to BSA was very small, about 23.89 mg/g; the maximum adsorption quantity of PGS-MPS reaches 65.12 mg/g; the adsorption quantity of PGS-g-PDMAEMA is the minimum, and is only 5.03 mg/g. The PGS-g-PDMAEMA as the blending additive of the polyvinylidene fluoride ultrafiltration membrane can not increase the adsorption of protein and can not cause direct pollution.

EXAMPLE 2 preparation of Ultrafiltration Membrane

Preparing a casting solution: adding 48 g of triethyl phosphate (TEP) serving as a solvent into a threaded reagent bottle, adding a certain mass of modified attapulgite, performing ultrasonic dispersion for 2 hours, adding 12g of polyvinylidene fluoride (PVDF) powder, mechanically stirring for 24 hours under an oil bath at 80 ℃, adding 3 g of PEG-400 serving as a pore-forming agent, and continuously stirring for 24 hours to form a uniform casting solution from the mixture.

Defoaming the casting solution: and (3) placing the prepared casting solution into a vacuum drying oven, and vacuumizing for 3 h at 80 ℃ for defoaming.

Film scraping and film forming: using an Elcometer 4340 film scraping machine, setting the temperature at 80 ℃, adjusting the index of a scraper to 200 mu m, casting the casting solution on a clean glass plate, forming a film by using an immersion precipitation phase inversion method, wherein the air evaporation time is 5 s, and the solidification bath temperature is 20 ℃.

Preparation of a dry film: and soaking the wet film in absolute ethyl alcohol, transferring the wet film to a normal hexane solution after 6 hours, taking out the wet film after 4 hours, and naturally drying the wet film.

The composition of the casting solution for different PGS-g-PDMAEMA addition amounts is shown in the following table:

characterization of Ultrafiltration membranes

(1) Film surface FTIR-ATR analysis: the surface chemical composition of the film is analyzed by an infrared spectrometer, the scanning precision is set to be 4, the scanning frequency is set to be 32 times, and the scanning range is set to be 4000 cm^-1~500 cm^-1. Infrared analysis of the film surface, as shown in FIG. 6, the PVDF mixed film was at 1650 cm as the amount of modifier added was increased^-1The peak intensity is gradually increased, and the characteristic absorption peak of C = O carbonyl in ester group is corresponded^[88]. This indicates that the modifier PGS-g-PDMAEMA was successfully incorporated into the membrane.

(2) Electron microscopy of the membrane, pore size porosity analysis: scanning Electron Microscopy (SEM): the appearance of the film surface and the section is analyzed by a cold field emission scanning electron microscope, and a sample needs to be quenched in liquid nitrogen when the section electron microscope is shot so as to obtain a complete and clear section diagram. FIG. 7 is a skin, floor and cross-sectional view of pure PVDF membrane and a mixed membrane, from which it can be seen that the PVDF/P0 membrane contains a large number of defective pores on the surface and a large number of macropores; the PVDF/P1 film has obviously reduced defect holes and uniform size; the number of macropores on the surfaces of PVDF/P4 and PVDF/P7 is gradually increased, and the number of defect pores of the mixed membrane added with modifier particles is smaller than that of the pure membrane. As can be seen from the pore size distribution diagram, the pore size distribution of PVDF/P0 is wide, the proportion of the average pore size is only 35%, the proportion of the average pore size of the mixed membrane after the modifier is added is greatly improved, and the percentage of the average pore size of PVDF/P1 is about 76%. This indicates that the modifier particles can be well mixed and compatible with the PVDF matrix, and the polymer chains on the PGS-g-PDMAEMA particles can be intertwined with the PVDF molecular chains, so that the mixed ultrafiltration membrane has a smaller number of macropores than a pure membrane. As can be seen from the bottom view of the film, with the increase of the amount of PGS-g-PDMAEMA particles, the crystal nuclei of the film gradually decrease, the number gradually increases, and the gaps among the crystal nuclei also gradually increase. This is because the increase in the modifier particles accelerates the precipitation rate and thus the crystallization of PVDF. As can be seen from the cross-sectional view, the cross-section of the membrane is in an asymmetric structure and consists of a skin layer, finger-shaped holes and sponge-shaped holes. The mixed film added with the modifier has finger-shaped hole length smaller than that of the pure film, and skin layer thickness smaller than that of the pure film. This is because the addition of the modifier increases the viscosity of the casting solution, and the kinetic exchange process between the solvent and the non-solvent during the phase inversion process is significantly slowed, so that the phenomenon of delayed phase separation is more significant, which is not favorable for the generation of finger-shaped pores. In addition, the faster the crystallization process, the faster the phase separation speed on the film surface, and the lower the thickness of the skin layer of the film produced. In the process of gelation, the casting solution is solidified into a membrane, and the contraction of the organic phase causes the generation of interfacial stress between the organic phase and the inorganic phase, which can increase the porosity of the membrane and form a proper pore structure, so that the increase of PGS-g-PDMAEMA particles also plays a role in pore-forming to a certain extent, and the porosity of the membrane is improved.

(3) And (3) pore diameter testing: the membrane pore size analyzer is used for analyzing the pore size and the distribution of the membrane by adopting a liquid-liquid displacement method, an isobutanol-water system is used in the experiment, and a saturated water phase is used as a mobile phase. Before testing, a membrane with a certain diameter needs to be cut, the membrane is soaked in a saturated alcohol phase solution for one day and then is tested, at least 5 samples are tested on each membrane, and an average value is obtained.

(4) And (3) porosity testing: the porosity of the film was analyzed by dry and wet methods. Shearing a wet film with a certain size, quickly wiping off water attached to the surface of the film by using filter paper, weighing to obtain the mass of the wet film, placing the wet film in the air for at least two days until the wet film is completely and naturally dried, and weighing to obtain the mass of a dry film. To reduce error, at least 9 samples were tested per film and the average calculated. The porosity was calculated by the following equation.

εThe porosity (%) of the membrane is referred to as,m ₁andm ₂respectively refer to the quality of the wet film and the dry film,ρ _wis the density of pure water (20 ℃, 1 g/cm)³）；ρ _mIs the density of the membrane (depending on the specific gravities and respective densities of PVDF and PGS, whereinρ _PVDF= 1.79 g/cm³；ρ _PGS= 2.05 g/cm³）。

The corresponding overall film thickness, skin thickness, average pore size, porosity data are shown in the following table:

figure 8 is a graph of pore size distribution for different membranes.

(5) Thermal stability of the film: performing thermal stability analysis on the membrane by thermogravimetric analysis (TG) and a thermogravimetric analyzer, wherein nitrogen is used as a protective atmosphere, the heating speed is 10 ℃/min, and the temperature range is 25-1000 ℃.

Fig. 9 is a thermogravimetric plot of pure PVDF ultrafiltration membranes and mixed membranes. As can be seen, the percentage of residual PVDF/P0 is the lowest, the percentage of residual PVDF/P1 is the highest, and the percentage of residual PVDF/P4 is slightly less than PVDF/P1. This is probably because as the amount of modifier was increased, the phase separation speed was increased, and more PVDF-g-PDMAEMA particles were carried out of the pore structure with the solvent flowing out during the film casting liquid phase separation, forming larger pores. The residual mass is mainly derived from the PVDF material, so the residual mass difference between PVDF/P0 and other composite films is related to the content of the PVDF-g-PDMAEMA particles serving as the modifying additive.

(6) Testing the mechanical strength of the film: the mechanical strength test includes elongation and tensile strength test, the dry film is cut into a rectangle with a certain size, and two ends of the film are respectively fixed on a tensile machine to enable the film to be in a natural straightening state. The actual test size of the sample was 10 mm × 50 mm, and the tensile speed of the tensile machine was set to 10 mm/min. The elongation and tensile strength calculations were performed using the following formulas. At least 5 samples of each film were prepared and tested, and the results averaged.

WhereinσtTensile strength (MPa);Fis the maximum load (N);bfilm sample width (mm);dis the film sample thickness (mm) (measured by section electron microscopy);Δcthe length (mm) of the sample when the sample is stretched at the moment of breaking during stretching;L ₀is the initial length (mm) of the sample.

The following table shows the mechanical strength test data for the pure PVDF film and the mixed film.

As can be seen from the table, the tensile strength and elongation of the film are improved with the increase of the amount of the modifier, the tensile strength of the PVDF/P7 mixed film is maximum and reaches 1.898 MPa, and the elongation of the PVDF/P4 is maximum and is 52.4%. This is probably because inorganic particles are added, and PDMAEMA molecular chains grafted on the surface of attapulgite are entangled with PVDF molecular chains during film formation, so that the toughness of the mixed film is stronger than that of a pure film, and the tensile strength of the film becomes higher, while the addition of inorganic particles simultaneously makes the film brittle, so that the elongation of the mixed film is reduced to some extent.

(7) Contact angle of film: contact angle is an important measure of the degree of wetting and contact angle is an important criterion for determining whether a substance is hydrophilic or not. In general principle, water forms an angle θ on the surface of the film, i.e. a contact angle. And (3) carrying out a static contact angle test on the surface of the membrane by using a pendant drop method, carrying out a dynamic contact angle test by using a DropMeter A-100p, wherein the size of each water drop is 2 mu L, the test time of the dynamic contact angle is 200 s, and each membrane is tested at least at 5 different positions in order to reduce errors. When the contact angle test is carried out on pure PVDF and the mixed membrane, as shown in FIG. 10, the contact angle of PVDF/P0 is 91.46 degrees, and after PGS-g-PDMAEMA particles are added, the contact angle of the mixed membrane is reduced, and the hydrophilicity of the membrane is improved. The contact angle of the PVDF/P1 membrane is the smallest, and the hydrophilicity is the best at 75.81 degrees. The membrane was further tested dynamically to study its permeability, and figure 11 is a screenshot during the dynamic contact angle determination, corresponding contact angle data being obtained by software analysis. As can be seen from FIG. 12, the mixed ultrafiltration membrane doped with PGS-g-PDMAEMA particles showed a faster decrease in contact angle than the pure membrane, indicating that the permeability of the membrane was improved. After 200 s, the contact angle of the PVDF/P1 membrane was about 20 ℃ lower than that of PVDF/P0, again indicating that the modified membrane had a greater increase in hydrophilicity. This is because in the phase inversion process, due to the induction effect of water, the hydrophilic chains are isolated and enriched towards the membrane surface, and the dimethylamino hydrophilic groups possessed by PGS-g-PDMAEMA are partially enriched on the membrane surface in the phase inversion process, so that the hydrophilicity of the membrane is enhanced.

(8) And (3) pure water flux test: the pure water flux test was performed using a Millipore model 8400 cup-style ultrafiltration cup using a dead-end filtration mode. At 25 ℃, the membrane is pre-pressed for 30 min under the pressure of 0.2 MPa, then the test is carried out under the pressure of 0.1 MPa, and the pure water flux is calculated by measuring the volume of penetrating fluid per minute. The pure water flux calculation formula is as follows:

whereinJShows the pure water flux (L.m)^-2·h^-1)；VRepresents permeate volume (L);Arepresents the effective membrane area (m)²)；tIndicates the filtration time (h).

FIG. 13 shows pure water fluxes of pure PVDF membrane and mixed membrane at different addition levels, and it can be seen that the pure water flux of PVDF/P0 membrane is at least about 123.28L/(m)²h) With the addition of the modifier PGS-g-PDMAEMA particles, the pure water flux gradually increased, and the pure water flux of the PVDF/P7 membrane was the maximum, about 271.23L/(m)²h) Twice as much as PVDF/P0 membrane. By combining electron microscope image analysis, the PVDF/P0 film surface contains a large number of defect holes, the number of the defect holes is small, the film hole diameter of the PGS-g-PDMAEMA particle is uniform, and the number of the film holes is increased. After PGS-g-PDMAEMA particles are added, the viscosity of the casting solution is increased, the mass transfer speed between a solvent and a non-solvent is reduced in the phase conversion film forming process, and the shrinkage difference between PVDF and PGS-g-PDMAEMA particles can cause the increase of film pores and porosity, so that the permeability of the film is improved. Furthermore, due to the hydrophilic nature of the PGS-g-PDMAEMA particles, PG is formed during the phase inversion processThe part of the dimethylamino hydrophilic group possessed by the S-g-PDMAEMA is enriched on the surface of the membrane, so that the hydrophilicity and the permeability of the membrane are enhanced.

(9) BSA static adsorption: a static adsorption experiment was performed using a phosphate buffer solution with a pH =7.4 to prepare a BSA solution with a mass concentration of 1 g/L. The wet membrane was cut into 15 mm x 60 mm size rectangles, soaked in phosphate buffer to balance the membrane surface charge, and after half an hour removed and transferred to plastic tubes containing 20 mL of BSA solution. The plastic test tube was sealed and placed in a 25 ℃ water bath shaker, and after 5 hours of adsorption, the absorbance of the solution in the tube was measured at a wavelength of 278 nm. At least four samples of the film were tested under each condition. The adsorption amount was calculated by the following formula.

Wherein the content of the first and second substances,q _srepresents the amount of static adsorption (. mu.g. cm) of the sample to be measured^-2)；C ₀AndC’respectively representing the concentration of the BSA solution in the initial state and the concentration (g/L) of the solution after static adsorption;Arepresents the area (m) of the wet film sample²)。

FIG. 14 is a static adsorption of pure PVDF membrane and mixed membrane. It can be seen from the figure that the adsorption capacity of the PVDF composite ultrafiltration membrane added with the modifier is less than that of the pure PVDF membrane, which shows that the anti-pollution performance of the membrane is enhanced by adding the modifier. The main reason is that in the static adsorption process, the modified attapulgite PGS-g-PDMAEMA gradually migrates to the membrane surface in the phase inversion process, a hydrophilic layer is formed on the membrane surface layer, so that certain energy is required for the hydrophobic BSA solute to destroy such a water layer, and the amount of BSA adsorbed on the membrane surface is reduced. Wherein the adsorption capacity of PVDF/P1 is the minimum, and is about 256.06 mg-cm^-2(ii) a The PVDF/P7 membrane had a slightly smaller static adsorption than PVDF/P4, probably due to the loss of PGS-g-PDMAEMA particles with the solvent flow out during the phase inversion process, and the reduction of modifier PGS-g-PDMAEMA migration to the membrane surface, consistent with the thermogravimetric results of the membrane.

(10) Membrane filtration BSA experiment: a Model 8400 type cup ultrafilter from Millipore, USA was used for the experiment. After the wet membrane is arranged in an ultrafilter, prepressing for half an hour under 0.2 MPa by using wakhaha purified water, adjusting the pressure to 0.1 MPa, replacing the feed liquid with 100 mL of BSA solution to carry out an ultrafiltration experiment (the mass concentration is 1 g/L, and the solvent is phosphoric acid buffer solution with pH = 7.4), collecting the volume of the penetrating fluid at different times, finishing protein filtration after 60 min, respectively sampling the penetrating fluid and the residual BSA to measure the absorbance value, and determining the protein concentration.

Protein retention was calculated by the following formula:

whereinRThe retention rate (%) is shown;C _prepresents the permeate concentration (g/L) after one hour of filtration;C ₀as initial concentration (g/L) of the raw material liquid

The dynamic adsorption quantity is defined as the quantity of BSA adsorbed by a unit membrane area per milliliter of penetrating fluid, and can reflect the protein adsorption condition of a membrane in a filtered protein solution and also reflect the blockage condition of a filter cake layer and pores laterally. The dynamic adsorption amount was calculated by the following formula.

Whereinq _dRepresents the dynamic adsorption (. mu.g. cm)^-2·mL^-1)；C ₀ 、C _pAndC ₂respectively showing the initial concentration of the raw material solution, the concentration of the penetrating fluid and the concentration (g/L) of the residual BSA solution;V、V ₁andV ₂respectively representing the initial volume of the raw material liquid, the volume of the penetrating fluid and the volume of the residual solution in the ultrafiltration cup;Adenotes the area (m) of the film sample²)。

The modified pollution index MFI is used for further analyzing the pollution condition of the membrane, and can reflect the pollution tendency of the membrane in the separation process, and the larger value of the pollution index MFI indicates the larger pollution tendency. According to the formula inThe following formula is firstly madet/V - VCurves, and then calculates the MFI.

AIs the membrane area;Vis the permeate volume;Δpis the transmembrane pressure difference;R _mresistance of the membrane itself;R _ctotal resistance to membrane face contaminants;μthe viscosity of the feed liquid is shown;tthe filtration time is indicated.

Membranes were filtered with BSA as contaminant and the permeation flux was examined as a function of time during the experiment. As shown in fig. 15, the permeation flux of the membrane decays with time, and finally tends to be stable, the flux drops fastest 5 min before filtration, and the permeation flux decays slowly by 20 min, and the last 30 min is the flux stable period. As can be seen from the figure, the permeation flux decay rate of the mixed membrane is smaller than that of a pure PVDF ultrafiltration membrane, and the stable permeation flux gradually increases with the increase of the addition amount of the modifier, and the flux decay rate is slow. The reason is that the surface of the membrane can adsorb part of BSA (bovine serum albumin) along with the change of time, the pores of the membrane are blocked to gradually form a filter cake layer, and the addition of the modified attapulgite PGS-g-PDMAEMA can improve the hydrophilicity of the membrane and enhance the anti-pollution performance of the membrane. As the addition amount of PGS-g-PDMAEMA is increased, the pore diameter and the porosity of the membrane are correspondingly increased. FIG. 16 shows the stable permeation flux of membrane-filtered BSA, the rejection rates of PVDF/P0 and PVDF composite ultrafiltration membranes in different addition amounts are both 100%, the stable flux of PVDF/P7 membrane is nearly twice that of PVDF/P0 membrane, and the water flux is increased under the condition of ensuring that the rejection rates are not changed. In the above filtration, the retention rate of BSA by each PVDF membrane was 100%.

FIG. 17 shows the amount of BSA adsorbed dynamically on the membrane surface during BSA filtration. It can be seen from the figure that the dynamic adsorption capacity of the pure PVDF membrane is obviously higher than that of the mixed matrix ultrafiltration membrane added with the modified attapulgite, which indicates that in the process of filtering BSA solution, the pores and the surface of the pure PVDF membrane can adsorb BSA molecules more easily, and the blockage of the pores and the accumulation on the surface of the membrane are serious. This is similar to the trend of static adsorption.

The modified pollution index MFI is used to analyze the pollution index of the membrane, as shown in fig. 18, the permeation flux V is plotted against the abscissa as t/V, and the slope in the linear region is the modified pollution index, which indicates the magnitude of the pollution tendency of the membrane in the process of separating the pollutants. It is evident from the figure that pure PVDF ultrafiltration membranes have the greatest slope, the greatest MFI index and the greatest tendency to fouling. The MFI index is gradually reduced along with the increase of the addition amount of the modified attapulgite, and the pollution tendency is gradually reduced in the process of separating and filtering BSA, which further shows that the pollution degree of the ultrafiltration membrane in the process of separating pollutants can be reduced by the addition of the modified attapulgite.

(11) Comparative experiment of easy-to-clean performance of the film: cleaning mode of the membrane: the effect of different cleaning modes of the contaminated membrane after BSA filtration on flux recovery was examined by washing the contaminated membrane with pure water, pH =10.35 and pH =4.03 solutions three times each, using ultrafiltration cup Millipore 8400 filters, and washing with solutions of different pH supplemented with ultrasonic washing. Washing with 20 mL of liquid for three times, each time for 10min, setting the rotation speed of the filter at 600 r/min, the ultrasonic time at 3 min, and the intensity at 40%. The flux recovery after washing was calculated using the following formula.

FRR The flux recovery (%) was expressed;J _w1andJ _w2the pure water flux (L.m) of the membrane before filtration and after washing are shown^-2·h^-1)。

And (3) examining the easy cleaning performance of the membrane, respectively cleaning with pure water, alkali with pH =10.35, acid with pH =4.03, adjusting different pH values and assisting with ultrasound, and cleaning a pure PVDF ultrafiltration membrane and a modified attapulgite mixed matrix ultrafiltration membrane PVDF/P4 with the addition of 4%. As shown in FIG. 19, the PVDF/P4 membrane has higher flux recovery rate after membrane cleaning than pure PVDF membrane in different modes, and the best effect is obtained by adjusting different pH values and adopting an ultrasonic cleaning mode. The result shows that the flux recovery rate of the pure membrane is the highest under the mode of adjusting different pH values, cleaning and assisting with ultrasound, and reaches 24 percent, while the flux of the PVDF/P4 mixed matrix ultrafiltration membrane can be recovered to 43.19 percent, which is approximately twice of that of the pure membrane. The pure PVDF membrane and the mixed matrix ultrafiltration membrane with different amounts of the modifier were cleaned in a cleaning manner with the highest flux recovery rate, and the flux recovery rate was measured, and the results are shown in fig. 20. As can be seen from the figure, the flux recovery rate of the mixed ultrafiltration membrane is greater than that of PVDF/P0, and the flux recovery rate of PVDF/P1 is the largest and reaches 54.17%, because the contact angle of PVDF/P1 is the smallest, the hydrophilicity is the best, and the roughness of the membrane surface is smaller, so the pollution resistance is the best, which is consistent with the trend of dynamic adsorption of the membrane.

(12) Membrane fouling resistance analysis: introduction of protein solution permeation fluxJpPure water flux of the membrane before filtration and after cleaningJ _w1AndJ _w2total resistance to contaminationR _tReversible contamination resistanceR _rAnd irreversible fouling resistanceR _irAnd (4) carrying out membrane fouling resistance analysis. After the protein solution is filtered, membrane cleaning is carried out, the pollution which can be removed in the cleaning process is called reversible pollution, the part of the pollution is mainly organic macromolecules and proteins deposited on the membrane surface, and the rest is called irreversible pollution, and is mainly denatured proteins adsorbed on the membrane surface and proteins blocked in membrane pores due to strong hydrophobic effect.

Total resistance to contaminationR _t

Reversible resistance to contaminationR _r

Irreversible resistance to contaminationR _ir

As can be seen from fig. 21, the total contamination resistance of the pure membrane and the composite membrane at different addition amounts was not much different, while the irreversible contamination index of the pure PVDF membrane was the largest, which was about 73.81%, indicating that the pure membrane was mainly affected by irreversible contamination during the BSA filtration process and the flux recovery rate was the lowest. The mixed matrix ultrafiltration membrane added with the modifier PGS-g-PDMAEMA has irreversible pollution index ratio smaller than that of PVDF/P0, and is beneficial to flux recovery. Among them, PVDF/P1 has the lowest irreversible fouling index, about 44.17%, and the highest flux recovery rate.

Claims

1. The application of the modified attapulgite in improving the porosity of the PVDF ultrafiltration membrane is characterized in that the surface of the modified attapulgite has a structure shown in a formula (I):

(I) (ii) a Wherein n is any integer between 1 and 10000;

in the application, the method also comprises the following steps:

step 1, grafting a coupling agent containing C = C bonds on the surface of attapulgite; step 2, carrying out polymerization reaction on N, N-dimethylaminoethyl methacrylate and the coupling agent grafted attapulgite obtained in the step 1; step 3, adding the modified attapulgite obtained in the step 2 into PVDF membrane casting solution, and preparing an ultrafiltration membrane by a phase inversion method;

in the step 1, the coupling agent containing a bond of C = C is a silane coupling agent MPS; the weight to volume ratio of attapulgite to coupling agent containing C = C bonds was 12 g: 5-15 mL;

in the step 3, the PVDF casting solution contains PVDF, an organic solvent and a pore-foaming agent, and the mass ratio of the modified attapulgite to the PVDF, the organic solvent and the pore-foaming agent is as follows: 0.05-0.42: 10-14: 1-5: 30-60.

2. The application of the method as claimed in claim 1, wherein in the step 1, the grafting reaction is carried out under the condition of an organic solvent, the organic solvent is toluene, the reaction temperature is 55-70 ℃, the reaction time is 2-6 h, and after the reaction is finished, a product is washed and dried.

3. The application of the attapulgite modified by the N, N-dimethylaminoethyl methacrylate as the component 2 is characterized in that the weight ratio of the N, N-dimethylaminoethyl methacrylate to the attapulgite grafted by the coupling agent is 0.5-6: 2; the polymerization reaction temperature is 75-85 ℃, the reaction time is 1-5 h, and after the reaction is finished, the product needs to be washed and dried.

4. Use according to claim 2, wherein the organic solvent is triethyl phosphate; the pore-foaming agent is PEG; in the step 3, the phase inversion method is to form a film by scraping PVDF casting film liquid on a flat plate and adopting an immersion precipitation phase inversion method.