CN112777689A - Method for efficiently treating oil-in-water emulsion by using super-hydrophilic nanofiber membrane with gradient structure - Google Patents
Method for efficiently treating oil-in-water emulsion by using super-hydrophilic nanofiber membrane with gradient structure Download PDFInfo
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Abstract
The invention provides a method for efficiently treating oil-in-water emulsion by using a super-hydrophilic nanofiber membrane with a gradient structure, which comprises the specific preparation steps of preparing the super-hydrophilic nanofiber membrane with the gradient structure through electrostatic spinning, filtering distilled water under the action of gravity for prewetting, then directly filtering the oil-in-water emulsion for oil-water separation, and directly reusing a filter membrane after being simply cleaned by using clean water. The invention aims at that the oil-in-water emulsion with stable surface active agent and droplet size from micron to nanometer can realize high-efficiency oil-water separation, the low-pressure driving can reach extremely high treatment flux, the oil retention rate is 92.51-99.52%, and the effect is not obviously attenuated after reusing for 6 times; the invention uses the super-hydrophilic nanofiber membrane material with the gradient structure to realize high-efficiency oil-water separation on various nanoscale oil-in-water emulsions, and has the advantages of convenient material preparation, mild operation conditions, high oil interception rate and treatment flux, strong pollution resistance and wide applicable treatment object range.
Description
Technical Field
The invention belongs to the technical field of oily wastewater treatment, and particularly relates to a method for efficiently treating an oil-in-water emulsion by using a super-hydrophilic nanofiber membrane with a gradient structure.
Background
The consumption of high-quality metal working fluid is increased along with the rapid development of the precision machining industry in China, the metal working fluid becomes waste emulsion after being recycled until the metal working fluid is invalid, and the waste emulsion has extremely high environmental pollution and ecological risk and belongs to dangerous waste (code HW 09). The waste emulsion of mechanical processing belongs to oil-in-water emulsion, contains a large amount of mineral oil and surfactant, oil drops in the emulsion are stably dispersed in water phase in micro/nano-scale liquid drops under the wrapping effect of the surfactant, the structural stability is extremely strong, and the difficulty of emulsion breaking and oil-water separation is greatly increased.
In recent years, the membrane separation technology is considered to be one of the methods for efficiently treating the emulsion wastewater due to the advantages of simple and convenient operation, good oil-water separation effect, low energy consumption, small medicament dosage, small secondary hazardous waste yield and the like. However, the existing membrane separation technology for treating oily wastewater mainly utilizes the interception effect of pore size screening to remove oil substances, and in order to realize effective interception of nano-scale oil drops, ultrafiltration membranes and even nanofiltration membranes are often used for filtration and separation. For example, the published chinese patent application CN 111701461a uses a polyacrylonitrile hydrogel ultrafiltration membrane prepared by a phase inversion method to process the emulsion, the membrane pores of the separation layer of the membrane material are dense, the porosity is relatively low, when processing 1000mg/L of soybean oil-water emulsion stabilized by sodium dodecyl sulfate, firstly, the emulsion needs to be pre-pressed with ultrapure water for 1h under the driving of high pressure of 2bar (200kPa), then the emulsion is filtered under the pressure of 1bar, the filtration flux of the pure water is between 500 LMH/bar and 600LMH/bar, and the processing flux of the emulsion is only about 370 LMH/bar; in addition, oil drops in the waste emulsion wastewater are easy to adhere to and block the surfaces and pore channels of the water-filtering membrane materials, irreversible membrane pollution is caused to the water-filtering membrane materials, severe membrane pollution fatally strikes the performance of the materials, the treatment performance and the service life of the materials are reduced, especially the influence on flux is caused, and the permeation flux after the membrane is severely polluted may be attenuated to only 2% -10% of the original flux. High pressure actuation and severe membrane fouling and flux attenuation severely limit the practical application of membrane separation technology in emulsion wastewater treatment.
From the limited factors of the membrane separation technology, the structural characteristics and the membrane surface properties of the membrane material have important influence on the separation performance, and the adjustment of the structural characteristics and the improvement of the anti-pollution performance of the membrane surface are of great importance for improving the oil-water separation efficiency. In recent years, much attention is paid to the preparation of the nanofiber membrane by using an electrostatic spinning method, the electrostatic spinning method is simple and convenient to synthesize, time-saving and labor-saving, the nanofiber membrane can be prepared, and the fiber structure is flexible and controllable; compared with a straight-hole membrane, the electrostatic spinning nanofiber membrane has mutually communicated pore diameter channels, is high in porosity, can reduce mass transfer resistance and scaling tendency, and is high in permeation flux; furthermore, in order to improve the anti-pollution performance of the membrane surface, researchers can modify or functionally load the surface of the fiber membrane to improve the hydrophilicity, or couple other technologies to improve the anti-pollution performance of the material, so that the phenomena of oil drop adhesion and membrane pollution can be effectively avoided, and the method has important significance in improving the treatment flux and the oil interception rate of the material. However, the current surface modification, functional modification or coupling technology is usually accompanied with a complicated modification method and loading process, and the research on directly preparing the membrane only by adjusting basic working parameters to regulate and control the structure and properties of the fiber membrane so as to construct the ultra-wetting membrane with good separation performance is less.
The super-hydrophilic membrane can realize high-efficiency oil-water separation based on superior structure and property characteristics, but at present, the processing objects of the super-hydrophilic membrane are mainly oil-water mixtures and simple oil-in-water emulsions, the oil phase viscosity of the emulsions is low, mineral oil with high viscosity is rarely involved, the concentration of the surfactant is low, even the surfactant is not contained, and the particle size of liquid drops is mainly distributed in a micron-scale range. For example, the oil phase of the emulsion treated by the super hydrophilic membrane disclosed in the Chinese patent CN 111871238A is toluene and cyclohexane with low viscosity, and does not contain surfactant, and the particle size distribution of the droplets is in the range of 5-30 μm. The use of the superhydrophilic nanofiber membrane for realizing efficient and rapid oil-water separation of the surfactant-containing nanoscale emulsion still has certain difficulty and challenge, and the superhydrophilic nanofiber membrane is rarely involved in the field of oil-water separation of actual waste emulsion, and needs to be researched and applied to popularization urgently.
Disclosure of Invention
Aiming at the defects, the invention provides a method for efficiently treating oil-in-water emulsion by using a super-hydrophilic nanofiber membrane with a gradient structure.
The invention provides the following technical scheme: a method for efficiently treating an oil-in-water emulsion by using a super-hydrophilic nanofiber membrane with a gradient structure comprises the following steps:
(1) selecting a stainless steel mesh with 500 meshes, sequentially performing ultrasonic treatment for 5min by using acetone, ethanol and distilled water, and drying for later use to serve as a receiving substrate on an electrostatic spinning roller;
(2) respectively dissolving polyacrylonitrile powder in N, N-dimethylformamide, and magnetically stirring at 500rpm at room temperature for 12h to prepare an electrostatic spinning working solution with the first concentration of the N, N-dimethylformamide dissolved with the polyacrylonitrile and an electrostatic spinning working solution with the second concentration of the N, N-dimethylformamide dissolved with the polyacrylonitrile, wherein the concentration of the polyacrylonitrile in the electrostatic spinning working solution with the first concentration is 8%, and the concentration of the polyacrylonitrile in the electrostatic spinning working solution with the second concentration is 3% -3.5%;
(3) spinning the electrostatic spinning working solution with the first concentration on the receiving substrate by using an electrostatic spinning machine to obtain the receiving substrate with the electrostatic spinning working solution film surface with the first concentration;
(4) spinning the second-concentration electrostatic spinning working solution on the receiving substrate with the first-concentration electrostatic spinning working solution membrane surface obtained in the step (3) by using an electrostatic spinning machine to obtain a super-hydrophilic nanofiber membrane with a gradient structure;
(5) placing the super-hydrophilic nanofiber membrane with the membrane surface diameter of 35mm obtained in the step (4) in a dead-end filtering device, pouring 200mL of distilled water into a filter cup, and enabling liquid to infiltrate and penetrate through the membrane surface of the super-hydrophilic nanofiber membrane under the action of gravity;
(6) pouring 20mL of oil-in-water emulsion into a filter cup, and filtering the oil-in-water emulsion under the drive of low negative pressure of vacuum filtration of 1.0 kPa-5.0 kPa for oil-water separation;
(7) and (3) taking out the super-hydrophilic nanofiber membrane after oil-water separation, simply washing for 1-3 times after soaking in clear water for several seconds, and directly repeating the steps (5) - (6) to filter the oil-in-water emulsion by using the washed super-hydrophilic nanofiber membrane.
Further, the working voltage of the electrostatic spinning machine in the step (3) is 25kV, the rotating speed of the roller is 50rpm, the length of a scanning path of a spinning needle is 200mm, the distance between the needle and the roller is 20cm, the liquid supply speed is 1.0mL/h, and the spinning time is 4 h.
Further, in the step (4), the working voltage of the electrostatic spinning machine is 25kV, the rotating speed of the roller is 50rpm, the length of a scanning path of a spinning needle is 200mm, the distance between the needle and the roller is 20cm, the liquid supply speed is 1.0mL/h, and the spinning time is 4 h.
Further, the super-hydrophilic nanofiber membrane with the gradient structure is provided with a first layer of fiber membrane and a second layer of fiber membrane from outside to inside, the pore size distribution range is 0.4-1.3 mu m, and the first layer of fiber membrane is a nanofiber membrane which is formed by spinning a second concentration of electrostatic spinning working solution on the receiving matrix with the first concentration of electrostatic spinning working solution membrane surface and comprises a spindle body and beads; the second layer of fiber membrane is a nanofiber membrane formed by spinning electrostatic spinning working solution with first concentration on the stainless steel mesh receiving matrix, and the first layer of fiber membrane has a micro-nano multistage coarse structure; the thickness of the super-hydrophilic nanofiber membrane with the gradient structure is 20.28 +/-1.00 mu m, the thickness of the membrane layer of the first layer of fiber membrane is 1.3 mu m-1.9 mu m, and the thickness of the membrane layer of the second layer of fiber membrane is 17.1 mu m-20.0 mu m; the first layer of fiber membrane and the second layer of fiber membrane of the super-hydrophilic nanofiber membrane with the gradient structure play different roles and cooperatively realize the high-efficiency oil-water separation of oil-in-water emulsion; the first layer of fiber membrane plays a role in efficiently intercepting nano-scale liquid drops in the emulsion; the second layer of fiber membrane has a certain mechanical supporting function and ensures rapid water passing and higher permeation flux.
Further, the width of a spindle body in the first layer of fiber membrane is 100 nm-500 nm, the length of the spindle body is 1 μm-2 μm, and the diameter of a bead ball in the first layer of fiber membrane is 1 μm-2 μm; the surface roughness Ra of the first layer of fiber membrane micro-nano multilevel rough structure is within the range of 21nm to 50 nm.
Further, the fiber diameter of the first layer of fiber membrane is 40 nm-150 nm, and the pore diameter range is 0.4 μm-0.7 μm.
Further, the super-hydrophilic nanofiber membrane with the gradient structure has super-hydrophilic and underwater super-oleophobic characteristics, the water contact angle of the super-hydrophilic nanofiber membrane is 16-28 degrees, and water drops completely infiltrate and permeate the super-hydrophilic nanofiber membrane after 5s, at the moment, the water contact angle is 0 degree; the underwater oil contact angle of the super-hydrophilic nanofiber membrane is 165-180 degrees, and the super-oleophobic state of more than 165 degrees is still kept after 10 minutes; the pure water filtration flux is 5000 LMH-6600 LMH, the oil invasion resistant pressure is 5 kPa-10 kPa, and the filter material has better permeability and oil invasion resistant characteristics.
Further, the treated oil-in-water emulsion is micron-sized or nano-sized emulsion with the droplet size smaller than the pore size of the super-hydrophilic nanofiber membrane, and the droplet size is 200 nm-10000 nm.
Further, the oil-in-water emulsion to be treated comprises laboratory-prepared oil-in-water model emulsion, complex waste emulsion generated in the field of actual machining, emulsion stabilized by anionic surfactant, emulsion stabilized by nonionic surfactant and oil-in-water emulsion stabilized by cationic surfactant; the oil phase of the oil-in-water model emulsion comprises low-viscosity short-chain alkane, hydrocarbon and high-viscosity mineral oil, and the concentration range of the surfactant is 0.1-3.0 g/L.
Further, the oil phase of the oil-in-water model emulsion comprises dichloromethane, trichloromethane, carbon tetrachloride, petroleum ether, hexadecane, soybean oil, liquid paraffin, vacuum pump oil, engine oil and the like, and the concentration of the oil phase is 1000ppm to 10000 ppm.
The invention has the beneficial effects that:
(1) in material preparation, the super-hydrophilic nanofiber membrane with the gradient structure is prepared by an electrostatic spinning method, the synthesis operation is simple and convenient, the time and the labor are saved, the consumption of the preparation raw materials is low, and the preparation method is green and economical; in addition, it is worth noting that: according to the preparation method, the flexible regulation and control of the fiber structure are realized only by changing the concentration gradient of the Polyacrylonitrile (PAN) working solution, so that a unique gradient structure with strong compatibility is constructed, and a micro-nano multi-level rough surface is created, thereby forming the super-hydrophilic nanofiber membrane with the gradient structure.
(2) From the aspect of treatment method, the method for treating the oil-in-water emulsion has mild operation conditions. The membrane material is pre-wetted by filtering distilled water only under the action of gravity, and then the oil-in-water emulsion is filtered, the driving pressure for realizing oil-water separation of various emulsions is only 1.0 kPa-5.0 kPa, the operation parameters are flexible and not strict, the operation is simple and convenient, the energy consumption is low, and the driving force is small.
(3) From the aspect of oil-water separation effect, the pure water filtration flux of the super-hydrophilic nanofiber membrane with the gradient structure is up to 5000-6600 LMH, the oil retention rate of oil-in-water emulsification stable to a surfactant is 92.51-99.52%, the treatment flux is 935.45-3783.78 LMH/bar, and the super-hydrophilic nanofiber membrane not only has good oil-water separation effect, but also has good permeability and extremely high treatment flux; in addition, the membrane material has the characteristics of super-hydrophilicity and super-lipophobicity under water, has extremely strong pollution resistance, has the flux recovery rate close to 100 percent, can be repeatedly used only by being simply cleaned by clear water, and has powerful practical application potential.
The super-hydrophilic nanofiber membrane with the gradient structure has the advantages that the membrane surface layer of the first layer of fiber membrane with the PAN concentration of 3% -3.5% has the functions of intercepting liquid drops and promoting liquid drop polymerization and emulsion breaking, the first layer of fiber membrane with the PAN concentration of 3% -3.5% creates a multi-stage micro-nano rough structure comprising a spindle body and a bead structure on the surface of the first layer of fiber membrane, the wettability of the membrane surface is remarkably increased based on a Cassie wetting model, so that the membrane surface has super-hydrophilic and underwater super-oleophobic properties, and rich nano-scale roughness in the micron scale of the surface can promote the collision polymerization and emulsion breaking of nano-scale emulsified liquid drops; and the second layer fiber membrane part with 8 percent of PAN concentration has a certain mechanical supporting function, and mass transfer resistance is reduced based on a highly communicated large-aperture channel and high porosity, so that rapid water passing is realized, and permeation flux is improved. In the gradient structure, the efficient interception and rapid demulsification effects of the gradient structure are combined with the mechanical support and low mass transfer resistance of the gradient structure, and the gradient structure and the mechanical support cooperatively realize the efficient oil-water separation of various nanoscale emulsions, so that the oil-water separation efficiency of the emulsions is remarkably improved.
(4) From the aspect of treatment objects and application range, the super-hydrophilic nanofiber membrane with the gradient structure has extremely wide application range, realizes high-efficiency oil-water separation on oil-in-water emulsions stabilized by various surfactants, has the surfactant concentration of 0.1-3.0 g/L, covers liquid drops with the particle size from micron-sized to nano-sized with higher treatment difficulty, and has very ideal oil-water separation effect on the nano-sized emulsions with the liquid drops with the particle size smaller than the material pore size; in addition, the method also plays an ideal oil-water separation effect aiming at the nanoscale waste emulsion with great treatment difficulty, expands the application range to the actual complex waste emulsion generated in the field of machining, and has important significance on the actual application and popularization of the super-hydrophilic nanofiber membrane.
Drawings
The invention will be described in more detail hereinafter on the basis of embodiments and with reference to the accompanying drawings. Wherein:
FIG. 1 is a schematic flow chart of a super-hydrophilic nanofiber membrane with a gradient structure prepared in example 1;
FIG. 2 is an SEM image of a super-hydrophilic nanofiber membrane with a gradient structure prepared in example 1;
FIG. 3 is a graph showing the experimental effect of the contact angle and the dynamic oil drop adhesion of the super-hydrophilic nanofiber membrane with the gradient structure prepared in example 1;
FIG. 4 is a graph showing the effect of oil-water separation of the nano-scale emulsion treated by the repeated use of the superhydrophilic nanofiber membrane having a gradient structure in example 2;
FIG. 5 is a graph showing the effect of the superhydrophilic nanofiber membrane having a gradient structure on oil-water separation of a nano-scale emulsion containing an anionic surfactant and a cationic surfactant in example 3;
FIG. 6 is a graph showing the effect of the superhydrophilic nanofiber membrane with a gradient structure on oil-water separation of nano-scale emulsion containing anionic surfactant with hexadecane as the oil phase in example 4;
FIG. 7 is a graph showing the oil-water separation effect of the superhydrophilic nanofiber membrane with a gradient structure on micron-sized emulsions containing different concentrations of surfactants in example 5;
FIG. 8 is a graph showing the effect of the superhydrophilic nanofiber membrane having a gradient structure on oil-water separation of a nano-scale actual waste emulsion in example 6;
fig. 9 is a graph comparing oil-water separation effects of the superhydrophilic nanofiber membrane having a gradient structure and the membrane material without a gradient structure in the comparative example.
Detailed description of the preferred embodiments
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The oil-in-water emulsion related in the embodiment of the invention comprises nano-scale emulsion prepared in a laboratory, micron-scale emulsion and actual waste emulsion in the field of mechanical processing, relevant parameters are shown in a table 1, and the preparation method comprises the following steps:
(a) surfactant-stabilized nano-emulsion: 500. mu.L of liquid paraffin and 0.05g of an anionic surfactant (sodium dodecylsulfate, SLS) were mixed and dispersed in 99.5mL of an aqueous phase, and the resulting solution was magnetically stirred at 1500rpm for 24 hours to obtain surfactant-stabilized nano-emulsion NE 1. Replacing an anionic surfactant with a cationic surfactant (cetyl trimethyl ammonium bromide, CTAB), and preparing NE2 without changing other parameters and operating conditions; NE3 was prepared by replacing liquid paraffin with hexadecane, still using anionic surfactant, and other parameters and operating conditions were unchanged.
(b) Surfactant-stabilized micron-size emulsions: mixing 2.0g of liquid paraffin and a proper amount of sodium dodecyl sulfate (SLS), dispersing the mixture into 200mL of water phase, and stirring the obtained solution at 13000rpm for 5min to obtain a micron-sized emulsion with stable surfactant; the mass of the added sodium dodecyl sulfate is 0.1g, 0.2g, 0.4g and 0.6g respectively, and the concentration of the surfactant of the prepared emulsion is 0.5g/L, 1g/L, 2g/L and 3g/L respectively, which are sequentially marked as ME0.5, ME1, ME2 and ME 3.
(c) And selecting 3 actual waste emulsions in the field of mechanical processing, and respectively recording the emulsions as W1, W2 and W3.
TABLE 1 characteristic parameters of different oil-in-water emulsions
Example 1
Referring to fig. 1, a superhydrophilic nanofiber membrane with a gradient structure is prepared. Selecting a 500-mesh stainless steel net, cutting into a size with the length of 300mm and the width of 210mm, sequentially placing the stainless steel net in acetone, ethanol and distilled water for ultrasonic treatment for 5min respectively, and drying for later use. Weighing 1.6g of PAN and dissolving in 20mL of N, N-dimethylformamide, magnetically stirring at 500rpm for 12h at room temperature to prepare a first-concentration electrostatic spinning working solution with PAN concentration of 8%, weighing 0.7g of PAN and dissolving in 20mL of N, N-dimethylformamide, and magnetically stirring at 500rpm for 12h at room temperature to prepare a second-concentration electrostatic spinning working solution with PAN concentration of 3.5%. Fixing a stainless steel mesh on a receiving roller of an electrostatic spinning machine by using an adhesive tape, spinning working solution with 8% of PAN concentration on the stainless steel mesh, wherein the working voltage of the electrostatic spinning machine is 25kV, the rotating speed of the roller is 50rpm, the length of a scanning path of a spinning needle is 200mm, the distance between the needle and the roller is 20cm, the liquid supply speed is 1.0mL/h, and the spinning time is 4 h. Spinning by directly using working solution with the PAN concentration of 3.5% on a roller with 8% PAN fiber membrane spun on the surface, wherein the working voltage is 25kV, the rotating speed of the roller is 50rpm, the scanning path length of a spinning needle is 200mm, the distance between the needle and the roller is 20cm, the liquid supply speed is 1.0mL/h, and the spinning time is 4 h. And after spinning is finished, taking down the adhesive tape for fixing the stainless steel net to obtain the electrostatic spinning fiber membrane containing the receiving matrix, and preparing the super-hydrophilic nanofiber membrane with the gradient structure. The super-hydrophilic nanofiber membrane with the gradient structure, which is prepared by processing oil-in-water emulsion, has micron-sized large pore diameter and extremely high porosity, and the pore diameter distribution range is 0.4-1.3 mu m; the porosity ranges from 90% to 94%.
The surface and the cross section of the membrane material are observed and characterized by adopting SEM, as shown in the attached figure 2. According to SEM characterization results, the thickness of the super-hydrophilic nanofiber membrane has a gradient structure, the total thickness of the membrane layers is 20.28 +/-1.00 mu m, the average thickness of the membrane layers of the first layer of fiber membrane with 8% of PAN concentration is 18.75 mu m, and the average thickness of the membrane layers of the second layer of fiber membrane with 3.5% of PAN concentration is 1.53 mu m. The fiber diameter of the super-hydrophilic nanofiber membrane has a gradient structure, the average fiber diameter of a first layer of fiber membrane with PAN concentration of 8% is 371.5nm, and the average fiber diameter of a second layer of fiber membrane with PAN concentration of 3.5% is 84.4 nm. The surface of the super-hydrophilic nanofiber membrane has unique spindle and bead structures, the width of the spindle is between 100nm and 500nm, the length of the spindle can reach 2 mu m, and the diameter of the bead is between 1 mu m and 2 mu m. And observing and characterizing the roughness of the membrane surface by adopting AFM, wherein the surface roughness Ra of the super-hydrophilic nanofiber membrane with the gradient structure is 23.2 +/-2.3 nm.
The super-hydrophilic nanofiber membrane with the gradient structure, which is prepared for treating oil-in-water emulsion, has the characteristics of super-hydrophilicity, super-oleophobicity under water and oil drop adhesion resistance, and is shown in figure 3. The water contact angle of the film surface is 22.2 degrees, and the water drop is completely infiltrated and permeated after 5 seconds, and the water contact angle is 0 degree; the underwater oil contact angle was 172.3 ° and remained in a superoleophobic state greater than 165 ° after 10 minutes. Dynamic oil drop adhesion experiments show that after liquid paraffin is used to approach the membrane surface and is extruded, the paraffin oil is not adhered to the membrane surface and can be completely separated; the filtration flux of the pure water is between 5000LMH and 6600LMH, and the oil intrusion resistant pressure can reach 10 kPa.
Example 2
The surfactant-stabilized nano-scale oil-in-water emulsion NE1 was treated with the super-hydrophilic nanofiber membrane having a gradient structure prepared in example 1, with a driving force of 3.0kPa, and oil-water separation was continuously performed using a plurality of cycles, and oil-water separation effect, contamination resistance and reusability were examined by oil rejection rate, treatment flux and flux recovery rate. The result is shown in figure 4, the nanometer oil-in-water emulsion NE1 is treated by the method, the treatment flux is 1143.33LMH, and the flux recovery rate is as high as 99.98%; in 8 continuous use processes, the oil retention rate is 94.16-97.20%, and the percentage of treatment flux to first flux (Ji/J)0) The flux recovery rate is still more than 86% after the membrane is reused for 6 times, and the super-hydrophilic nanofiber membrane with the gradient structure has higher treatment flux, ideal oil-water separation effect and extremely strong pollution resistance, and the oil-water separation effect is not obviously attenuated after the membrane is continuously used for multiple times and can be reused for at least 6 times.
Example 3
The super-hydrophilic nanofiber membrane with the gradient structure prepared in example 1 is used for treating anionic or cationic surfactant-stabilized nanoscale oil-in-water emulsions NE1 and NE2, the driving force is 3.0kPa, and the oil-water separation effect is examined through the oil retention rate, the filtrate light transmittance and the treatment flux. As shown in the attached figure 5, the oil retention rate and the light transmittance of NE1 of the nano-scale emulsion stabilized by the anionic surfactant (SLS) are 95.68% and 94.3%, respectively, and the treatment flux is 1143.33LMH/bar, while the oil retention rate and the light transmittance of NE2 of the nano-scale emulsion stabilized by the cationic surfactant (CTAB) are 92.51% and 92.4%, respectively, and the treatment flux is 935.45 LMH/bar. Therefore, the method can achieve ideal oil-water separation effect when used for treating the nano-scale oil-in-water emulsion stabilized by the anionic/cationic surfactant, but has better effect on the nano-scale oil-in-water emulsion stabilized by the anionic surfactant.
Example 4
The anionic surfactant-stabilized nano-scale hexadecane/water-emulsion NE3 was treated by the super-hydrophilic nanofiber membrane with a gradient structure prepared in example 1, the driving force was 1.0kPa, and the oil-water separation effect was examined by the oil retention rate, the filtrate transmittance and the treatment flux. As shown in figure 6, when the original emulsion and the filtrate are compared, the method for treating the nano-scale hexadecane/water-emulsion NE3 stabilized by the anionic surfactant can realize excellent oil-water separation effect, the oil retention rate and the light transmittance of the filtrate are respectively 99.52 percent and 97.2 percent, and the treatment flux is up to 3741.81 LMH/bar.
Example 5
The super-hydrophilic nanofiber membrane with the gradient structure prepared in example 1 is used for treating micron-sized emulsion (ME 0.5-ME 3) with different concentrations of stable surfactants, the driving force is 5.0kPa, and the oil-water separation effect is examined through the oil retention rate, the filtrate light transmittance and the treatment flux. The results are shown in fig. 7, and show that the pollutant interception effect and the treatment flux of the membrane material are reduced with the increase of the concentration of the surfactant; but the oil retention rate of the super-hydrophilic nanofiber membrane on micron-sized emulsion containing a surfactant is more than 98.8 percent and even reaches as high as 99.68 percent, and the treatment flux is 1829.52-3783.78 LMH/bar, so that the super-hydrophilic nanofiber membrane with the gradient structure has an excellent oil-water separation effect on the micron-sized oil-in-water emulsion.
Example 6
The super-hydrophilic nanofiber membrane with the gradient structure prepared in example 1 was used for treating actual nanoscale waste emulsion (W1-W3) in the machining field, the driving force was 1.0kPa, and the oil-water separation effect was examined through the light transmittance and the treatment flux of the filtrate. The result shows that the membrane material can still exert a very ideal oil-water separation effect on complex actual waste emulsion, and the light transmittance of filtrate obtained after oil-water separation of three nanoscale waste emulsions is 83.1%, 98.5% and 99.5% in sequence, as shown in figure 8; besides, the treatment flux of the three nanoscale waste emulsions is 756.68 LMH/bar, 972.87LMH/bar and 4107.68LMH/bar respectively. Therefore, the super-hydrophilic nanofiber membrane with the gradient structure can realize ideal oil-water separation aiming at the nano-scale waste emulsion generated in the field of mechanical processing, and has high pollutant retention rate and treatment flux.
Comparative example
The super-hydrophilic nanofiber membrane with a gradient structure (denoted as "M1") and the membrane material without a gradient structure (denoted as "M0") obtained by spinning with PAN concentration of 3.5% were used for treating the surfactant-stabilized nano-scale oil-in-water emulsion NE1, the oil-water separation effect was examined by oil retention and filtrate transmittance, and three cycles were continuously used to compare the oil-water separation effect and reusability. The specific operation method comprises the following steps: placing the membrane material in a dead-end filter device, wherein the diameter of the membrane surface is 35mm, pouring 200mL of distilled water into a filter cup, and enabling liquid to infiltrate and penetrate through the membrane surface under the action of gravity; then pouring 20mL of emulsion NE1 into a filter cup, filtering the emulsion under the negative pressure drive of vacuum filtration of 3.0kPa, carrying out oil-water separation, measuring the oil concentration and the volume of filtrate of the filtrate, calculating the oil retention rate and the treatment flux, and representing the light transmittance of the filtrate; and taking out the membrane after oil-water separation, soaking in clear water for several seconds, and then simply washing for 1-3 times, so that the filtering oil-in-water emulsion can be directly reused. The result is shown in figure 9, the nanometer oil-in-water emulsion NE1 treated by the method has the oil retention rate of 95.68-97.20 percent, the light transmittance of the filtrate of 94.3-95.6 percent, and the oil retention rate and the transmittance can be kept extremely high after three cycles of continuous use; the oil interception rate and the light transmittance of the M0 have certain effects in the first filtration, but are inferior to those of the M1, and the oil interception rate and the light transmittance after repeated use are reduced, so that the good oil-water separation effect cannot be maintained. Therefore, the super-hydrophilic nanofiber membrane with the gradient structure has a significantly higher oil-water separation effect on the nano-scale emulsion than a membrane material without the gradient structure.
It should be understood that the detailed description of the invention is merely illustrative of the invention and is not intended to limit the invention to the specific embodiments described. It will be appreciated by those skilled in the art that the present invention may be modified or substituted equally as well to achieve the same technical result; as long as the use requirements are met, the method is within the protection scope of the invention.
Claims (10)
1. A method for efficiently treating an oil-in-water emulsion by using a super-hydrophilic nanofiber membrane with a gradient structure is characterized by comprising the following steps:
(1) selecting a stainless steel mesh with 500 meshes, sequentially performing ultrasonic treatment for 5min by using acetone, ethanol and distilled water, and drying for later use to serve as a receiving substrate on an electrostatic spinning roller;
(2) respectively dissolving polyacrylonitrile powder in N, N-dimethylformamide, and magnetically stirring at 500rpm at room temperature for 12h to prepare an electrostatic spinning working solution with the first concentration of the N, N-dimethylformamide dissolved with the polyacrylonitrile and an electrostatic spinning working solution with the second concentration of the N, N-dimethylformamide dissolved with the polyacrylonitrile, wherein the concentration of the polyacrylonitrile in the electrostatic spinning working solution with the first concentration is 8%, and the concentration of the polyacrylonitrile in the electrostatic spinning working solution with the second concentration is 3% -3.5%;
(3) spinning the electrostatic spinning working solution with the first concentration on the receiving substrate by using an electrostatic spinning machine to obtain the receiving substrate with the electrostatic spinning working solution film surface with the first concentration;
(4) spinning the second-concentration electrostatic spinning working solution on the receiving substrate with the first-concentration electrostatic spinning working solution membrane surface obtained in the step (3) by using an electrostatic spinning machine to obtain a super-hydrophilic nanofiber membrane with a gradient structure;
(5) placing the super-hydrophilic nanofiber membrane with the membrane surface diameter of 35mm obtained in the step (4) in a dead-end filtering device, pouring 200mL of distilled water into a filter cup, and enabling liquid to infiltrate and penetrate through the membrane surface of the super-hydrophilic nanofiber membrane under the action of gravity;
(6) pouring 20mL of oil-in-water emulsion into a filter cup, and filtering the oil-in-water emulsion under the drive of low negative pressure of vacuum filtration of 1.0 kPa-5.0 kPa for oil-water separation;
(7) and (3) taking out the super-hydrophilic nanofiber membrane after oil-water separation, simply washing for 1-3 times after soaking in clear water for several seconds, and directly repeating the steps (5) - (6) to filter the oil-in-water emulsion by using the washed super-hydrophilic nanofiber membrane.
2. The method for efficiently treating the oil-in-water emulsion by using the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 1, wherein the working voltage of the electrostatic spinning machine in the step (3) is 25kV, the roller speed is 50rpm, the length of a scanning path of a spinning needle is 200mm, the distance between the needle and the roller is 20cm, the liquid supply speed is 1.0mL/h, and the spinning time is 4 h.
3. The method for efficiently processing oil-in-water emulsion by using the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 1, wherein the working voltage of the electrostatic spinning machine in the step (4) is 25kV, the roller rotation speed is 50rpm, the length of a scanning path of a spinning needle is 200mm, the distance between the needle and the roller is 20cm, the liquid supply speed is 1.0mL/h, and the spinning time is 4 h.
4. The method for efficiently treating the oil-in-water emulsion by using the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 1, wherein the super-hydrophilic nanofiber membrane with the gradient structure is provided with a first layer of fiber membrane and a second layer of fiber membrane from outside to inside, the pore size distribution ranges from 0.4 μm to 1.3 μm, the first layer of fiber membrane is a nanofiber membrane which is formed by spinning the second concentration of the electrospinning working solution on the receiving matrix with the first concentration of the electrospinning working solution membrane surface and comprises a spindle body and beads; the second layer of fiber membrane is a nanofiber membrane formed by spinning electrostatic spinning working solution with first concentration on the stainless steel mesh receiving matrix, and the first layer of fiber membrane has a micro-nano multistage coarse structure; the thickness of the super-hydrophilic nanofiber membrane with the gradient structure is 20.28 +/-1.00 mu m, the thickness of the membrane layer of the first layer of fiber membrane is 1.3 mu m-1.9 mu m, and the thickness of the membrane layer of the second layer of fiber membrane is 17.1 mu m-20.0 mu m.
5. The method for efficiently processing the oil-in-water emulsion through the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 4, wherein the width of a spindle in the first layer of fiber membrane is 100 nm-500 nm, the length of the spindle is 1 μm-2 μm, and the diameter of a bead in the first layer of fiber membrane is 1 μm-2 μm; the surface roughness Ra of the first layer of fiber membrane micro-nano multilevel rough structure is within the range of 21nm to 50 nm.
6. The method for efficiently processing oil-in-water emulsion through the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 4, wherein the fiber diameter of the first layer of fiber membrane is 40 nm-150 nm, and the pore diameter is in the range of 0.4 μm-0.7 μm.
7. The method for efficiently treating the oil-in-water emulsion by using the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 4, wherein the super-hydrophilic nanofiber membrane with the gradient structure has super-hydrophilic and underwater super-oleophobic characteristics, the water contact angle of the super-hydrophilic nanofiber membrane is 16-28 degrees, and a water drop completely infiltrates and permeates the super-hydrophilic nanofiber membrane after 5s, and the water contact angle is 0 degree; the underwater oil contact angle of the super-hydrophilic nanofiber membrane is 165-180 degrees, and the super-oleophobic state of more than 165 degrees is still kept after 10 minutes; the filtration flux of pure water is 5000 LMH-6600 LMH, and the oil intrusion resistance pressure is 5 kPa-10 kPa.
8. The method for efficiently processing the oil-in-water emulsion through the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 1, wherein the oil-in-water emulsion to be processed is a micron-scale to nano-scale emulsion with a droplet size smaller than the pore size of the super-hydrophilic nanofiber membrane, and the droplet size is 200nm to 10000 nm.
9. The method for efficiently treating the oil-in-water emulsion by using the super-hydrophilic nanofiber membrane with the gradient structure as claimed in claim 1, wherein the oil-in-water emulsion to be treated comprises a laboratory-prepared oil-in-water model emulsion, a complex waste emulsion generated in the field of actual machining, an emulsion stabilized by an anionic surfactant, an emulsion stabilized by a nonionic surfactant and an oil-in-water emulsion stabilized by a cationic surfactant; the oil phase of the oil-in-water model emulsion comprises low-viscosity short-chain alkane, hydrocarbon and high-viscosity mineral oil, and the concentration range of the surfactant is 0.1-3.0 g/L.
10. The method for efficiently processing the oil-in-water emulsion through the super-hydrophilic nanofiber membrane with the gradient structure as recited in claim 9, wherein the oil phase of the oil-in-water model emulsion comprises dichloromethane, chloroform, carbon tetrachloride, petroleum ether, hexadecane, soybean oil, liquid paraffin, vacuum pump oil, engine oil and the like, and the concentration of the oil phase is 1000ppm to 10000 ppm.
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