CN113289498B - Positively charged nanofiltration membrane and preparation method thereof - Google Patents
Positively charged nanofiltration membrane and preparation method thereof Download PDFInfo
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- B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
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- B01D61/02—Reverse osmosis; Hyperfiltration ; Nanofiltration
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
- B01D69/12—Composite membranes; Ultra-thin membranes
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- B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
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- B01D2325/16—Membrane materials having positively charged functional groups
Abstract
The invention relates to a positively charged nanofiltration membrane and a preparation method thereof, wherein the positively charged nanofiltration membrane comprises a supporting base membrane and a polyamide functional separation layer, and the polyamide functional separation layer is prepared by carrying out polymerization reaction on the surface of the supporting base membrane by using a water-phase solution containing polyamine and an oil-phase solution of polyacyl chloride; the positively charged nanofiltration membrane also comprises a protective layer, wherein a compact membrane layer is formed on the surface of the polyamide functional separation layer by using polyvinyl alcohol and glutaraldehyde to carry out a cross-linking reaction. The invention also discloses a preparation method of the positively charged nanofiltration membrane, which comprises the steps of preparing the water phase solution and the oil phase solution, preparing the polyamide functional separation layer by interfacial polymerization of the water solution and the oil phase solution, preparing the protective layer, and cleaning with deionized water to obtain the positively charged nanofiltration membrane. The preparation method is simple and easy to control, the raw materials are easy to obtain, and the cost is low. The positive charge nanofiltration membrane prepared by the invention has higher rejection rate on multivalent cations, and the stability of positive charge is greatly improved.
Description
Technical Field
The invention relates to the technical field of nanofiltration composite membranes, in particular to a positively charged nanofiltration membrane and a preparation method thereof.
Background
Nanofiltration is a membrane separation technology between ultrafiltration and reverse osmosis, has the advantages of low operation pressure, high separation efficiency, low operation cost and the like, is widely applied to the fields of drinking water preparation, seawater desalination, brackish water, sewage treatment, biopharmaceuticals, food, chemical engineering and the like, and plays an important role in recycling domestic sewage and industrial wastewater. At present, the market mostly gives priority to negatively charged nanofiltration membranes, but with the increase of the complexity of the separation system and the improvement of the requirements on the membrane separation performance, such as the separation of multivalent cations and some small cationic molecules, the negatively charged nanofiltration membranes have poor effects, and therefore, positively charged nanofiltration membranes are more and more paid attention by researchers.
The method mainly comprises two different modes of preparing the positively charged nanofiltration membrane, namely, the polysulfone nanofiltration membrane is modified by adopting an ultraviolet light grafting method, namely, a monomer is introduced under a certain condition, and double bonds in the monomer can be subjected to polymerization reaction with the polysulfone membrane under the irradiation of ultraviolet light to prepare the positively charged nanofiltration membrane; and secondly, adopting an interfacial polymerization method to prepare the positively charged nanofiltration membrane by using a positively charged polymer, for example, preparing the positively charged nanofiltration membrane by using quaternized chitosan and derivatives thereof. However, the first method, the ultraviolet grafting technology, is difficult to realize industrialization, and the effect of improving positive charge in the process of grafting monomers is not obvious and complicated, and is not beneficial to industrialization; the nanofiltration membrane prepared by the second method has low water flux and is not suitable for commercial production. Although different research on preparation of positively charged nanofiltration membranes exists, according to the results reported by the research, the effect of positive charge is not obvious, the property of positive charge of the prepared positively charged nanofiltration membrane in the operation process is unstable or even disappears, and the prepared positively charged nanofiltration membrane has large fluctuation on the separation effect of multivalent cations or small cationic molecules, so that the industrial production of the positively charged nanofiltration membrane is restricted.
Disclosure of Invention
Technical problem to be solved
In view of the defects and shortcomings of the prior art, the invention provides a positively charged nanofiltration membrane and a preparation method thereof, and solves the technical problems that the preparation and use stability of the conventional positively charged nanofiltration membrane is poor and the commercial production cannot be realized.
(II) technical scheme
In order to achieve the purpose, the invention adopts the main technical scheme that:
in a first aspect, an embodiment of the present invention provides a positively charged nanofiltration membrane, which includes a supporting base membrane and a polyamide functional separation layer, where the polyamide functional separation layer of the positively charged nanofiltration membrane is prepared by a polymerization reaction of a polyamine-containing aqueous phase solution and a polyacyl chloride-containing oil phase solution on the surface of the supporting base membrane; the aqueous phase solution is dissolved with hydrophilic high polymer, hydrophilic surfactant, polyamine and water-soluble divalent cation; the oil phase solution is a solution obtained by dissolving polybasic acyl chloride in an organic solvent.
Furthermore, the positively charged nanofiltration membrane also forms a protective layer on the surface of the polyamide functional separation layer; the protective layer is produced by the cross-linking reaction of polyvinyl alcohol and glutaraldehyde, and prevents the loss of the positively charged complex formed by the water-soluble divalent cations with the hydrophilic high polymer and the hydrophilic surfactant.
In a second aspect, an embodiment of the present invention provides a method for preparing a positively-charged nanofiltration membrane, including the following steps:
s1, preparing a solution, namely adding a hydrophilic high polymer, a hydrophilic surfactant, polyamine and water-soluble divalent salt into water, and uniformly mixing to obtain an aqueous phase solution; dissolving polyacyl chloride in an organic solvent to obtain an oil phase solution;
s2, preparing a polyamide functional separation layer, wherein the polyamide functional separation layer is prepared by carrying out interfacial polymerization reaction on the water phase solution and the oil phase solution prepared in the step S1 on a supporting base film and carrying out heat treatment;
s3, preparing a protective layer, wherein the protective layer is formed by a cross-linking reaction of polyvinyl alcohol and glutaraldehyde, and a dense film layer is formed on the surface of the polyamide functional separation layer;
and S4, putting the membrane processed in the step S3 into deionized water, cleaning and drying to finally obtain the positively charged nanofiltration membrane.
When preparing the aqueous solution, adding the hydrophilic high polymer, the hydrophilic surfactant, the polyamine and the water-soluble divalent salt into water to enable divalent cations to form a complex with the hydrophilic surfactant and the hydrophilic high polymer; these complexes are incorporated into the polyamide functional separation layer when interfacial polymerization occurs, thereby positively charging the filter membrane surface.
The hydrophilic high polymer has good reactivity, contains hydrophilic groups such as hydroxyl, amino, carboxyl and the like, can form strong hydrogen bond interaction with water, can also form a stable complex with divalent cations, and can be intertwined with polypiperazine amide molecules when a high molecular weight polymer and divalent salt form the complex; the hydrophilic surfactant can reduce the interfacial tension of the aqueous phase solution, so that the aqueous phase solution is more uniformly spread on the surface of the supporting base membrane, the adhesion of the aqueous phase monomer on the supporting base membrane is increased, and the wettability of the membrane surface is higher; on the other hand, the surfactant and the divalent cation salt form a complex which is used as a carrier for positively charging sources, and positive charges are uniformly distributed on the membrane surface.
Further, step S2 includes: firstly, coating aqueous solution on a supporting basement membrane, removing redundant aqueous solution and then airing; and then the film coated with the water phase solution is placed into the oil phase solution or the oil phase solution is coated on the supporting base film, and after the redundant oil phase solution is removed, the film is treated at the temperature of 90-130 ℃ to carry out interfacial polymerization reaction.
Preferably, in step S1, the hydrophilic polymer is one or more of polyacrylamide, sodium alginate and polyvinylpyrrolidone, and the mass fraction of the hydrophilic polymer is 0.1% -5%.
Optionally, in step S1, the hydrophilic surfactant is one or more of sodium camphorsulfonate, sodium citrate, sodium dodecyl sulfate, and sodium hexadecyl sulfate, and the mass fraction of the hydrophilic surfactant is 0.1% to 5%.
Preferably, in step S1, the polybasic amide is piperazine, and the added mass fraction of piperazine is 0.1-1%.
Preferably, in step S1, the water-soluble divalent salt is a water-soluble salt of magnesium ion or calcium ion, and the added mass fraction thereof is 0.1-1%.
Optionally, in step S1, the poly-acyl chloride in the oil phase solution is trimesoyl chloride, and the mass fraction of the trimesoyl chloride is 0.1-1.0%, and more preferably, the mass fraction of the trimesoyl chloride in the oil phase solution is 0.1-0.5%.
Preferably, in step S3, the polymerization degree of the polyvinyl alcohol is 1500-1900, and the alcoholysis degree is more than 98%; the mass concentration of the solution of glutaraldehyde is 0.7-0.9%, and the crosslinking time is 7-15 min.
Preferably, the protective layer is prepared by crosslinking reaction of polyvinyl alcohol and glutaraldehyde, and is dried at 60-80 deg.C for no less than 30 min.
Preferably, in step S3, the concentration of the polyvinyl alcohol solution is 3.5-4.5%, and the mass fraction of the sulfuric acid catalyst is 1%.
Optionally, the support basement membrane is a polysulfone basement membrane, and the basement membrane can separate ovalbumin with a substance molecular weight of above 40000.
(III) advantageous effects
The invention has the beneficial effects that:
1. according to the preparation method, in an aqueous phase solution, a hydrophilic surfactant (such as sodium citrate) and a hydrophilic high polymer (such as polyvinylpyrrolidone) can both carry out a complex reaction with water-soluble divalent metal ions to generate a stable compound with complex ions, and the hydrophilic high polymer is a macromolecular substance, so that the complexed macromolecules can be intertwined with the generated polypiperazine amide in an interfacial polymerization process and are mutually doped in a separation layer formed by an aqueous phase complex layer and an interfacial polymerization, so that the positively charged nanofiltration composite membrane is prepared;
2. after the preparation of the positively charged nanofiltration membrane is completed through interfacial polymerization, a compact structure is formed on the surface of a polyamide functional separation layer of the membrane through further cross-linking reaction of polyvinyl alcohol and glutaraldehyde, and under the condition that a sealing layer is formed on the surface of the separation layer, macromolecules with positive charge cannot flow away along with an aqueous solution in the experiment process, so that the stability of the positively charged nanofiltration membrane is enhanced;
3. the water phase adopts piperazine as a monomer and the oil phase monomer trimesoyl chloride to prepare the nanofiltration membrane through interfacial polymerization, the method is simple and easy to control, the technology of the patent has better reproducibility and the preparation cost is low.
4. The positively charged nanofiltration membrane prepared by the method can improve the retention rate of multivalent cations, particularly divalent cations, and the performance of the positively charged nanofiltration membrane is greatly improved.
Drawings
FIG. 1 is a scanning electron microscope image of a polysulfone base film;
FIG. 2 is a scanning electron microscope image of the positively charged nanofiltration membrane of example 1;
FIG. 3 is a scanning electron microscope image of a nanofiltration membrane of comparative example 1;
FIG. 4 is a scanning electron micrograph of the nanofiltration membrane of comparative example 2;
FIG. 5 is a scanning electron micrograph of the nanofiltration membrane of comparative example 3;
figure 6 is a scanning electron microscope image of the nanofiltration membrane of comparative example 5.
Detailed Description
For the purpose of better explaining the present invention and to facilitate understanding, the present invention will be described in detail by way of specific embodiments with reference to the accompanying drawings.
The positively charged nanofiltration membrane provided by the embodiment of the invention comprises a supporting base membrane and a polyamide functional separation layer, wherein the polyamide functional separation layer of the positively charged nanofiltration membrane is prepared by carrying out polymerization reaction on a water phase solution containing polyamine and an oil phase solution containing polyacyl chloride on the surface of the supporting base membrane; the aqueous phase solution is dissolved with hydrophilic high polymer, hydrophilic surfactant, polyamine and water-soluble divalent cation; the oil phase solution is a solution obtained by dissolving polybasic acyl chloride in an organic solvent.
The positively charged nanofiltration membrane also comprises a protective layer formed on the surface of the polyamide functional separation layer; the protective layer is produced by a cross-linking reaction of polyvinyl alcohol and glutaraldehyde.
The positively charged nanofiltration membrane provided by the embodiment of the invention is prepared by the following preparation method, and specifically comprises the following steps:
s1, preparing a solution, namely adding a hydrophilic high polymer, a hydrophilic surfactant, polyamine and water-soluble divalent salt into water, and uniformly mixing to obtain an aqueous phase solution; dissolving polyacyl chloride in an organic solvent to obtain an oil phase solution;
the hydrophilic high polymer is one or more of polyacrylamide, sodium alginate and polyvinylpyrrolidone, and the mass fraction of the hydrophilic high polymer is 0.1-5%;
the hydrophilic surfactant is one or more of sodium camphorsulfonate, sodium citrate, sodium dodecyl sulfate and sodium hexadecyl sulfate, and the mass fraction of the hydrophilic surfactant is 0.1-5%;
the polyamine in the aqueous phase solution is piperazine, and the mass fraction of the added piperazine is 0.1-1%;
the water-soluble divalent salt is water-soluble salt of magnesium ions or calcium ions, and the mass fraction of the water-soluble divalent salt is 0.1-1%; dissolving polybasic acyl chloride in Isopar L (1 isoalkane solvent) to obtain oil phase solution, wherein the polybasic acyl chloride in the oil phase solution is trimesoyl chloride, the mass fraction of the trimesoyl chloride is 0.1-1.0%, and more preferably, the mass fraction of the trimesoyl chloride in the oil phase solution is 0.1-0.5%.
S2, preparing a polyamide functional separation layer, wherein the polyamide functional separation layer is prepared by carrying out interfacial polymerization reaction on the water phase solution and the oil phase solution prepared in the step S1 on a supporting base film and carrying out heat treatment at 90-130 ℃;
the method specifically comprises the following steps: firstly, coating aqueous solution on a supporting basement membrane, removing redundant aqueous solution and then airing; then the film coated with the water phase solution is put into the oil phase solution or the oil phase solution is coated on the film coated with the water phase solution, and after the redundant oil phase solution is removed, the interfacial polymerization reaction is carried out at the temperature of 90-130 ℃.
A nanofiltration membrane prepared by piperazine and trimesoyl chloride through interfacial polymerization reaction is mostly a negatively charged nanofiltration membrane which cannot have a good separation effect on multivalent salts and small cationic molecules (part of dyes, antibiotics and the like), but a functional layer of the nanofiltration membrane is doped with a high-molecular complex containing water-soluble divalent metal ions, so that after the nanofiltration membrane generated by winding the high-molecular complex and a polypiperazine amide functional layer mutually detects a potential, the nanofiltration membrane is found to have certain positive charge, the problem of separation of the multivalent salts and the small cationic molecules can be well solved, and the interception sequence of inorganic salts with the same concentration is MgCl2＞MgSO4＞Na2SO4＞NaCl。
S3, preparing a protective layer, wherein the protective layer is formed by a cross-linking reaction of polyvinyl alcohol and glutaraldehyde, and a dense film layer is formed on the surface of the polyamide functional separation layer;
the polymerization degree of the polyvinyl alcohol is 1500-1900, and the alcoholysis degree is more than 98 percent; the mass concentration of the solution of glutaraldehyde is 0.7-0.9%, and the crosslinking time is 7-15 min;
the longer the cross-linking reaction time of the polyvinyl alcohol and the glutaraldehyde is, the thickness of a protective layer formed by the polyvinyl alcohol and the glutaraldehyde is increased, but the water flux of the nanofiltration membrane is reduced, and the backwashing and recovery performance of the nanofiltration membrane is reduced; if the crosslinking time is short, the degree of crosslinking is reduced, the thickness of the film is insufficient, and the properties cannot be maintained for a long time. Thus, proper cross-linking time can increase the hydrophilic effect of the membrane while maintaining the strength of the membrane.
The protective layer is prepared by cross-linking reaction of polyvinyl alcohol and glutaraldehyde, and is dried at 60-80 deg.C for no less than 30 min.
And S4, putting the membrane processed in the step S3 into deionized water, cleaning and drying to finally obtain the positively charged nanofiltration membrane.
The step S4 is intended to remove the remaining unreacted material.
In order to better understand the above technical solutions, exemplary embodiments of the present invention may be described in more detail below by referring to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
The following is a detailed description of embodiments of the invention:
in the invention, the polysulfone base film can be any base film provided by manufacturers, and the performance difference of the base film and the type of the base film have no direct influence on the result of the invention, so that the commercial polysulfone base film can be selected or made by self, which also provides possibility for common application and commercial application of the invention.
The polysulfone base film used in the following examples is a self-made base film. The film production date was less than 30 days to the experimental date, during which time it was stored in a 1.5% aqueous solution of sodium bisulfite. Before the interfacial polymerization reaction is carried out to prepare the composite membrane, the polysulfone base membrane is soaked in pure water 24 hours in advance.
The membrane performance of a nanofiltration membrane was evaluated in the following examples under the following conditions: the testing pressure is 0.75MPa, the flow of concentrated water is 1.0GPM, the concentrated water is 2000ppm saline solution, the pH value of the concentrated water is 6.5-7.5, and the environmental temperature is 25 ℃;
evaluation indexes are as follows: magnesium chloride, magnesium sulfate, sodium chloride, sodium sulfate 4 salt desalination rate and water flux.
In the following examples, the salt rejection is defined as the difference between the concentrations of concentrate and product water divided by the concentrate concentration; the water flux is defined as the volume of water per unit time that permeates the composite separation membrane per unit area in the above test procedure and is expressed in L/m2H (LMH). Each data point above was averaged from 9 samples.
In the following examples and comparative examples, the components of the aqueous phase solution and the oil phase solution are referred to by mass percent if not otherwise specified.
Example 1
A preparation method of a positively charged nanofiltration membrane comprises the following steps,
(1) preparing a solution: 0.15% of piperazine, 2.5% of sodium camphorsulfonate, 0.1% of polyvinyl alcohol, 2.5% of polyvinylpyrrolidone, 1% of sodium citrate and 1% of magnesium chloride, and uniformly mixing to obtain an aqueous phase solution;
dissolving 0.15% trimesoyl chloride in Isopar L (1 isoalkane solvent) to obtain oil phase solution;
(2) preparation of a polyamide functional separation layer: coating the water phase solution obtained in the step (1) on a polysulfone basement membrane, pouring off the redundant solution after 60s, and placing and drying in the shade; coating the membrane treated by the water phase solution with the oil phase solution obtained in the step (1), and pouring off the redundant oil phase solution after 30 seconds; then the membrane is dried for 4min at the temperature of 100 ℃;
(3) preparing a protective layer, coating a polyvinyl alcohol (1799 type) solution containing a sulfuric acid catalyst and with the mass fraction of 4% on a nanofiltration membrane, pouring off after 3min, drying the membrane in the shade, and pouring 0.8% glutaraldehyde (50% solution) on the membrane surface containing the polyvinyl alcohol for crosslinking for 10 min; the cross-linked positively charged nanofiltration membrane is subjected to heat treatment at 70 ℃ for 30min, and the mechanical property of the membrane can be further enhanced through the heat treatment;
(4) and (4) putting the membrane treated in the step (3) into deionized water, cleaning, and airing to obtain the positively charged nanofiltration membrane.
Example 2
Example 2 is based on example 1, the composition of the aqueous phase solution in step (1) is adjusted to be 0.15% of piperazine, 2.5% of sodium camphorsulfonate, 0.1% of polyvinyl alcohol, 2.5% of polyvinylpyrrolidone and 0.5% of magnesium chloride, and the mixture is mixed uniformly; other steps and conditions were the same as in example 1.
Example 3
Example 3 is based on example 1, the composition of the aqueous phase solution of step (1) is adjusted to 0.15% piperazine, 2.5% sodium camphorsulfonate, 0.1% polyvinyl alcohol, 2% polyvinyl amide and 0.5% magnesium chloride, and the mixture is mixed uniformly; other steps and conditions were the same as in example 1.
Example 4
Example 4 is based on example 1, the composition of the aqueous phase solution in step (1) is adjusted to 0.15% piperazine, 2.5% sodium camphorsulfonate, 0.1% polyvinyl alcohol, 0.5% sodium alginate and 0.5% magnesium chloride, and the mixture is mixed uniformly; other steps and conditions were the same as in example 1.
Example 5
Example 5 the composition of the aqueous solution of step (1) was adjusted to 0.15% piperazine, 2.5% sodium camphorsulfonate, 0.1% polyvinyl alcohol, 1% sodium citrate, 0.5% magnesium chloride, and mixed well based on example 1; other steps and conditions were the same as in example 1.
Example 6
Example 6 is based on example 1, the composition of the aqueous phase solution of step (1) is adjusted to 0.15% piperazine, 2.5% sodium camphorsulfonate, 0.1% polyvinyl alcohol, 1% sodium citrate, 2% polyvinyl amide and 1% magnesium chloride, and the mixture is mixed uniformly; other steps and conditions were the same as in example 1.
Example 7
Example 7 is based on example 1, the composition of the aqueous phase solution in step (1) is adjusted to 0.15% piperazine, 2.5% sodium camphorsulfonate, 0.1% polyvinyl alcohol, 1% sodium citrate, 0.5% sodium alginate and 1% magnesium chloride, and the mixture is mixed uniformly; other steps and conditions were the same as in example 1.
Example 8
Example 8 is based on example 1, the composition of the aqueous phase solution of step (1) is adjusted to 0.15% piperazine, 2.5% sodium camphorsulfonate, 0.1% polyvinyl alcohol, 2.5% polyvinyl pyrrolidone, 2% polyvinyl amide, 1% magnesium chloride, and mixed uniformly; other steps and conditions were the same as in example 1.
Example 9
Example 9 is based on example 2, the drying conditions of step (2) were changed to 90 ℃ for 5 min; other steps and conditions were the same as in example 1.
Example 10
Example 10 is based on example 2, the drying conditions in step (2) were changed to 130 ℃ for 3 min; other steps and conditions were the same as in example 2.
Comparative example 1
Comparative example 1 on the basis of example 1, when preparing an aqueous phase solution, a compound containing complex ions (polyvinylpyrrolidone, polyacrylamide, sodium alginate and sodium citrate) is not added to a water phase system, the mass fraction of polyamine in the aqueous phase is 0.15%, and the mass fraction of hydrophilic surfactant camphor sodium sulfonate is 2.5%; other steps and conditions were the same as in example 1.
Comparative example 2
Comparative example 2 on the basis of example 1, when preparing an aqueous phase solution, a water-soluble divalent salt magnesium chloride was not added to the aqueous phase system; other steps and conditions were the same as in example 1.
Comparative example 3
Comparative example 3 on the basis of example 1, the hydrophilic polymer and the water-soluble divalent salt were not added when preparing the aqueous solution; other steps and conditions were the same as in example 1.
Comparative example 4
Comparative example 4 on the basis of example 1, the protective layer treatment of step (3) was not performed; other steps and conditions were the same as in example 1.
Comparative example 5
Comparative example 5 is based on example 3, the crosslinking time in step (3) was changed to 15 min; other steps and conditions were the same as in example 3.
1. The positively charged nanofiltration membranes prepared in examples 1 to 10 and the membranes prepared in comparative examples 1 to 5 were subjected to membrane performance evaluation, and the salt rejection and water flux of 4 salts of magnesium chloride, magnesium sulfate, sodium chloride and sodium sulfate were used for characterization, and the specific experimental results are shown in table 1.
Table 1 evaluation results of film properties
As can be seen from the data results in Table 1, the positively charged nanofiltration membrane prepared by the method has a good separation effect on divalent salts, and in a preferred embodiment, the desalination rate on magnesium chloride is as high as 99.2% and 98.6%, and the corresponding water flux is 61LMH and 55 LMH; in a preferred embodiment, the salt rejection rate of the magnesium sulfate reaches 96.1 percent and 98.6 percent, and the corresponding water flux is 51LMH and 59 LMH; the flux of water is higher. In comparative examples 1 to 5, it can be seen that the addition of the water-soluble polymer increases the water flux of the nanofiltration membrane by comparing the substances added to the different aqueous solutions, and in comparative example 2, the separation effect of the membrane is poor compared to that of example 1, with the water-soluble polymer and the water-soluble surfactant but without the water-soluble divalent salt; in comparative example 4, since there is no protective layer in the preparation process of the membrane, the membrane has no influence on the performance of the positively charged nanofiltration membrane initially when the membrane is subjected to the inorganic salt rejection test, but gradually shows a decrease in the positively charged property with the running of the experiment, and compared with example 1, after the average value of 9 samples is taken, the salt rejection rate of the membrane is lower than that of example 1, and the effect is poor. In comparative example 5, the water flux was significantly reduced in the treatment of the four substances as compared with example 3 because the crosslinking time of the protective layer was longer.
From this, it can be seen that,
1) the simultaneous existence of the divalent water-soluble salt, the corresponding water-soluble polymer with the complexing effect and the surfactant is very important in the preparation process of the positively charged membrane, and the divalent water-soluble salt, the corresponding water-soluble polymer with the complexing effect and the surfactant are not necessary. The high polymer which has the complexing effect does not exist, the water-soluble divalent salt is easy to lose along with the treated concentrated water, the divalent salt does not exist, the retention rate of the prepared nanofiltration membrane on inorganic salt is not changed greatly, only the water flux is slightly improved, and the high polymer only has the hydrophilic performance;
2) the protective layer is used for preventing macromolecular substances complexed in the water phase from gradually losing along with the test solution in the running process of the experiment, and the performance of the nanofiltration membrane can be kept from being influenced by the running time;
3) if the protective layer is excessively crosslinked, the thickness of the membrane increases, the water flux of the membrane decreases, and the recovery capability of the membrane decreases, which is disadvantageous in the case of backwashing of the subsequent membrane.
2. The polysulfone base film and the films prepared by the methods of example 1, comparative example 2, comparative example 3 and comparative example 5 were subjected to electron microscope scanning, and the specific scanning results are shown in fig. 1 to fig. 6.
From the results of the scanning electron microscope,
1) compared with the prior art shown in the figure 1 and the figure 2, the polysulfone base film has a loose structure, large gaps and narrow and uneven cracks; compared with an untreated polysulfone bottom membrane, the prepared positively-charged nanofiltration membrane is a uniform and compact porous three-dimensional net structure through coating preparation of a separation layer (a complex layer of macromolecules and divalent salts and a polypiperazine amide layer) and treatment of a sealing layer (formed by crosslinking polyvinyl alcohol and glutaraldehyde). The positively charged complex layer molecules are tightly immobilized inside the membrane layer.
2) Compared with the water-soluble divalent salt or the aqueous solution high polymer which is not complexed with the water-soluble divalent salt in the aqueous solution, the nanofiltration membrane prepared by the method has limited coverage or blocking degree on the polysulfone bottom membrane in the aqueous solution in fig. 2, 3, 4 and 5.
3) Compared with fig. 2 and fig. 6, the excessive crosslinking of the sealing layer forms a dense and fine membrane on the surface of the separation layer, and the pore diameter of the membrane is finer, which brings about the reduction of water flux, and is consistent with the results of the experiment.
3. Stability testing of membranes
Comparing the positively charged nanofiltration membrane prepared by the method in the embodiment 1 with the positively charged nanofiltration membrane prepared by the method in the comparative example 4, intercepting 2000ppm of magnesium chloride, and observing the change of the desalination rate along with time;
the testing pressure is 0.75MPa, the flow of concentrated water is 1.0GPM, the pH value of the concentrated water is 6.5-7.5, and the environmental temperature is 25 ℃; the specific test results are shown in table 2,
table 2 stability testing of positively charged nanofiltration membranes
As can be seen from table 2, with the electropositive membrane prepared in example 1, the initial retention rate for magnesium chloride was 99.1% and the water flux was 61LMH, and the experimental run was 16h, the retention rate for magnesium chloride was 98.5% and the water flux was 52 LMH. The nanofiltration membrane is charged positively after the membrane is operated for 16 hours;
when a magnesium chloride interception experiment is carried out after the positively-charged nanofiltration membrane is prepared by adopting the method of the comparative example 4, the initial interception rate is 89.5 percent, the water flux is 40LMH, and with the increase of the operation time, the interception rate of magnesium chloride after 2 hours is 84.7 percent, and the water flux is 46 LMH; after the operation is carried out for 4 hours, the retention rate of magnesium chloride is 78.2%, the water flux is 55LMH, the retention rate of the nano-filtration membrane on the magnesium chloride is reduced along with the increase of the operation time, the water flux is improved, which indicates that the selective permeability capability of the nano-filtration membrane is deteriorated, macromolecular substances possibly generated by complexation in a water phase gradually permeate into a test solution from the surface of the membrane, so that the positive charge of the nano-filtration membrane is gradually reduced, the water flux is improved along with the increase of the water flux, and further indicates that a protective layer is added on the surface of the nano-filtration membrane, and the positive charge complex can be effectively retained in the functional separation layer.
From the results, the positively charged nanofiltration membrane prepared by the method has a good divalent salt separation effect, has good stability and long service life, and can meet the requirements of commercial production and use.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (9)
1. The positively charged nanofiltration membrane is characterized by comprising a supporting base membrane and a polyamide functional separation layer, wherein the polyamide functional separation layer of the nanofiltration membrane is prepared by carrying out polymerization reaction on a water phase solution containing polyamine and an oil phase solution containing polybasic acyl chloride on the surface of the supporting base membrane;
the aqueous phase solution is dissolved with hydrophilic high polymer, hydrophilic surfactant, polyamine and water-soluble divalent cation;
the oil phase solution is obtained by dissolving polyacyl chloride in an organic solvent;
a protective layer is further formed on the surface of the polyamide functional separation layer, and the protective layer is generated by crosslinking reaction of polyvinyl alcohol and glutaraldehyde; the protective layer prevents the loss of the positively charged complex formed by the water-soluble divalent cations with the hydrophilic polymer and the hydrophilic surfactant.
2. The preparation method of the positively charged nanofiltration membrane is characterized by comprising the following steps:
s1, preparing a solution, namely adding a hydrophilic high polymer, a hydrophilic surfactant, polyamine and water-soluble divalent salt into water, and uniformly mixing to obtain an aqueous phase solution; dissolving polyacyl chloride in an organic solvent to obtain an oil phase solution;
s2, preparing a polyamide functional separation layer, wherein the polyamide functional separation layer is prepared by carrying out interfacial polymerization reaction on the water phase solution and the oil phase solution prepared in the step S1 on a supporting base film and carrying out heat treatment;
s3, preparing a protective layer, wherein the protective layer is formed by a cross-linking reaction of polyvinyl alcohol and glutaraldehyde, and a dense film layer is formed on the surface of the polyamide functional separation layer;
and S4, putting the membrane processed in the step S3 into deionized water, cleaning and drying to finally obtain the positively charged nanofiltration membrane.
3. The method of claim 2, wherein step S2 includes: firstly, coating aqueous solution on a supporting basement membrane, removing redundant aqueous solution and then airing; and then the film coated with the water phase solution is placed into the oil phase solution or the oil phase solution is coated on the supporting base film, and after the redundant oil phase solution is removed, the film is treated at the temperature of 90-130 ℃ to carry out interfacial polymerization reaction.
4. The method according to claim 2, wherein in step S1, the hydrophilic polymer is one or more selected from polyacrylamide, sodium alginate and polyvinylpyrrolidone, and the mass fraction of the hydrophilic polymer is 0.1-5%.
5. The method of claim 2, wherein in step S1, the hydrophilic surfactant is one or more selected from sodium camphorsulfonate, sodium citrate, sodium lauryl sulfate and sodium cetyl sulfate, and the mass fraction of the hydrophilic surfactant is 0.1-5%.
6. The method according to claim 2, wherein in step S1, the polyamine is piperazine with a mass fraction of 0.1-1%.
7. The method according to claim 2, wherein in step S1, the water-soluble divalent salt is a water-soluble salt of magnesium ion or calcium ion, and the mass fraction thereof is 0.1 to 1%.
8. The method of claim 2, wherein in step S1, the polybasic acid chloride in the oil phase solution is trimesoyl chloride, and the mass fraction of trimesoyl chloride is 0.1-1.0%.
9. The method according to claim 2, wherein in step S3, the degree of polymerization of the polyvinyl alcohol is 1500-1900, and the degree of alcoholysis is > 98%; the mass concentration of the solution of glutaraldehyde is 0.7-0.9%, and the crosslinking time is 7-15 min.
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