CN111569661B

CN111569661B - Preparation method of cellulose flat nanofiltration membrane with stable structure

Info

Publication number: CN111569661B
Application number: CN202010438257.0A
Authority: CN
Inventors: 汪东; 黄六莲; 林珊; 张慧; 曹石林; 马晓娟; 陈礼辉; 倪永浩; 李建国; 苗庆显
Original assignee: Fujian Agriculture and Forestry University
Current assignee: Fujian Agriculture and Forestry University
Priority date: 2020-05-21
Filing date: 2020-05-21
Publication date: 2022-06-14
Anticipated expiration: 2040-05-21
Also published as: CN111569661A

Abstract

The invention discloses a preparation method of a cellulose flat nanofiltration membrane with a stable structure, which is characterized in that a regenerated cellulose membrane is used as a supporting layer and is immersed in a dopamine solution for surface modification, and then the regenerated cellulose membrane with the surface modification is sequentially put into a piperazine aqueous solution and a n-hexane solution of trimesoyl chloride for reaction, so that a polyamide active layer is formed on the surface of the membrane. The use of dopamine increases the interaction force between the base membrane and the polyamide active layer, so that the support layer and the active layer are tightly combined, and the prepared nanofiltration membrane has good stability.

Description

Preparation method of cellulose flat nanofiltration membrane with stable structure

Technical Field

The invention belongs to the field of natural high polymer materials, and particularly relates to a preparation method of a cellulose flat nanofiltration membrane with a stable structure.

Background

The separation membrane plays a very important role in membrane technology, and can separate and treat organic matters, microorganisms, metal ions and the like in wastewater so as to achieve the effect of purifying water. The membrane pores of the separation membrane are regulated and controlled by the preparation process, and can be divided into microfiltration, ultrafiltration and nanofiltration according to the pore diameter characteristics of the membrane, and substances filtered by different pore diameters are different, so that the separation membrane is also applied to different fields. The cut-off molecular weight of the nanofiltration membrane is 200-1000 Da, the aperture range is 1-10 nm, and the nanofiltration membrane can be effectively applied to removing heavy metals, reducing total dissolved solids, softening water quality and the like. Because most surfaces of the nanofiltration membranes are charged, the nanofiltration membranes have the functions of selectively intercepting ions in water, intercepting high-valence ions and permeating low-valence ions, and retaining substances beneficial to human bodies. The interfacial polymerization method is widely used in the industrial production of the nanofiltration membrane, and the prepared nanofiltration membrane is a composite nanofiltration membrane. The interfacial polymerization method is mainly characterized in that a proper water phase monomer (such as piperazine, polyethyleneimine and the like) and an organic phase monomer (such as 1,3, 5-trimesoyl chloride and the like) are selected to carry out polymerization reaction on a supporting layer to generate a thin active layer. The mature raw materials of the supporting layer in the market are mainly organic synthetic polymer membranes such as polyethers and polyvinylidene fluorides, however, the organic synthetic polymer membranes have great environmental pollution and high material cost in the synthetic process, and researchers begin to seek natural, environment-friendly, economic and efficient raw materials to replace the organic synthetic polymer membranes. Cellulose is widely sourced, has the advantages of low price, easy obtainment, good reproducibility and the like, and cellulose is gradually developed as a raw material to prepare cellulose membranes. The regenerated cellulose membrane has the advantages of good hydrophilicity, air permeability, solvent resistance and the like, and has the irreplaceable advantage of an organic synthetic polymer membrane.

The existing cellulose-based nanofiltration membrane takes a regenerated cellulose membrane as a supporting layer, and piperazine solution with certain concentration and 1,3, 5-trimesoyl chloride are selected to carry out interfacial polymerization reaction on the supporting layer to form an active layer composite membrane with selective filtration. However, since a stable chemical bond is not established between the active layer and the support layer, and only by the physical action of mutual adhesion, the support layer and the active layer are separated from each other in long-term use, so that the separation performance of the nanofiltration membrane is reduced, and the separation membrane needs to be replaced periodically, so that the service life is short. Therefore, an economical and effective method is required to be selected for treating the supporting layer, and a stable chemical bond is constructed between the supporting layer and the active layer, so that the interaction force between the two layers is enhanced, and the active layer is prevented from easily falling off from the supporting layer.

Disclosure of Invention

The invention aims to provide a preparation method of a cellulose flat nanofiltration membrane with stable structure, which improves the structural stability of the cellulose nanofiltration membrane through strong adhesion and chemical bonds between a polyamide active layer and PDA-RCM.

In order to achieve the purpose, the invention adopts the following technical scheme:

a preparation method of a cellulose flat nanofiltration membrane with stable structure is characterized by comprising the following steps: the method comprises the following steps:

1) crushing a cellulose raw material, dissolving the crushed cellulose raw material in 70-90 wt% of NMMO (N-methylmorpholine-N-oxide) aqueous solution, adding 2-3 wt% of N-propyl gallate of the cellulose raw material, stirring and reacting at 100-120 ℃ for 1-3 h, stopping heating, cooling to 90 ℃, closing a stirrer, vacuumizing or standing and defoaming for 2-5 h to obtain 4-10 wt% of cellulose casting solution, pouring the cellulose casting solution on a glass plate, scraping the cellulose casting solution into a film by using a coating machine, quickly immersing the cellulose casting solution in deionized water, and removing the film to obtain a regenerated cellulose film;

2) preparing 1-5 g/L dopamine solution by using Tris-HCl buffer solution (50 mM, pH = 7-9), then shearing the obtained regenerated cellulose membrane into a circular sheet with the diameter of 5-10 cm, placing the circular sheet in the obtained dopamine solution, oscillating at constant temperature for 1-5 h to enable dopamine to generate self-polymerization reaction, and fully washing the circular sheet with deionized water to obtain a surface-modified regenerated cellulose membrane;

3) soaking the obtained surface-modified regenerated cellulose membrane in 0.01-0.50 wt% piperazine (PIP) aqueous solution for 10-40 min, taking out, drying residual liquid drops on the surface by using filter paper, and then placing on a glass plate for fixation;

4) soaking the film material fixed on the glass plate in a n-hexane solution of 0.01-0.50 wt% of trimesoyl chloride (TMC) for reaction for 1-5 min, so that PIP and TMC monomers are subjected to interface polymerization reaction, and a polyamide active layer is formed on the surface of the film;

5) and removing residual organic phase solution after the reaction is finished, and airing the obtained membrane material in air at room temperature for 1-3 h to obtain the cellulose flat nanofiltration membrane with the stable structure.

The cellulose raw material in the step 1) is any one of wood pulp, cotton pulp, hemp pulp, bamboo pulp, straw pulp, bagasse pulp, mulberry bark pulp or reed pulp, preferably wood pulp, cotton pulp, bamboo pulp and the like, and more preferably a raw material of which the content of alpha cellulose is more than or equal to 92% and the polymerization degree of cellulose is more than or equal to 480. If the cellulose raw material with low cellulose content (alpha cellulose content is less than or equal to 92%) and low polymerization degree (less than or equal to 480) is adopted, the prepared regenerated cellulose membrane is not easy to form, has poor mechanical property and is not beneficial to industrial production of the nanofiltration membrane.

The surface of the regenerated cellulose membrane contains a large amount of hydroxyl groups, which is beneficial to more Polydopamine (PDA) to deposit on the surface of the membrane; the polydopamine deposited on the surface of the membrane can perform Michael addition reaction with amino in piperazine to form a firm chemical bond, so that the interaction force between the base membrane and the polyamide active layer is increased, the support layer and the active layer are tightly combined, and the prepared nanofiltration membrane has good stability.

The invention has the following advantages:

1) the cellulose raw material has wide source and low cost, and is dissolved by adopting the NMMO solvent, so that the method has no pollution, the solvent can be recovered, and the production cost is reduced to a great extent.

2) The regenerated cellulose membrane keeps good hydrophilicity and biocompatibility of cellulose, and the problem of poor hydrophilicity is effectively solved by using the regenerated cellulose membrane as a base membrane, so that a good basis is provided for further preparing the nanofiltration membrane.

3) The regenerated cellulose membrane has more hydroxyl groups, is easy to chemically modify, has high hydrophilicity, and provides better water flux and anti-pollution effect.

4) The mode of dopamine modification is mild, does not cause any damage to the supporting layer, provides good hydrophilicity, and can be used as a secondary reaction platform to promote further reaction.

5) The cellulose membrane prepared by the method has good stability, has the characteristics of good interception rate, high water flux and strength, pollution resistance, easy degradation and the like, and can be used in large-scale commercialization.

Drawings

FIG. 1 is an XPS map of surface chemical composition analysis of RCM, PDA-RCM and NF-PDA-RCM, in which (a) is a broad-band XPS peak map, and (b), (C) and (d) are C1s peak maps of RCM, PDA-RCM and NF-PDA-RCM, respectively.

FIG. 2 is an infrared spectrum of bamboo pulp fiber, RCM, PDA-RCM, NF-PDA-RCM and NF-RCM.

FIG. 3 is a graph of the separation performance of ethanol soaked NF-PDA-RCM (a) and NF-RCM (b) versus magnesium sulfate solution.

FIG. 4 is a graph comparing the stability of NF-PDA-RCM (a) and NF-RCM (b) in ethanol solution.

Detailed Description

In order to make the present invention more comprehensible, the technical solutions of the present invention are further described below with reference to specific embodiments, but the present invention is not limited thereto.

Examples

1) Crushing bamboo pulp, dissolving the bamboo pulp in 80wt% of NMMO aqueous solution, adding 3 wt% of n-propyl gallate, stirring and reacting for 3 hours at 100 ℃, stopping heating, cooling to 90 ℃, closing a stirrer, vacuumizing or standing and defoaming for 3 hours to obtain 6 wt% of cellulose casting solution, pouring the cellulose casting solution on a glass plate, scraping the cellulose casting solution into a film by using a coater, quickly immersing the film in deionized water, and removing the film to obtain a regenerated cellulose film (RCM);

2) preparing 2 g/L dopamine solution by using Tris-HCl buffer solution (50 mM, pH = 7-9), then cutting the obtained regenerated cellulose membrane into a circular sheet with the diameter of 5 cm, placing the circular sheet in the obtained dopamine solution, oscillating at constant temperature for 4 h to enable dopamine to generate self-polymerization reaction, and fully washing the dopamine solution by using deionized water to obtain a surface-modified regenerated cellulose membrane (PDA-RCM);

3) soaking the obtained surface-modified regenerated cellulose membrane in 0.1 wt% PIP water solution for 20 min, taking out, drying the residual liquid drops on the surface with filter paper, and fixing on a glass plate of 10cm × 10 cm;

4) soaking the film material fixed on the glass plate in 0.3 wt% TMC n-hexane solution for reaction for 3.5 min, so that PIP and TMC monomers are subjected to interfacial polymerization reaction, and a polyamide active layer is formed on the surface of the film;

5) and removing residual organic phase solution after the reaction is finished, and airing the obtained membrane material in the air at room temperature for 2 hours to obtain the cellulose flat nanofiltration membrane (NF-PDA-RCM) with stable structure.

The surface chemical compositions of RCM, PDA-RCM and NF-PDA-RCM were evaluated by XPS, and the results are shown in FIG. 1. As can be seen from the graph (a), the N1s peak was clearly observed in the PDA-RCM, compared to the peak pattern of RCM, indicating that the polydopamine-modified regenerated cellulose film was not only physically deposited but also chemically bonded, i.e., C-N bond was present (FIG. 1 a). As can be seen from (b) in the figure, the RCM retains the original structure of cellulose, and C has three connection modes of C-C (284.6 eV), C-O (286.4 eV) and O-C-O (288.5 eV); as can be seen from (C) in the figure, the relative intensity of the C-C group is significantly increased when the electron energy is 284.6 eV, and a new peak 285.6 eV (C-N) appears in the XPS spectrum of the PDA-RCM, further confirming the successful combination of the PDA and the RCM; as can be seen from (d) in the figure, the X-ray photoelectron spectrum of NF-PDA-RCM shows four significant strong peaks of 200.2 eV, 284.9 eV, 400.1 eV and 532.7 eV, which represent Cl 2P, C1s, N1s and O1, respectively, the presence of Cl indicates that a polyamide active layer was successfully formed on the regenerated cellulose film, and the new peak corresponding to C = O bond was observed at 288.9 eV for NF-PDA-RCM, which also indicates that some polyamide active layers were formed on PDA-RCM, and polydopamine was chemically bonded between the support layer and the active layer. The experimental result shows that the RCM surface is coated with a large amount of PDA, the amino group of PIP successfully reacts with PDA, and the supporting layer and the polyamide active layer are connected in a chemical bond mode.

Comparative example

2) cutting the obtained regenerated cellulose membrane into a circle with the diameter of 5 cm, soaking the regenerated cellulose membrane in 0.1 wt% of PIP aqueous solution for 20 min, taking out the regenerated cellulose membrane, sucking residual liquid drops on the surface of the regenerated cellulose membrane by using filter paper, and then fixing the regenerated cellulose membrane on a glass plate with the thickness of 10cm multiplied by 10 cm;

3) and (3) soaking the membrane material fixed on the glass plate in 0.3 wt% TMC n-hexane solution for reaction for 3.5 min, so that PIP and TMC monomers are subjected to interfacial polymerization reaction, and a polyamide active layer is formed on the surface of the membrane, thus obtaining the cellulose-based nanofiltration membrane (NF-RCM).

The functional group characteristics of RCM, PDA-RCM, NF-PDA-RCM, and NF-RCM were analyzed by FT-IR analysis, and the results are shown in FIG. 2. As can be seen from fig. 2, the FT-IR spectrum of RCM obtained from the dissolution of NMMO is similar to that of cellulose, indicating that the process of dissolving cellulose by NMMO does not cause significant changes in the chemical composition of cellulose, which is a physical process. PDA-RCM at 1610 cm compared to FT-IR spectrum of RCM^-1A characteristic peak of C = C belonging to the benzene ring appears, indicating that polydopamine is successfully linked to the regenerated cellulose membrane. C = O tensile vibration peak (1625 cm) appearing on the spectrum of NF-PDA-RCM and NF-RCM^-1) And C-N tensile vibration Peak (1450 cm)^-1) This indicates that the amino groups in PIP and the carboxyl groups in TMC polymerize to form a large number of amide bonds on the regenerated cellulose film, further demonstrating that the RCM surface forms a thin layer of polyamide, consistent with the results of XPS.

Soaking NF-PDA-RCM and NF-RCM in ethanol solution, performing filtration performance treatment at intervals, and detecting water flux and rejection rate of the prepared membrane. The results of the separation performance of the NF-PDA-RCM and NF-RCM treatment of the 1000 ppm magnesium sulfate solution are shown in FIG. 3. As can be seen, the separation performance of NF-PDA-RCM was only slightly changed after soaking in ethanol for 5 h. Under the condition of 0.4 MPa, the water flux is only 25.06L/(m)²H) increased to 40.1L/(m)²·h），MgSO₄The retention rate of the catalyst is reduced to 60.9% from 75.6%. However, after 5h of NF-RCM soaking, MgSO₄The retention rate is sharply reduced from 73.8 percent to 12.3 percent, and the water flux is reduced from 17.54L/(m)²H) increases abruptly to 85.94L/(m)²H) that isThe separation performance thereof is remarkably decreased. The phenomenon shows that the NF-PDA-RCM has good structural stability in an ethanol solution, namely that the modification of dopamine enhances the interfacial force between the active polymerization layer and the supporting layer, thereby improving the stability of the nanofiltration membrane.

The stability of NF-RCM and NF-PDA-RCM was evaluated by further immersing them in an ethanol solution, and the results are shown in FIG. 4. As can be seen in the figure, a part of the polymer layer of the NF-RCM is separated from the supporting layer after soaking for 9 hours, and the polymer layer is completely separated from the supporting layer after 36 hours; and after the NF-PDA-RCM is soaked for 36 hours, the separation of the polymeric layer and the supporting layer is not obvious. This further demonstrates that the bonding force of NF-PDA-RCM between the polymer layer and the support layer is significantly stronger than NF-RCM.

In the invention, polydopamine is adhered to the surface of RCM through strong covalent bonds (such as pi-pi action, hydrogen bonds and the like), so that the reactivity of the membrane surface is improved; and then taking PDA-RCM as a support layer, and forming a polyamide active layer by immobilizing interfacial polymerization reaction of PIP and TMC, wherein a quinone group in the PDA and an amino group in the PIP can generate Michael addition reaction to immobilize the PIP. Therefore, the polyamide active layer is connected to the PDA-RCM through covalent bonds, and other intermolecular forces are combined, so that the interface compatibility and the structural stability of the support layer are enhanced.

The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.

Claims

1. The utility model provides a stable in structure's dull and stereotyped nanofiltration membrane of cellulose which characterized in that: the preparation method comprises the following steps:

1) placing a circular flaky regenerated cellulose membrane with the diameter of 5-10 cm in a Tris-HCl buffer solution of 1-5 g/L dopamine, oscillating at constant temperature for 1-5 h to enable the dopamine to generate a self-polymerization reaction, and fully washing with deionized water to obtain a surface-modified regenerated cellulose membrane;

2) soaking the obtained surface-modified regenerated cellulose membrane in 0.01-0.50 wt% piperazine water solution for 10-40 min, taking out, drying residual liquid drops on the surface by using filter paper, and then placing on a glass plate for fixing;

3) soaking the membrane material fixed on the glass plate in a 0.01-0.50 wt% n-hexane solution of trimesoyl chloride for reaction for 1-5 min;

4) removing residual organic phase solution after the reaction is finished, and airing the obtained membrane material in air at room temperature for 1-3 h to obtain the cellulose flat nanofiltration membrane with the stable structure;

the regenerated cellulose membrane is prepared by crushing a cellulose raw material, dissolving the crushed cellulose raw material in 70-90 wt% of N-methylmorpholine-N-oxide aqueous solution, adding 2-3 wt% of N-propyl gallate of the cellulose raw material, stirring and reacting at 100-120 ℃ for 1-3 h, stopping heating, cooling to 90 ℃, closing a stirrer, vacuumizing or standing for defoaming for 2-5 h to obtain 4-10 wt% of cellulose membrane casting solution, pouring the cellulose membrane casting solution on a glass plate, scraping to form a membrane, quickly immersing in deionized water, and removing the membrane.

2. The structurally stable cellulose planar nanofiltration membrane of claim 1, wherein: the cellulose raw material is any one of wood pulp, cotton pulp, hemp pulp, bamboo pulp, straw pulp, bagasse pulp, mulberry bark pulp or reed pulp.