Application of graphene oxide sub-nano channel membrane in anion and cation separation

Abstract

The invention discloses application of a graphene oxide sub-nanometer channel membrane in anion and cation separation, and belongs to the technical field of membranes. The application method comprises the following steps: providing a graphene oxide nano solution; carrying out vacuum filtration on the graphene oxide nano solution to prepare a film, so as to obtain an unsupported graphene oxide nano film or reduced graphene oxide nano film; the method comprises the following steps: cutting an unsupported graphene oxide nano-film or reduced graphene oxide nano-film into square long strips, and encapsulating and embedding the square long strips by using a solid material; and slicing to obtain the reduced graphene oxide or graphene oxide sub-nano channel film. The selectivity of the graphene oxide sub-nano-channel membrane to zwitterions is tested by an electrochemical method, so that the zwitterions are accurately screened. The sub-nano channel membrane is simple in preparation method and good in anti-swelling effect, and specific functional groups do not need to be additionally introduced. The membrane shows better zwitterion selectivity in various salt solutions, and a new strategy and a new method are provided for realizing efficient zwitterion screening.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, and in particular to the application of a graphene oxide subnanochannel membrane in the separation of anions and cations. Background Technology

[0002] Anion and cation separation technologies have important applications in wastewater treatment, energy conversion, and resource recovery. Traditional separation technologies, such as adsorption and ion exchange, are difficult to achieve long-term effective separation of anions and cations due to their large footprint and difficulties in material regeneration. Compared to these methods, membrane separation technology has attracted widespread attention due to its advantages such as low energy consumption, small footprint, no phase change, and ease of integration. Among them, ion exchange membranes possess excellent selective separation capabilities for anions and cations, making them an important means of achieving efficient anion and cation screening. However, existing ion exchange membranes mainly achieve anion and cation separation by introducing functionalized groups, a method with complex preparation processes and stringent reaction conditions. Furthermore, the raw materials for ion exchange membrane preparation often use fluoropolymer monomers, which can easily cause environmental harm.

[0003] With the rapid development of nanotechnology, membrane materials constructed from low-dimensional materials (referring to zero-dimensional, one-dimensional, and two-dimensional nanomaterials, such as carbon nanotubes (CNTs), graphene oxide (GO), and covalent organic frameworks (COFs)) have shown broad application prospects in wastewater treatment, seawater desalination, and resource recovery due to their excellent physicochemical properties, structural tunability, and modifiability. Compared to traditional membrane materials, low-dimensional material membranes can be directly prepared by vacuum filtration or scraping membrane methods, making the synthesis methods simpler. Furthermore, through physicochemical modification and structural orientation design, low-dimensional material membranes can exhibit excellent anion and cation selectivity, and some research has been conducted on salinity gradient energy recovery. However, due to their relatively low mechanical strength, low-dimensional material membranes struggle to achieve long-term stable and efficient anion and cation separation. Additionally, some low-dimensional materials, such as GO, swell upon contact with water, making it difficult to maintain their structural stability. Given that existing low-dimensional material membranes cannot meet the treatment requirements for efficient anion and cation selective sieving, there is an urgent need to develop a low-dimensional material membrane with a simple preparation process, stable physicochemical properties, and excellent anion and cation separation performance to meet these treatment needs. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides an application of a GO subnanochannel membrane (c-GO membrane) in anion and cation separation, achieving highly efficient selective sieving of anions and cations. The low-dimensional material subnanochannel membrane described in this invention has a simple preparation method, good anti-swelling effect, and does not require the introduction of additional specific functional groups. Furthermore, this membrane exhibits good anion and cation selectivity in various salt solutions, providing a new strategy and method for achieving highly efficient anion and cation separation.

[0005] The purpose of this invention is to provide an application of graphene oxide subnanochannel membrane in anion and cation separation, comprising the following steps: Graphene oxide (GO) nanomaterials were mixed with a reducing agent and a reduction reaction was carried out to prepare a reduced graphene oxide nanosolution. The graphene oxide GO nano solution or the reduced graphene oxide rGO solution was prepared into a membrane by vacuum filtration, dried and removed from the substrate membrane to obtain an unsupported graphene oxide nano GO membrane or a reduced graphene oxide nano rGO membrane. Unsupported graphene oxide nanofilms or reduced graphene oxide nanofilms are cut into rectangular strips and encapsulated with a solid material to restrict pore swelling. By slicing, the reduced graphene oxide (GO) or graphene oxide (rGO) sub-nanometer channel membranes (i.e., c-GO membranes or c-rGO membranes) are finally obtained. The selectivity of the reduced graphene oxide or graphene oxide subnanochannel membrane to anions and cations is tested by electrochemical methods to achieve accurate sieving of anions and cations.

[0006] In some embodiments of the present invention, the reducing agent comprises one or more of hydrazine hydrate, L-ascorbic acid, and sodium borohydride.

[0007] In some embodiments of the present invention, the mass ratio of the graphene oxide (GO) nanomaterial to the reducing agent is 25:(0~250); the mixing method is magnetic stirring at a speed of 200~600 rpm, and the reduction reaction time is 4~6 h.

[0008] In some embodiments of the present invention, the substrate membrane in the vacuum filtration is selected from one or more of polyethersulfone, polyvinylidene fluoride and anodic aluminum oxide membrane; the pore size is 0.22 μm.

[0009] In some embodiments of the present invention, the drying temperature is 25~40 °C and the drying time is 0.5~1 h. In some embodiments of the present invention, the solid material is selected from one or more of epoxy resin, acrylate and polydimethylsiloxane.

[0010] In some embodiments of the present invention, the separation of cations and anions refers to the separation of monovalent cations and anions.

[0011] In some embodiments of the present invention, the monovalent cation includes Li + Na + and K + One or more of the following, wherein the anion is Cl - .

[0012] In some embodiments of the present invention, the width of the square strip is 2 to 8 mm; the diameter of the slice is greater than the width of the square strip; and the diameter and thickness of the slice are both on the order of centimeters.

[0013] In some embodiments of the present invention, the electrochemical method test is performed using a four-electrode method; In the four-electrode method, the electrolytic cell is a commercial H-type electrolytic cell; the KCl solutions on both sides of the electrolytic cell have concentrations of 0.1 M and 0.01 M, respectively; and the electrochemical workstation is a commercial electrochemical workstation. The working electrode and counter electrode are both 2×2 cm platinum sheet electrodes, and the reference electrode and working sensing electrode are both standard Ag / AgCl electrodes containing a 3 M KCl solution salt bridge. The membrane potential is determined by scanning voltammetry, with the applied voltage range from -200 mV to 200 mV. The voltage corresponding to the zero system current is the membrane potential. Calculation of intramembrane cation transport number based on Nernst formula The calculation formula is as follows: in, and These represent the membrane potential and the ratio of electrolyte solution concentrations on both sides during the electromigration test, respectively. , , These represent the Faraday constant, the standard gas constant, and the Kelvin temperature, respectively. The closer the calculated result is to 1, the better the selectivity of the membrane material for cations, and the higher the cation transport number. It is a commonly used method for measuring the anion and cation selectivity of membrane materials.

[0014] In some embodiments of the present invention, the cation mobility test includes the following: The prepared membrane was placed in an H-type electrolytic cell, and the same electrolyte solution of the same concentration was added to the electrolytic cells on both sides of the cell. The volt-ampere characteristic curve of the system was determined by using an electrochemical workstation and the four-electrode method to determine the membrane ionic conductivity.

[0015] The concentration on both sides of the electrolytic cell is 0.1 M; the voltage applied during the test process is in the range of -200 mV to 200 mV, and the slope of the obtained IV curve is the membrane ionic conductivity.

[0016] Furthermore, the formula for calculating membrane ionic conductivity is as follows: , in, and The ionic conductances of membrane-containing and non-membrane-containing systems are respectively. and These represent the membrane thickness and cross-sectional area, respectively.

[0017] Furthermore, based on membrane ionic conductivity and cation transport number, the intramembrane ion mobility is calculated using the following formula: in, , , , and These represent the charge per unit electron, Avogadro's constant, solute concentration, cation mobility, and anion mobility, respectively.

[0018] The technical solution of the present invention has the following advantages compared with the prior art: (1) The preparation method is simple and the membrane structure is stable. This invention prepares reduced graphene oxide (rGO) by chemical reduction, which is simple and the reaction process is mild and controllable. By embedding the membrane with epoxy resin to limit the swelling phenomenon of the material structure, the interlayer porosity of the membrane material is maintained at the sub-nanometer level, which helps to improve ion selectivity.

[0019] (2) High cation and anion selectivity, and wide applicability. Based on the ionic double layer theory, this invention ensures a high degree of overlap of the double layers within the membrane pores by strictly limiting the spacing between the graphene oxide (GO) membrane layers, thereby achieving efficient cation and anion sieving and rapid mass transfer. The sub-nanometer channel membrane provided by this invention exhibits excellent ion selectivity for most monovalent cations and anions, and the test results show good parallelism, making it suitable for a wide range of applications.

[0020] (3) Provides a new approach to selective ion-anion separation. Compared with traditional ion exchange membranes, the sub-nanochannel membrane provided by this invention can achieve good ion-anion selectivity without the introduction of functional groups, providing a new approach for the development of novel ion-selective separation membranes. Attached Figure Description

[0021] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a schematic diagram of the overall process of the present invention; Figure 2 These are scanning electron microscope (SEM) images of the c-GO film obtained in Example 1 and the c-rGO film obtained in Example 2 of the present invention. Figure 3 The diagram shows the interlayer porosity of the c-GO membrane obtained in Example 1 and the c-rGO membrane obtained in Example 2 of this invention. Figure 4 The images show the X-ray photoelectron spectroscopy (XPS) spectra of the c-GO film obtained in Example 1 and the c-rGO film obtained in Example 2 of this invention. Figure 5 The c-GO membrane obtained in Example 1, the c-rGO membrane obtained in Example 2, and the membranes obtained in Comparative Examples 1-4 of this invention are compared with K. + Cl - Comparison chart of ion selectivity results; Figure 6 The KCl ion conductivity of the c-GO membrane obtained in Example 1, the c-rGO membrane obtained in Example 2, and the membranes obtained in Comparative Examples 1-4 are shown. Figure 7 The c-GO membrane obtained in Example 1, the c-rGO membrane obtained in Example 2, and the K in the membranes obtained in Comparative Examples 1-4 are examples of the present invention. + Ion mobility and K + The ratio of the migration rates of ion-containing aqueous solutions. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0023] Furthermore, unless otherwise specified, the reagents and devices described in the embodiments are all commercially available.

[0024] Example 1 This embodiment provides a graphene oxide subnanochannel membrane for anion and cation separation and its preparation method, the specific steps of which are as follows: (1) The graphene oxide GO solution was prepared into a membrane by vacuum filtration, dried at 40 °C for 1 h and then peeled off from the base membrane to form an unsupported graphene oxide GO membrane, which was stored at room temperature.

[0025] (2) The unsupported graphene oxide membrane was cut into a rectangular strip with a width of 8 mm. The sample was vertically fixed using a commercial sample holder and poured into a commercial cylindrical mold containing the fixed sample for encapsulation and embedding. After the epoxy resin solidified, it was processed using a low-speed precision cutting machine to produce a solid material sheet (i.e., GO sub-nanochannel c-GO membrane) containing a graphene oxide sub-nanochannel membrane with a diameter and thickness in the centimeter range. The commercial epoxy resin and its curing agent were mixed in a mass ratio of 3:1. The obtained graphene oxide sub-nanochannel membrane (c-GO membrane) was characterized, and the results are shown in the figure. Figures 2-4 .

[0026] Depend on Figure 2 As shown, the surface of the prepared c-GO membrane is regular and without defects, and the average thickness of the membrane is 8.206 μm.

[0027] Depend on Figure 3 As shown, the interlayer porosity of the fabricated c-GO film is 4 angstroms, which is in the sub-nanometer range.

[0028] Depend on Figure 4 As shown, the prepared c-GO membrane has abundant oxygen-containing functional groups, mainly existing in the form of ether bonds.

[0029] Example 2 This embodiment provides a c-rGO membrane for anion and cation separation and its preparation method, the specific steps of which are as follows: (1) GO and L-ascorbic acid were blended at a mass ratio of 25 mg:250 mg. The reduction reaction was carried out at 600 rpm for 6 h by magnetic stirring to prepare rGO solution.

[0030] (2) The above rGO solution was prepared into a membrane by vacuum filtration, dried at 40 °C for 1 h and then peeled off from the base membrane to form an unsupported rGO membrane, which was stored at room temperature.

[0031] (3) The unsupported rGO membrane was cut into a rectangular strip with a width of 8 mm. The sample was vertically fixed using a commercial sample holder and poured into a commercial cylindrical mold containing the fixed sample for encapsulation and embedding. After the epoxy resin solidified, it was processed using a low-speed precision cutting machine to produce a solid material sheet (i.e., c-rGO membrane) containing a reduced graphene oxide sub-nanochannel membrane with a diameter and thickness in the centimeter range. The commercial epoxy resin and its curing agent were mixed in a mass ratio of 3:1. The structure of the obtained c-rGO sub-nanochannel membrane was characterized, and the results are shown in […]. Figures 2-4 .

[0032] Depend on Figure 2 As shown, the surface of the prepared c-rGO membrane is flat and uniform, and the average thickness of the membrane is 7.040 μm.

[0033] Depend on Figure 3 As shown, the interlayer porosity of the fabricated c-rGO film is 3.6 angstroms, which is in the sub-nanometer range.

[0034] Depend on Figure 4 As shown, the number of oxygen-containing functional groups in the prepared c-rGO membrane is reduced, and they are mainly connected by carbon-carbon double bonds (i.e., benzene ring structure).

[0035] Comparative Example 1 This comparative example is similar to Example 1, except that the commercial nanofiltration membrane NF90 was used for anion and cation separation testing.

[0036] Comparative Example 2 This comparative example is similar to Example 1, except that a commercial cation exchange membrane FKB-PK-130 was used for anion and cation separation tests.

[0037] Comparative Example 3 This comparative example is similar to Example 1, except that the commercial proton exchange membrane Nafion-211 was used for anion and cation separation tests.

[0038] Comparative Example 4 This comparative example is similar to Example 1, except that the GO membrane was not embedded and sliced.

[0039] Performance Test 1: Cation Migration Ability The cation migration capacity of the c-GO membrane obtained in Example 1 and the c-rGO membrane obtained in Example 2 of this invention, the commercial nanofiltration membrane NF90 obtained in Comparative Example 1, the commercial cation exchange membrane FKB-PK-130 obtained in Comparative Example 2, the commercial proton exchange membrane Nafion-211 obtained in Comparative Example 3, and the unencapsulated GO membrane obtained in Comparative Example 4 were tested as follows: (1) Place the prepared membrane in a commercial H-type electrolytic cell with a volume of 40 mL, and add KCl solutions with concentrations of 0.1 M and 0.01 M to the two electrolytic cells respectively; (2) The membrane potential was determined using a four-electrode method on a commercial electrochemical workstation. The working electrode and counter electrode were placed on a 2×2 cm platinum sheet electrode, while the reference electrode and working sensing electrode were placed on a standard Ag / AgCl electrode containing a 3 M KCl solution salt bridge. The membrane potential was determined by scanning voltammetry, with an applied voltage range of -200 mV to 200 mV. The voltage corresponding to zero system current was the membrane potential.

[0040] (3) Calculate the intramembrane cation transport number based on the Nernst formula The calculation formula is as follows: , in, and These represent the membrane potential and the ratio of electrolyte solution concentrations on both sides during the electromigration test, respectively. , , These represent the Faraday constant, the standard gas constant, and the Kelvin temperature, respectively. The closer the calculated result is to 1, the better the selectivity of the membrane material for cations. Cation transport number. It is a commonly used method for measuring the anion and cation selectivity of membrane materials.

[0041] The experimental results are shown in Figure 5 ,Depend on Figure 5 As shown, the K selected in this invention +The transport numbers of the ions in the c-GO membrane obtained in Example 1 and the c-rGO membrane obtained in Example 2 were 0.88 and 0.95, respectively, close to 1, which is similar to that of the cation exchange membrane FKB-PK-130 in Comparative Example 2 and the commercial proton exchange membrane Nafion-211 in Comparative Example 3, indicating that the membranes obtained in the embodiments of the present invention have excellent anion and cation selectivity. Compared with the commercial nanofiltration membrane NF90 in Comparative Example 1 and the unencapsulated GO membrane in Comparative Example 4, the c-GO and c-rGO membranes obtained in Examples 1 and 2 have even better anion and cation selectivity.

[0042] Performance Test 2 Cation mobility determination The cation mobility of the c-GO membrane obtained in Example 1 and the c-rGO membrane obtained in Example 2 of this invention, the commercial nanofiltration membrane NF90 obtained in Comparative Example 1, the commercial cation exchange membrane FKB-PK-130 obtained in Comparative Example 2, the commercial proton exchange membrane Nafion-211 obtained in Comparative Example 3, and the uncoated GO membrane obtained in Comparative Example 4 were measured. The experimental results are shown in […]. Figure 6 ~ 7, as detailed below: (1) Place the prepared membrane in a commercial H-type electrolytic cell with a volume of 40 mL, and add KCl solutions with concentrations of 0.1 M and 0.1 M to the two sides of the electrolytic cell respectively; (2) The membrane potential was determined using a commercial electrochemical workstation via a four-electrode method. The working and counter electrodes were placed on a 2×2 cm platinum sheet electrode, while the reference and working sensing electrodes were placed on a standard Ag / AgCl electrode containing a 3 M KCl solution salt bridge. The membrane potential was determined by scanning voltammetry, with an applied voltage range of -200 mV to 200 mV. The slope of the resulting IV curve represents the membrane ionic conductivity.

[0043] (3) Based on Ohm's law, membrane ionic conductivity The calculation formula is as follows: , in, and The ionic conductances of membrane-containing and non-membrane-containing systems are respectively. and These represent the membrane thickness and cross-sectional area, respectively.

[0044] (4) Based on the membrane ion conductivity and cation transport number, the intramembrane ion mobility is calculated, and the formula is as follows: , in, , , , and These represent the charge per unit electron, Avogadro's constant, solute concentration, cation mobility, and anion mobility, respectively.

[0045] Depend on Figure 5 As shown, in this invention, the cation transport number (t) is used. + The above indicators are used to evaluate the anion and cation selectivity in membrane materials. The calculation formulas for these indicators are as follows: , in, and These represent the membrane potential and the ratio of electrolyte solution concentrations on both sides during the electromigration test, respectively. , , These represent the Faraday constant, the standard gas constant, and the Kelvin temperature, respectively. The closer the calculated result is to 1, the better the selectivity of the membrane material for cations.

[0046] Depend on Figure 6 As shown, the KCl ion conductivity of the c-GO membrane obtained in Example 1 and the c-rGO membrane obtained in Example 2 are 0.24 and 0.60 S / m, respectively. -1 It exhibits better ion transport performance than the selected Comparative Example 1 commercial nanofiltration membrane NF90, Comparative Example 2 commercial cation exchange membrane FKB-PK-130, Comparative Example 3 commercial proton exchange membrane Nafion-211, and the unencapsulated GO membrane obtained in Comparative Example 4.

[0047] Depend on Figure 7 As shown, the Kc of the c-GO membrane obtained in Example 1 and the c-rGO membrane obtained in Example 2 are... + The ion mobilities were 2.18 × 10⁻⁶. -8 and 5.92×10 -8 m 2 s -1 V -1 Among them, K + The ion mobility in c-rGO membranes is significantly higher than that of other commercial membranes, namely, the commercial nanofiltration membrane NF90, the commercial cation exchange membrane FKB-PK-130, the commercial proton exchange membrane Nafion-211, and the uncoated GO membrane (2 × 10⁻⁶). 5 The increases of 2.3 times, 9.4 times, and 371.2 times indicate that this membrane preparation method can significantly improve Kc. + Rapid ion transmembrane mass transfer capability.

[0048] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An application of a graphene oxide subnanochannel membrane in the separation of anions and cations, characterized in that, Includes the following steps: Graphene oxide nanomaterials were mixed with a reducing agent and a reduction reaction was carried out to prepare a reduced graphene oxide nanosolution. The graphene oxide nanosolution or reduced graphene oxide solution is prepared into a membrane by vacuum filtration, dried and removed from the base membrane to obtain an unsupported graphene oxide nanofilm or a reduced graphene oxide nanofilm. Unsupported graphene oxide nanofilms or reduced graphene oxide nanofilms are cut into rectangular strips and encapsulated with a solid material to restrict pore swelling; by slicing, the reduced graphene oxide or graphene oxide subnanometer channel membrane is finally obtained. The selectivity of the reduced graphene oxide or graphene oxide subnanochannel membrane to anions and cations is tested by electrochemical methods to achieve accurate sieving of anions and cations.

2. The application according to claim 1, characterized in that, The reducing agent comprises one or more of hydrazine hydrate, L-ascorbic acid, and sodium borohydride.

3. The application according to claim 1, characterized in that, The mass ratio of the graphene oxide nanomaterial to the reducing agent is 25:(0~250); the mixing method is magnetic stirring at a speed of 200~600 rpm, and the reduction reaction time is 4~6h.

4. The application according to claim 1, characterized in that, The substrate membrane used in vacuum filtration is selected from one or more of polyethersulfone, polyvinylidene fluoride and anodic aluminum oxide membrane, with a pore size of 0.22 μm.

5. The application according to claim 1, characterized in that, The drying temperature is 25~40 ℃, and the drying time is 0.5~1h.

6. The application according to claim 1, characterized in that, The solid material is selected from one or more of epoxy resin, acrylate and polydimethylsiloxane.

7. The application according to claim 1, characterized in that, The separation of cations and anions refers to the separation of monovalent cations and anions.

8. The application according to claim 7, characterized in that, The monovalent cations include Li + Na + and K + One or more of the following, wherein the anion is Cl - .

9. The application according to claim 1, characterized in that, The width of the rectangular strip is 2-8 mm; the diameter of the slice is greater than the width of the rectangular strip; the diameter and thickness of the slice are both on the order of centimeters.

10. The application according to claim 1, characterized in that, Electrochemical testing was performed using a four-electrode method. In the four-electrode method, the electrolytic cell is a commercial H-type electrolytic cell; the electrochemical workstation is a commercial electrochemical workstation; The working electrode and counter electrode are platinum sheet electrodes, and the reference electrode and working sensing electrode are standard Ag / AgCl electrodes of KCl solution salt bridge. The membrane potential is determined by scanning voltammetry, with an applied voltage range of -200 mV to 200 mV. The voltage corresponding to zero system current is the membrane potential. Calculation of intramembrane cation transport number based on Nernst formula The calculation formula is as follows: ， in, and These represent the membrane potential and the ratio of electrolyte solution concentrations on both sides during the electromigration test, respectively. , , These are Faraday constant, standard gas constant, and Kelvin temperature, respectively. The closer the calculated result is to 1, the better the selectivity of the membrane material for cations, and the higher the cation transference number. It is a commonly used method for measuring the anion and cation selectivity of membrane materials.

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