A method for the preparation of graphene oxide membranes for lithium isotope separation

Abstract

The application relates to the technical field of isotope separation, and particularly relates to a preparation method of a graphene oxide film for lithium isotope separation, which comprises the following steps: S1, oxidation reaction: S1.1, taking a natural graphite sample, grinding and crushing the sample sufficiently, then slowly adding the sample into concentrated sulfuric acid, continuously stirring until a uniform reaction liquid is formed; S1.2, grinding potassium permanganate into powder, and adding the powder into the reaction liquid; S1.3, then adding ultrapure water drop by drop into the reaction liquid for dilution, uniformly stirring at normal temperature, and covering the reaction liquid with a preservative film; S2, centrifugation and washing; S3, film preparation; and S4, regulation of layer spacing. The preparation strategy of first carrying out film plating and then regulating the layer spacing is adopted, the uniformity of the prepared film structure is ensured, the regulation of the layer spacing is more accurate, and the purity is higher; the technical problem that in the prior art, the film structure uniformity is poor due to uneven distribution of the graphene oxide solution in the film plating process is solved, and the dispersion problem in the film plating process in the prior art is fundamentally avoided.

Description

Technical Field

[0001] This invention relates to the field of isotope separation technology, and more specifically to a method for preparing a graphene oxide membrane for lithium isotope separation. Background Technology

[0002] Two main isotopes of lithium exist in nature: 6Li and 7Li. 6Li has a relative abundance of 7.45%, while 7Li has a relative abundance of 92.55%. Both isotopes have significant industrial applications. 7Li is characterized by its small neutron cross-section, playing a crucial role in regulating nuclear fission reactions and maintaining equipment. 7Li can be used as a coolant in next-generation molten salt reactors and as a pH adjuster in pressurized water reactors. 6Li is a raw material in nuclear fusion reactors; thermal neutron bombardment of 6Li produces tritium. Furthermore, 6Li itself is an important thermonuclear weapon material, possessing significant defense value.

[0003] The lithium amalgam method is currently the main industrial production method for separating lithium isotopes. The lithium amalgam method is characterized by a high separation coefficient, fast exchange rate, and easy interconversion and phase-to-phase flow within the system, which is beneficial for process design. However, the lithium amalgam method requires the use of large amounts of mercury, a highly toxic heavy metal pollutant that poses a significant threat to the environment. The United States discontinued this separation method in the 1970s. With increasing emphasis on environmental protection in my country, finding an environmentally friendly method for separating lithium isotopes has become increasingly important.

[0004] Membrane separation technology offers advantages such as energy saving, environmental friendliness, simple operation, low maintenance costs, and small footprint, and is gradually gaining attention in the field of isotope separation. Especially in nanoscale confined mass transfer systems, the migration behavior of ions within confined channels is influenced by size effects, hydration structures, and interfacial interactions, potentially amplifying minute differences between isotopes and achieving selective separation. Against this backdrop, membrane materials based on two-dimensional layered structures are considered to have potential advantages, as their sub-nanometer channels can provide finely controlled space for ion sieving.

[0005] Subsequent researchers proposed membrane separation methods, primarily focusing on gel polymer electrolyte membranes for lithium isotope separation, their preparation methods, and applications (invention patent, application number: 202211415565.7), and a method for enrichment. 6 Li isotope composite separation membrane and its application (Invention Patent, Application No.: 202410212444.5).

[0006] For lithium isotope-separated gel polymer electrolyte membranes and their preparation and applications, the core ionic liquids and chelating agents (such as 10-hydroxybenzoquinoline and azonaphthols) in the formulation are specialized fine chemicals. In particular, these organic chelating agents with specific spatial configurations involve complex synthesis steps and are expensive, significantly increasing the cost of large-scale industrial production of membrane materials. Furthermore, precise mass ratios of the components are required (e.g., monomer:ionic liquid:chelating agent approximately 20-40:40-50:20-30). Finding the optimal ratio window in actual production is very time-consuming. Inappropriate proportions of ionic liquids or chelating agents can lead to uneven polymer precipitation, the formation of bubbles within the membrane, or obstruction of lithium-ion conduction channels. Moreover, the reagents used in the preparation process have certain toxicity and safety risks. Methyl methacrylate has an irritating odor, and long-term exposure may affect the skin and respiratory tract. Benzoyl peroxide is a strong oxidizing agent that is easily decomposed by heat or friction, posing a potential explosion hazard. Strict safety control is required during storage and handling. Some chelating agents (such as azo compounds) may have certain biotoxicity or sensitizing properties, posing environmental challenges in the later stages of membrane material recycling and waste disposal.

[0007] For a type of enrichment 6 Although the composite separation membrane for Li isotopes and its application improved the separation flux of lithium isotopes in the examples, its separation coefficient was about 1.03-1.06, which is similar to the separation coefficient of other membrane separation technologies and did not represent a breakthrough. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a method for preparing graphene oxide films for lithium isotope separation that utilizes the layered structure of graphene and the method of adjusting the interlayer spacing to enhance the difference in migration rates between different isotopes, improve the lithium isotope separation coefficient, and thus achieve efficient isotope separation.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a method for preparing a graphene oxide membrane for lithium isotope separation, comprising the following steps: S1, Oxidation reaction: S1.1 Take a natural graphite sample, grind it thoroughly, and slowly add it to concentrated sulfuric acid while stirring continuously until a uniform reaction solution is formed. S1.2 Grind potassium permanganate into powder and add it to the reaction solution; S1.3. Then, add ultrapure water dropwise to the reaction solution for dilution, stir evenly at room temperature, and cover with plastic wrap. S2, Centrifugation and washing: The graphene oxide solution obtained in S1.3 was centrifuged and washed multiple times to obtain the final precipitate dispersed in ultrapure water, and then ultrasonically treated to obtain a graphene oxide dispersion. S3. Membrane preparation: The graphene oxide dispersion prepared in S2 is used as a membrane precursor. The graphene oxide dispersion is slowly poured into a vacuum filtration device. Under negative pressure, graphene sheets are deposited layer by layer along the surface of the substrate membrane to form a continuous and dense membrane layer. S4. Adjusting the interlayer spacing: Immerse the membrane layer filtered in S3 in a lithium nitrate solution to adjust the interlayer spacing, and then dry and clean it to obtain a graphene oxide membrane that separates lithium isotopes.

[0010] Further, prior to step S1, the natural graphite sample undergoes a pre-oxidation treatment. The specific steps include: dissolving potassium persulfate and phosphorus pentoxide in concentrated sulfuric acid, dissolving them uniformly with ultrasonic assistance, then adding fully ground natural graphite powder, stirring evenly, diluting with ultrapure water, allowing it to stand until solid-liquid separation, then discarding the supernatant, filtering the resulting precipitate, and finally drying it in a vacuum drying oven to obtain a pre-oxidized graphite sample.

[0011] Furthermore, during the pre-oxidation process, natural graphite powder is added using a water bath at a constant temperature until the solution gradually becomes a paste and exhibits strong viscosity, indicating that the graphite has undergone partial oxidation. Then, it is cooled to room temperature and ultrapure water is added for dilution.

[0012] Furthermore, in steps S1.2 and S1.3, the addition of potassium permanganate solution and the dropwise addition of ultrapure water are both carried out under ice-water bath conditions.

[0013] Further, after step S1.3, the reaction solution is transferred to a beaker, stirred continuously, and deionized water is slowly added. Hydrogen peroxide is then added dropwise. At this point, the reaction solution produces a large number of bubbles, and its color gradually changes from brown to bright yellow, indicating that the oxidation reaction has been successfully completed.

[0014] Furthermore, in S2, the specific steps of multiple centrifugations and washing include: S2.1. Centrifuge the graphene oxide solution obtained in S1.3 and discard the supernatant to obtain a preliminary precipitate; S2.2. The initial precipitate is redispersed in diluted hydrochloric acid solution, and centrifuged a second time to discard the supernatant, resulting in a further precipitate. S2.3. The further precipitate obtained in S2.2 is dispersed again with deionized water and centrifuged for a third time to obtain the final precipitate; S2.4. The final precipitate is dispersed in ultrapure water and subjected to ultrasonic treatment to obtain a graphene oxide dispersion.

[0015] Furthermore, in step S3, the pretreated base membrane is flattened and fixed in the filtration device as a support layer. The specific steps of the pretreatment are as follows: before filtration, the base membrane is fully wetted with deionized water.

[0016] Furthermore, the base membrane is made of metal, ceramic, or fiber material.

[0017] Furthermore, prior to step S4, the filtered membrane layer is subjected to multiple cleaning processes, each lasting no more than 1 hour, to remove residual acidic substances, metal ions, or other soluble impurities from the membrane formation process.

[0018] Furthermore, in S4, the concentration of the lithium nitrate solution does not exceed 1 mol / L, the soaking process is carried out at room temperature, and the soaking time does not exceed 1 hour.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention adopts a preparation strategy of first depositing a film and then controlling the interlayer spacing to ensure that the prepared film structure is uniform, thereby making the interlayer spacing control more precise and the purity higher; it solves the technical problem that the uneven distribution of graphene oxide solution during the deposition process in the prior art leads to poor film structure uniformity, and fundamentally avoids the dispersion problem in the deposition process in the prior art. 2. This invention effectively removes residual acidic substances, metal ions, or other soluble impurities from the membrane fabrication process by performing multiple cleaning processes on the membrane layer after filtration. Furthermore, through precise parameter control and repeated rinsing and drying procedures after soaking, free lithium ions and other residual impurities can be thoroughly removed, ensuring that the subsequent separation effect is not affected. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1: A method for preparing a graphene oxide membrane for lithium isotope separation includes the following steps: S1, Oxidation reaction: The pre-oxidation treatment of natural graphite samples includes the following steps: dissolving potassium persulfate and phosphorus pentoxide in concentrated sulfuric acid, using ultrasonic-assisted dissolution to achieve uniformity, adding fully ground natural graphite powder in a water bath at a constant temperature until the solution gradually becomes a paste and exhibits strong viscosity, indicating that the graphite has undergone partial oxidation, cooling to room temperature, adding ultrapure water for dilution, allowing it to stand until solid-liquid separation, then discarding the supernatant, filtering the resulting precipitate, and finally drying it in a vacuum drying oven to obtain the pre-oxidized graphite sample; S1.1 After thoroughly grinding and crushing the dried pre-oxidized graphite sample, slowly add it to concentrated sulfuric acid and continue stirring until a uniform reaction solution is formed. S1.2. Grind potassium permanganate into a fine powder and slowly add it to the reaction solution in small batches under ice-water bath conditions. As potassium permanganate is added, the system gradually turns dark green. Then, heat the system and stir it. The system gradually changes from dark green to a viscous paste. S1.3. Under ice-water bath conditions, slowly add ultrapure water dropwise to the system for dilution. Each addition should be allowed to proceed only after the system temperature and bubbles have stabilized. After stirring thoroughly at room temperature, cover with plastic wrap. Transfer the reaction solution to a beaker, and while stirring, slowly add deionized water, then rapidly add hydrogen peroxide dropwise. At this point, the mixture will produce a large number of bubbles, and the color will gradually change from brown to bright yellow, indicating that the oxidation reaction has been successfully completed. Continue stirring thoroughly and let stand overnight at room temperature. S2, Centrifugation and washing: The graphene oxide solution obtained in S1.3 was centrifuged and washed multiple times to obtain the final precipitate dispersed in ultrapure water, and then ultrasonically treated to obtain a graphene oxide dispersion. S2.1. Centrifuge the graphene oxide solution obtained in S1.3 and discard the supernatant to obtain a preliminary precipitate; S2.2. The initial precipitate is redispersed in diluted hydrochloric acid solution, and centrifuged a second time to discard the supernatant, resulting in a further precipitate. S2.3. The further precipitate obtained in S2.2 is dispersed again with deionized water and centrifuged for a third time to obtain the final precipitate; S2.4. The final precipitate is dispersed in ultrapure water and subjected to ultrasonic treatment to obtain a graphene oxide dispersion. S3. Membrane preparation: The vacuum filtration device consists of a vacuum filtration flask, a filter membrane clamp, and a vacuum pump. During the vacuum filtration process, a stable negative pressure condition (approximately -0.1 MPa) is maintained. The pretreated base membrane is flattened and fixed in the vacuum filtration device as a support layer. The selected base membrane is a cellulose membrane with a pore size of 0.2-0.5 μm. Before vacuum filtration, the base membrane is fully wetted with deionized water to ensure that the graphene oxide dispersion can be evenly spread and tightly bound. The graphene oxide dispersion prepared in S2 was used as a film-forming precursor. The graphene oxide dispersion was slowly poured into a vacuum filtration device. Under negative pressure, graphene sheets were deposited layer by layer along the surface of the substrate membrane to form a continuous and dense film layer. After filtration, the wet film and the substrate membrane were taken out together and allowed to stand at room temperature to release residual moisture. Then, it was placed in a constant temperature drying oven to dry, so that the film layer and the substrate membrane were tightly bonded to obtain a film with a complete structure. The obtained film surface was smooth and without obvious cracks, and the film layer and the substrate membrane were firmly bonded. S4. Adjusting the interlayer spacing: The membrane after filtration is washed multiple times, with each wash lasting no more than 1 hour, to remove residual acidic substances, metal ions, or other soluble impurities from the membrane fabrication process. After washing, the membrane is dried. The dried membrane is then immersed in a lithium nitrate solution to adjust the interlayer spacing. The concentration of the lithium nitrate solution does not exceed 1 mol / L. The immersion process is carried out at room temperature for no more than 1 hour. During the immersion process, the membrane remains intact and does not exhibit obvious warping or delamination. After immersion, the membrane is removed and its surface is gently rinsed with a small amount of deionized water to remove unadsorbed or weakly bound free lithium ions. It is then dried again to constant weight to finally obtain a graphene oxide membrane that separates lithium isotopes.

[0022] Lithium isotope separation process: 1. The feed solution is an aqueous lithium chloride solution with an initial concentration of 1.0 mol / L, serving as the raw material system for lithium isotope separation. 6 Li and 7 Li maintains its natural abundance distribution in solution. 6 The abundance of Li was 8.09%, which is higher than that of R. f for =0.088, and deionized water was used as the extractant to construct a stable osmotic pressure difference driven system; All solutions were prepared using analytical grade reagents and deionized water to ensure that no other interfering ions in the system would affect the migration behavior of isotopes. 2. The separation device consists of two symmetrical glass reaction tanks, each with a volume of 100 mL. They are separated by a membrane module to form two independent fluid spaces: the feed side and the absorption side. The membrane module is in the form of a disc membrane and uses the graphene oxide membrane prepared in the example. During the experiment, both solutions were kept agitated by magnetic stirrers to reduce the influence of concentration polarization on the mass transfer process. The system was operated at room temperature without additional pressure, and the separation process was driven entirely by the osmotic pressure difference between the solutions. The device operates in an intermittent static mode, meaning that after each stage of separation is completed within a fixed time, the solution is transferred and the membrane is replaced before proceeding to the next stage of separation. 3. Add lithium chloride solution to the feed side (high concentration side) and add an equal volume of deionized water to the draw side. The system is run under stirring conditions to ensure sufficient water migration and solute mass transfer. 4. Separation Results: After the operation is completed, the solution from the aspirated side is taken for separation testing. 6 The abundance of Li was 11.25%, which was higher than that of R. p for =0.1268, that is 6 The single-stage separation coefficient of Li is Rp / R f =1.44.

[0023] Conclusion: The separation coefficient of lithium isotopes using the graphene oxide film prepared by this invention is much higher than that of existing technologies. This is because this invention changes the preparation process of the graphene oxide film, adopting a strategy of coating first and then regulating, which fundamentally avoids the dispersion problem in the coating process. It also avoids the problem of poor film structure uniformity caused by uneven distribution of graphene oxide solution during the coating process in the existing technology of regulating first and then coating, thus ensuring the uniformity of the film structure. As a result, the interlayer spacing can be controlled more precisely and the purity is better. Furthermore, this invention introduces multiple rinsing and drying processes before and after coating, which removes the influence of impurities to the maximum extent and has a shorter processing cycle.

[0024] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a graphene oxide membrane for lithium isotope separation, characterized in that, Includes the following steps: S1, Oxidation reaction: S1.1 Take a natural graphite sample, grind it thoroughly, and slowly add it to concentrated sulfuric acid while stirring continuously until a uniform reaction solution is formed. S1.2 Grind potassium permanganate into powder and add it to the reaction solution; S1.

3. Then, add ultrapure water dropwise to the reaction solution for dilution, stir evenly at room temperature, and cover with plastic wrap. S2, Centrifugation and washing: The graphene oxide solution obtained in S1.3 was centrifuged and washed multiple times to obtain the final precipitate dispersed in ultrapure water, and then ultrasonically treated to obtain a graphene oxide dispersion. S3. Membrane preparation: The graphene oxide dispersion prepared in S2 is used as a membrane precursor. The graphene oxide dispersion is slowly poured into a vacuum filtration device. Under negative pressure, graphene sheets are deposited layer by layer along the surface of the substrate membrane to form a continuous and dense membrane layer. S4. Adjusting the interlayer spacing: Immerse the membrane layer filtered in S3 in a lithium nitrate solution to adjust the interlayer spacing, and then dry and clean it to obtain a graphene oxide membrane that separates lithium isotopes.

2. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, Before step S1, the natural graphite sample is pre-oxidized. The specific steps include: dissolving potassium persulfate and phosphorus pentoxide in concentrated sulfuric acid, dissolving evenly with ultrasonic assistance, then adding fully ground natural graphite powder, stirring evenly, diluting with ultrapure water, letting stand until solid-liquid separation, then discarding the supernatant, filtering the obtained precipitate, and finally drying in a vacuum drying oven to obtain the pre-oxidized graphite sample.

3. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 2, characterized in that, During the pre-oxidation process, natural graphite powder is added using a water bath at a constant temperature until the solution gradually becomes a paste and exhibits strong viscosity, indicating that the graphite has undergone partial oxidation. Then, it is cooled to room temperature and diluted with ultrapure water.

4. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, In steps S1.2 and S1.3, the addition of potassium permanganate solution and the addition of ultrapure water were both carried out under ice-water bath conditions.

5. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, After step S1.3, the reaction solution is transferred to a beaker, stirred continuously, and deionized water is slowly added. Hydrogen peroxide is then added dropwise. At this point, the reaction solution produces a large number of bubbles, and the color gradually changes from brown to bright yellow, indicating that the oxidation reaction has been successfully completed.

6. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, In S2, the specific steps of multiple centrifugation and washing include: S2.

1. Centrifuge the graphene oxide solution obtained in S1.3 and discard the supernatant to obtain a preliminary precipitate; S2.

2. The initial precipitate is redispersed in diluted hydrochloric acid solution, and centrifuged a second time to discard the supernatant, resulting in a further precipitate. S2.

3. The further precipitate obtained in S2.2 is dispersed again with deionized water and centrifuged for a third time to obtain the final precipitate; S2.

4. The final precipitate is dispersed in ultrapure water and subjected to ultrasonic treatment to obtain a graphene oxide dispersion.

7. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, In step S3, the pretreated base membrane is flattened and fixed in the filtration device as a support layer. The specific steps of the pretreatment are as follows: before filtration, the base membrane is fully wetted with deionized water.

8. The method for preparing graphene oxide membrane for lithium isotope separation according to claim 7, characterized in that, The base membrane is made of metal, ceramic, or fiber material.

9. The method for preparing a graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, Before step S4, the filtered membrane layer is cleaned multiple times, with each cleaning time not exceeding 1 hour, to remove residual acidic substances, metal ions, or other soluble impurities from the membrane formation process.

10. The method for preparing a graphene oxide membrane for lithium isotope separation according to claim 1, characterized in that, In step S4, the concentration of the lithium nitrate solution does not exceed 1 mol / L, the soaking process is carried out at room temperature, and the soaking time does not exceed 1 hour.