CN107261857B

CN107261857B - Modified graphene membrane for separating monovalent and polyvalent metal ions and preparation method thereof

Info

Publication number: CN107261857B
Application number: CN201710325859.3A
Authority: CN
Inventors: 奚月恒; 刘壮; 褚良银; 汪伟; 谢锐; 巨晓洁
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2017-05-10
Filing date: 2017-05-10
Publication date: 2020-06-30
Anticipated expiration: 2037-05-10
Also published as: CN107261857A

Abstract

The invention discloses a modified graphene membrane for separating monovalent and polyvalent metal ions, wherein the membrane material is slightly reduced graphene oxide with the carbon-oxygen mass ratio of 1.2-1.4, and a two-dimensional nano mass transfer channel formed by slightly reduced graphene oxide sheet layers in the membrane can enable monovalent metal ions to pass through and intercept polyvalent metal ions. The modified graphene film may be prepared by a method comprising the process steps of: uniformly dispersing the slightly reduced graphene oxide in water to form a slightly reduced graphene oxide casting solution; filtering the casting solution by using a substrate membrane to form a wet-state modified graphene membrane on the substrate membrane; and (3) drying the wet modified graphene film and the substrate film at normal temperature, and peeling the dried film from the substrate film to obtain the modified graphene film.

Description

Modified graphene membrane for separating monovalent and polyvalent metal ions and preparation method thereof

Technical Field

The invention belongs to the field of graphene oxide membranes, and particularly relates to a modified graphene membrane for separating monovalent and polyvalent metal ions and a preparation method thereof.

Background

The separation of monovalent/polyvalent metal ions is widely applied in the fields of resources, environment, agriculture, life health and the like. Monovalent metal ions such as potassium, sodium and lithium ions are ions which have positive significance for human production and life. Potassium ions are the main metal ions in the intracellular fluid of human and animals, and directly affect the health of the human and animals; potassium ions are also one of three major nutrients for plant growth, and the use of potassium fertilizer can promote the maturity of crops. Sodium ions are the main metal ions in human and animal extracellular fluids, and adults need to take about 10 g of salt every day to supplement sodium chloride required by human bodies; sodium compounds such as sodium hydroxide and sodium chloride are also important industrial raw materials. Lithium ions are monovalent metal ions with the smallest atomic weight, have great energy density as the anode of the battery, and are indispensable metal ions for preparing clean and efficient lithium ion batteries. Most of these monovalent metal ion resources are extracted from natural seawater or brine, but seawater and brine are often mixed with high content of polyvalent metal ions, such as alkaline earth metal ions of magnesium, calcium, etc., and in the process of extracting monovalent metal ions by purification, polyvalent metal ion impurities are removed from monovalent metal ions.

In addition, polyvalent metal ions such as lead, cadmium and chromium, which cause great damage to the nervous system of human and animals, are indispensable raw materials in many industrial productions or are often present together, so that they are inevitably contained in the relevant industrial discharge wastewater, and the polyvalent metal ions should be strictly removed in the treatment of the wastewater. Of course, the above monovalent metal ions contained in the wastewater do not need to be removed because they are beneficial to animals and plants. Therefore, the separation of univalent and multivalent metal ions is realized, the multivalent metal ions are intercepted, and the univalent metal ions are returned to be natural resources, thereby having important significance for environmental protection and resource recycling.

Membrane separation techniques are important means for separating monovalent and polyvalent metal ions. Heretofore, separation membranes have been largely classified into two major categories, i.e., (1) charge-repelling membranes and (2) size-sieving membranes, according to the mechanism of separation of monovalent and polyvalent metal ions.

According to the Dow's south effect, the electrostatic repulsion force of the positively charged charge-repelling membrane to the polyvalent metal ions is greater than that of the monovalent metal ions, so that the permeation of the polyvalent metal ions can be greatly hindered, and the separation of the monovalent and polyvalent metal ions is embodied. These charge-rejection membranes combine electrodialysis techniques to drive mass transfer with electric forces, which simultaneously enhance the permeability and selectivity of ion separation. However, the positively charged functional layers of these membranes are prepared with weak electrostatic interactions, are easily destroyed by changes in current and solution pH, and cannot achieve stable separations over long periods of time. Moreover, the limiting current density is too low to be stably controlled in practical use, and if it is slightly exceeded, by-products are electrolyzed out, the membrane structure is destroyed, and the separation performance is impaired.

Based on size effect, the separation of ions or molecules with different sizes can be realized by constructing membrane pores with different sizes. Monovalent metalThe hydration diameter of the ion is in the range of 0.658-0.764nm, and the hydration diameter of the polyvalent metal ion is in the range of 0.802-0.950nm, so that the uniform membrane pores or channels with the diameter of about 0.8nm are constructed to have the best separation effect on the monovalent and polyvalent metal ions. Currently, membrane pores are prepared by the following methods: (1) interface polymerization is carried out by utilizing high molecules such as TMC/MPD (1,3, 5-benzene trimethyl acyl chloride/1, 3-phenylenediamine) and the like to construct nanofiltration membrane pores; (2) punching holes on a PET (polyethylene terephthalate) film and a graphene film by heavy ion irradiation; (3) punching a graphene film by using electric pulses; (4) using graphene oxide, Ti₃C₂T_xTwo-dimensional nanochannels are built by two-dimensional materials (two-dimensional transition metal carbide or carbonitride) and the like, and membrane pores/channels with different sizes can be built for mass transfer and separation of ions or molecules. However, the pore diameter distribution range of the polymeric nanofiltration membrane is too wide (0.4-2.0nm), the membrane pores are irregular and uniform, and the accurate separation of monovalent and polyvalent metal ions cannot be realized. The pore size of PET and graphene membranes prepared by the heavy ion irradiation method is distributed in the range of 0.4-0.6nm, but the perforation density of the membranes is low, so that the permeation flux of ion separation is small. The graphene membrane prepared by the electric pulse perforating method has a large aperture (2-20nm), and almost all monovalent and polyvalent metal ions can not be separated.

A two-dimensional nano-channel with the height adjustable between 0.64 and 0.98nm can be designed by using a physical confinement graphene oxide nano-sheet layer method, the permeation rate of divalent metal ions in the channel is far lower than that of monovalent metal ions, but the nano-channel is longer in length and longer in mass transfer distance, so that the ion permeation flux is lower. Further, Ti₃C₂T_xThe height of the two-dimensional nano channel can be enlarged/reduced along with the change of the valence of the metal ions through the electrostatic repulsion/attraction effect between the ions and the nano sheet layers, but under the condition that monovalent and polyvalent metal ions coexist, the two-dimensional nano structure is in the mixed combined action of electrostatic repulsion and attraction, and the efficient separation of the monovalent and polyvalent metal ions is difficult to realize. The two-dimensional nanochannel height constructed by graphene oxide is 0.9nm, and most of monovalent and polyvalent metal ions with the hydration diameter less than 0.9nm cannot be separated. If the two-dimensional nanochannel height is to be reduced, it is generally desirable to eliminate a certain amountAs a supporting oxidizing group on the nanosheet layer, thereby reducing the interlayer spacing of the two-dimensional nanochannel. To date, the common methods for eliminating the oxidized group are mainly two methods, i.e., (1) thermal reduction and (2) chemical reduction, performed on graphene oxide. However, thermal reduction can destroy the structure of the graphene oxide nanosheet layer, form a large number of defects and holes, and is not favorable for precise separation of ions and molecules. The chemical reduction under the high-temperature hydrothermal condition can cause fluctuation and wrinkling of the sheet layer, so that the constructed nano structure is not flat, and the uniformity of the height of the two-dimensional nano channel is influenced. None of these methods produces two-dimensional nanosheets suitable for separation of monovalent/polyvalent metal ions.

None of the above methods can achieve highly efficient and accurate separation of monovalent and polyvalent metal ions. At present, how to achieve the aim is a great challenge to construct nano membrane pores or nano channels with regular and uniform sizes, low mass transfer resistance and good separation precision.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a modified graphene membrane with flat and regular two-dimensional nano-channels and uniform lamella spacing and a preparation method thereof, so that monovalent metal ions and polyvalent metal ions can be efficiently and accurately separated.

The basic idea of the invention is to control the interlayer spacing of the graphene two-dimensional nano sheets by adjusting the modification degree of graphene, design a two-dimensional nano mass transfer channel about 0.8nm and realize the accurate screening of polyvalent metal ions with the hydration diameter larger than 0.8nm and monovalent metal ions with the hydration diameter smaller than 0.8 nm.

The membrane material of the modified graphene membrane for separating monovalent metal ions from polyvalent metal ions is slightly reduced graphene oxide with the carbon-oxygen mass ratio of 1.2-1.4, and monovalent metal ions can pass through the two-dimensional nano mass transfer channel formed by the slightly reduced graphene oxide sheet layers in the membrane and the polyvalent metal ions are intercepted.

In the technical scheme of the modified graphene membrane, the interlayer distance between the slightly reduced graphene oxide sheets forming the two-dimensional nano mass transfer channel is 0.765 nm-0.801 nm.

In the technical scheme of the modified graphene film, the thickness of the modified graphene film can be 0.5-5.0 μm; the preferred film thickness is 1.5 μm to 2.5. mu.m.

In the technical scheme of the modified graphene membrane, the modified graphene membrane is preferably formed by stacking slightly reduced graphene oxide platelets layer by layer through suction filtration. The slightly reduced graphene oxide is prepared by chemically reducing graphene oxide at the temperature of normal temperature (10-25 ℃). The modification degree of the slightly modified graphene refers to the degree of reduction and oxidation, depends on the reduction and oxidation reaction time at normal temperature, and is characterized by the carbon-oxygen mass ratio. The carbon-oxygen mass ratio of the common graphene oxide is 0.86-0.91.

The modified graphene film can be prepared by a method comprising the following process steps:

(1) uniformly dispersing the slightly reduced graphene oxide in water to form a casting solution with the concentration of the slightly reduced graphene oxide of 0.20 mg/mL-0.30 mg/mL;

(2) filtering the casting solution obtained in the step (1) by using a base material membrane to form a wet-state modified graphene membrane on the base material membrane;

(3) and (2) drying the wet modified graphene film and the substrate film at normal temperature (10-25 ℃), and then stripping the dried modified graphene film from the substrate film to obtain the modified graphene film.

The modified graphene film prepared by the method can be prepared by selecting slightly reduced graphene oxide prepared by proper reduction time at normal temperature, namely selecting slightly reduced graphene oxide with different carbon-oxygen mass ratios, and the wet effective interlamellar spacing of the modified graphene oxide film can be adjusted to be about 0.8 nm.

In the technical scheme for preparing the modified graphene membrane, the substrate membrane is preferably an organic membrane with an average pore diameter of 200 nm-230 nm. It is further preferable to use a circular organic substrate film having a diameter of 45mm to 50 mm. The organic substrate film is a commercially available organic substrate film directly available from the market.

In the technical scheme for preparing the modified graphene film, the amount of the casting solution is such that the modified graphene film with the designed thickness can be formed; for a circular organic substrate membrane with the diameter of 45 mm-50 mm, the dosage of the casting solution is 38 mL-42 mL.

Graphene oxide is commercially available or may be prepared by reference to existing methods, for example: the preparation of graphene oxide can be referred to in (1) y.xu, h.bai, g.lu, c.li, g.shi, j.am.chem.soc.2008,130, 5856-5857; (2) hummers, R.E.Offeman, J.Am.chem.Soc.1958,80, 1339-1339; the slightly reduced graphene oxide can be prepared by slightly reducing graphene oxide under mild conditions according to the requirement of reduction degree in practical application. The preparation method can be referred to as the following documents: D.Li, M.B.Muller, S.Gilje, R.B.Kaner, G.G.Wallace, Nature Anotechnol.2008,3, 101-.

The principle of the invention is that the separation of the univalent and multivalent metal ions can be realized accurately, efficiently and stably, and the principle is as follows: the slightly reduced graphene oxide is modified graphene obtained by partially reducing and oxidizing graphene under mild chemical reduction conditions, partial oxidation groups on the surface of a sheet layer of the modified graphene are eliminated, the layer-to-layer distance of the modified graphene can be adjusted through different reduction degrees, thermal fluctuation and wrinkling of the sheet layer are avoided under mild temperature conditions, and a two-dimensional nano channel of the modified graphene membrane is very flat and regular, so that a two-dimensional nano mass transfer channel with uniform size and height of about 0.8nm can be obtained, monovalent metal ions with a hydration diameter range of 0.658-0.764nm and polyvalent metal ions with a hydration diameter range of 0.802-0.950nm can be accurately separated based on size screening effect, and the separation stability and repeatability are good.

Compared with the prior art, the invention has the following beneficial effects:

1. the invention provides a novel modified graphene oxide film prepared by slightly reducing graphene oxide. Because the number of oxidation groups on the slightly reduced graphene oxide nanosheet layer is less than that of common graphene oxide, and the number of supports is reduced, the interlayer spacing can be reduced to be less than 0.9nm, and the construction of a two-dimensional nano mass transfer channel for separating monovalent and polyvalent metal ions and having a height of about 0.8nm can be realized by adjusting the reduction degree.

2. The modified graphene adopted by the invention is chemically reduced and oxidized by using a normal-temperature condition, so that a high-temperature condition is avoided, the obtained slightly reduced graphene oxide sheet layer is very flat, thermal fluctuation caused by hydrothermal conditions can not occur, the nano structure of the modified graphene oxide film prepared by the method is very uniform and regular, and the structure is favorable for constructing a two-dimensional nano structure for accurately screening ions.

3. Because the oxidation area on the slightly reduced graphene oxide lamella is less compared with that of graphene oxide, the reduction of the oxidation area can reduce the hydration action of the lamella and water molecules, and can reduce the interlayer spacing to generate pi-pi action between the lamellae, thereby effectively increasing the stability of the modified graphene film in water, acid solution or alkali solution and being beneficial to the application of the film in the field of ion separation under acid. Experiments show that the modified graphene membrane prepared by the invention has good stability in ion separation application, and can still maintain the stable separation performance of the membrane for more than 6 months. Meanwhile, the method has good repeatability, is beneficial to the stable operation of the method in the long-term ion separation process, and is beneficial to the industrial application and development of the technology.

4. In the process of separating monovalent and polyvalent metal ions, the modified graphene oxide membrane provided by the invention can obtain higher ion permeation flux only by utilizing osmotic pressure without adding an external pressure field or an external electric field, and keeps good ion separation performance, and the energy-saving characteristic is favorable for further industrial application of the technology.

Drawings

Fig. 1 is an X-ray photoelectron spectrum of 12-hour modified graphene prepared in example 1;

fig. 2 is an atomic force microscope image of 12 hour modified graphene prepared in example 1;

FIG. 3 is a scanning electron micrograph of a cross-section of a 12 hour modified graphene film prepared in example 1;

FIG. 4 is a graph of elemental analysis C/O ratio for films prepared in comparative example 1 and examples 1-3;

FIG. 5 is a graph of the effective interlayer spacing of films prepared in comparative example 1 and examples 1-3 in a wet X-ray diffraction test;

fig. 6 is an X-ray photoelectron spectrum of 24-hour modified graphene prepared in example 2;

fig. 7 is an atomic force microscope image of 24 hour modified graphene prepared in example 2;

FIG. 8 is a scanning electron micrograph of a cross-section of a 24 hour modified graphene film prepared in example 2;

fig. 9 is an X-ray photoelectron spectrum of 48-hour modified graphene prepared in example 3;

fig. 10 is an atomic force microscope image of 48 hour modified graphene prepared in example 3;

FIG. 11 is a scanning electron micrograph of a cross section of a 48 hour modified graphene film prepared in example 3;

fig. 12 is a cross-sectional scanning electron micrograph of the modified graphene film prepared in comparative example 1;

FIG. 13 is an X-ray diffraction energy spectrum of the high-temperature reduced graphene oxide film prepared in comparative examples 2 to 3 in a wet state;

fig. 14 is a photograph of the graphene oxide film prepared in comparative example 1 immersed in pure water, a hydrochloric acid solution, and a sodium hydroxide solution for various times;

fig. 15 is a photograph of a 24-hour modified graphene film prepared in example 2 immersed in pure water, a hydrochloric acid solution, a sodium hydroxide solution for various times;

fig. 16 is a photograph of a 48 hour modified graphene film prepared in example 3 immersed in pure water, a hydrochloric acid solution, a sodium hydroxide solution for various times;

FIG. 17 is a stress-strain curve for the graphene oxide films and modified graphene films prepared in comparative example 1 and examples 1-3;

FIG. 18 is a schematic view of an H-type apparatus used in the separation test of monovalent and polyvalent metal ions;

FIG. 19 is a graph of the separation permeability performance for 9 mixed metal ions for a 24 hour modified graphene membrane prepared in example 2;

FIG. 20 is a graph of the separation permeability performance of 24-hour modified graphene oxide membranes and 50%, 80% high temperature reduced graphene oxide membranes prepared in example 2, comparative example 3, respectively, for 3 mixed metal ions;

fig. 21 is a graph of separation stability and reproducibility test performance of the 24-hour modified graphene film and the high-temperature reduced graphene oxide film prepared in example 2 for 9 mixed metal ions;

Detailed Description

The modified graphene oxide membrane for separation of monovalent and polyvalent metal ions and the preparation method thereof according to the present invention are further illustrated by the following examples.

In the following examples and comparative examples, the graphite flakes were obtained from Nanjing Xiancheng nanotechnology Co., Ltd, concentrated sulfuric acid, potassium persulfate, sodium hydroxide, and hydrogen chloride were obtained from Duchen Kelong chemical reagent works, phosphorus pentoxide and potassium permanganate were obtained from Ducheng chemical reagent Co., Ltd, hydrazine hydrate and ammonia water were obtained from Duchen Kelong chemical reagent works, the mixed cellulose membrane was obtained from Hangzhou (torch) Xidoumen membrane industry Co., Ltd, the anodic aluminum oxide membrane (AAO membrane) was obtained from Whatman Co., UK, and the deionized water was produced from Millipore purified water system.

Example 1

In this embodiment, the preparation method of the slightly reduced graphene oxide lamella and the modified graphene film is as follows:

(1) preparation of graphene oxide by improved Hummer method

① adding 3g of scaly graphite, 12mL of concentrated sulfuric acid, 2.5g of potassium persulfate and 2.5g of phosphorus pentoxide into a single-neck flask, reacting at 80 ℃ for 4.5h under the condition of stirring, cooling the obtained reaction liquid to room temperature, adding 500mL of deionized water, stirring overnight, filtering, rinsing the filter cake with 1500mL of deionized water, and airing at room temperature for 24h to remove water in the filter cake, thus completing the pretreatment of graphite;

② adding 120mL of concentrated sulfuric acid with the temperature of 0 ℃ into a conical flask, then adding ① pretreated graphite, then adding 15g of potassium permanganate, controlling the adding speed of the potassium permanganate to be as slow as possible so that the temperature of a mixture in the conical flask does not exceed 20 ℃, reacting for 2 hours at 35 ℃ after the potassium permanganate is added, adding the obtained reaction solution into 250mL of deionized water for dilution, controlling the temperature to be not more than 50 ℃ through an ice bath during the dilution process, stirring after the dilution is finished, adding 700mL of deionized water after stirring for 2 hours, then adding 20mL of 30 wt% hydrogen peroxide, generating a large amount of bubbles in the solution, centrifuging the obtained solution at the rotation speed of 8000r/min when no bubbles are generated, washing the centrifuged and washed solid with 10 wt% hydrochloric acid, repeating the centrifuging-washing operation for 3 times, uniformly dispersing the obtained solid in 500mL of deionized water, filling the deionized water into a dialysis bag with the molecular weight cutoff of 14000, and dialyzing for 10 days in the deionized water.

③, taking 20mL of the graphene oxide solution obtained in the step ②, diluting the graphene oxide solution with 980mL of deionized water, treating the solution for 0.5h under the ultrasonic condition with the power of 100W, centrifuging the solution at the rotating speed of 3000r/min to remove the graphite particles which are not peeled off, and finally diluting the solution with deionized water to obtain a graphene oxide aqueous solution with the graphene oxide concentration of 0.5 mg/mL;

(2) preparation of slightly reduced graphene oxide

① A70 mL aqueous solution of graphene oxide obtained in step (1) was taken, and 70mL deionized water, 48. mu.L hydrazine hydrate ((51.2 wt%) and 550. mu.L ammonia (25 wt%) were added, followed by reaction at 25 ℃ for 12 hours with stirring.

② after the reaction was completed, the solution was titrated to pH 7 with HCl solution.

③ and centrifuging and washing the obtained solution for three times by using deionized water under the centrifugation conditions of 20000 revolutions per minute, 20 minutes and 20 ℃, adding deionized water into the precipitate obtained after the last centrifugation to adjust the concentration of the slightly reduced graphene oxide in the precipitate to be 0.25mg/mL, thus obtaining the 12-hour slightly reduced graphene oxide aqueous solution.

Carrying out X-ray photoelectron spectroscopy (XPS) detection on the 12-hour slightly reduced graphene oxide sample prepared in the step (2), as shown in figure 1; the atomic force microscopy image of the slightly reduced graphene oxide prepared in step (2) for 12 hours is shown in fig. 2, and it can be seen from fig. 2 that the thickness of the slightly reduced graphene oxide nanosheet layer is about 1 nm.

(3) Treating modified graphene oxide membrane casting solution

And (3) taking 40mL of the slightly reduced graphene oxide aqueous solution prepared in the step (2), and treating for 0.5h under the ultrasonic condition with the power of 100W to obtain the slightly modified graphene oxide film casting solution.

(4) Preparation of 12-hour modified graphene oxide film

And (2) taking a mixed cellulose membrane (CN-CA membrane) with the average pore diameter of 220nm as a base material membrane, filtering the membrane casting solution obtained in the step (3) by using the base material membrane to form a wet-state modified graphene oxide membrane on the base material membrane, drying the wet-state 12-hour modified graphene oxide membrane together with the base material membrane at normal temperature for 12 hours, and stripping the dried 12-hour modified graphene oxide membrane from the base material membrane to obtain the 12-hour modified graphene oxide membrane with the thickness of about 2 microns.

The 12-hour modified graphene oxide film prepared in this example can be completely peeled off from the substrate film, and a scanning electron microscope image of a cross section of the 12-hour modified graphene oxide film is shown in fig. 3, and it can be seen from fig. 3 that the 12-hour modified graphene oxide film has a good layer-by-layer stacking structure.

And (3) taking 5 parts of the 12-hour modified graphene oxide film sample prepared in the step (4) for Element Analysis (EA) detection, wherein the result shows that the carbon-oxygen ratio of the 12-hour modified graphene oxide film prepared in the step is 1.219-1.251 as shown in FIG. 4. The wet sample was subjected to X-ray diffraction (XRD) testing, as shown in fig. 5, and the average wet effective interlayer distance of the 12-hour modified graphene oxide film prepared in this example was 0.8933 nm.

Example 2

In this example, the preparation method of the modified graphene oxide film is substantially the same as the operation of example 1, except that: the reaction time at normal temperature in the step (2) is 24 hours. The carbon-oxygen ratio of the 24-hour modified graphene oxide film prepared by the embodiment is 1.206-1.285, and the average wet effective interlamellar spacing is 0.7736 nm.

Example 3

In this example, the preparation method of the modified graphene oxide film is substantially the same as the operation of example 1, except that: the reaction time in step (2) was 48 hours at room temperature. The carbon-oxygen ratio of the 48-hour modified graphene oxide film prepared by the embodiment is 1.346-1.403, and the average wet effective interlamellar spacing is 0.7340 nm.

Comparative example 1

In this embodiment, the graphene oxide sheets and the graphene oxide film are prepared by the following steps:

(1) preparation of the casting solution

And (2) taking 20mL of the graphene oxide aqueous solution prepared in the step (1) in the embodiment 1, and treating for 0.5h under the ultrasonic condition with the power of 100W to obtain a casting solution.

(2) And (2) taking an anodic aluminum oxide film (AAO film) with the average pore diameter of 220nm as a base material film, filtering the casting film liquid obtained in the step (1) by using the base material film to form a wet graphene oxide film on the base material film, drying the wet graphene oxide film and the base material film at the normal temperature for 12 hours, and stripping the dried graphene oxide film from the base material film to obtain the graphene oxide film with the thickness of about 2 microns.

The graphene oxide film prepared in this comparative example was completely peeled off from the substrate film, and the scanning electron micrograph of the cross section of the graphene oxide film is shown in fig. 12, and it can be seen from fig. 12 that the graphene oxide film had a good layer-by-layer stacking structure. As shown in fig. 5, the graphene oxide film prepared in the present comparative example had an average wet effective interlayer distance of 1.073 nm.

Comparative example 2

In this embodiment, the preparation method of the high-temperature reduced graphene oxide lamella and the 50% high-temperature reduced graphene oxide film is as follows:

(1) preparation of high temperature reduced graphene oxide

70mL of the aqueous graphene oxide solution obtained in step (1) of example 1 was added with 70mL of deionized water, 48. mu.L of hydrazine hydrate ((51.2 wt%) and 550. mu.L of aqueous ammonia (25 wt%), and reacted at 65 ℃ for 20 minutes with stirring.

(2) Preparation of the casting solution

And (3) mixing 20mL of the high-temperature reduced graphene oxide aqueous solution prepared in the step (1) with 10mL of the graphene oxide aqueous solution prepared in the step (1) in the example 1, and treating for 0.5h under the ultrasonic condition with the power of 100W to obtain a casting solution.

(3) And (2) taking a mixed cellulose membrane (CN-CA membrane) with the average pore diameter of 220nm as a base material membrane, filtering the membrane casting solution obtained in the step (2) by using the base material membrane to form a wet 50% high-temperature reduction-oxidation graphene membrane on the base material membrane, placing the wet 50% high-temperature reduction-oxidation graphene membrane and the base material membrane together at normal temperature for drying for 12 hours, and stripping the dried 50% high-temperature reduction-oxidation graphene membrane from the base material membrane to obtain the 50% high-temperature reduction-oxidation graphene membrane with the thickness of about 2 microns. As shown in fig. 13, the 50% high temperature reduced graphene oxide film prepared in this comparative example had an average wet effective interlayer spacing of 0.763 nm.

Comparative example 3

In this comparative example, the preparation method of the 80% high temperature reduced graphene oxide film was substantially the same as the operation of comparative example 2, except that: the casting solution in step (2) was prepared by mixing 32mL of the aqueous solution of high-temperature reduced graphene oxide with 4mL of the aqueous solution of graphene oxide prepared in step (1) in example 1. As shown in fig. 13, the average wet effective interlayer spacing of the 80% high temperature reduced graphene oxide film prepared in this comparative example was 0.758 nm.

Performance testing

Stability of graphene oxide film and modified graphene oxide film in pure water, hydrochloric acid solution and sodium hydroxide solution

1. The graphene oxide film in comparative example 1 was immersed in pure water (pH 6.8), a hydrochloric acid solution (pH 1.2), and a sodium hydroxide solution (pH 13.2) for 1 minute to 1 week, respectively, and the film was as shown in fig. 14, and it was found that the graphene oxide film was not broken when immersed in pure water for 1 week, but was dissolved after being immersed in the hydrochloric acid solution and the sodium hydroxide solution for 1 day. It is demonstrated that the graphene oxide film has good stability in pure water, but has poor stability in an acid solution or an alkali solution.

2. The 24-hour modified graphene oxide film prepared in example 2 was immersed in pure water (pH 6.8), a hydrochloric acid solution (pH 1.2), and a sodium hydroxide solution (pH 13.2) for 1 day to 1 month, respectively, and the film was as shown in fig. 15. From the figure, the 24-hour modified graphene oxide film prepared in example 2 can be stable for a long time under three conditions, and shows good stability.

3. The 48-hour modified graphene oxide film prepared in example 3 was immersed in pure water (pH 6.8), a hydrochloric acid solution (pH 1.2), and a sodium hydroxide solution (pH 13.2) for 1 day to 1 month, respectively, and the film was as shown in fig. 16. From the figure, the 48-hour modified graphene oxide film prepared in example 3 can be stable for a long time under three conditions, and shows good stability.

Stress-strain curve

The stress-strain curves of the films prepared in comparative example 1, example 2, and example 3 are shown in fig. 17, and it can be seen from fig. 17 that the breaking stress of the modified graphene oxide films prepared in example 1, example 2, and example 3 is improved to some extent and has better mechanical strength than the graphene oxide film prepared in comparative example 1.

Separation effect test of (III) univalent and multivalent metal ions

1. The separation test of monovalent and polyvalent metal ions was carried out using the apparatus shown in FIG. 18. The 24 hour modified graphene oxide film prepared in example 2 was inserted into an H-type apparatus for testing. The solution on the supply side is 9 mixed metal ion solutions, namely potassium nitrate, sodium nitrate, lithium nitrate, lead nitrate, copper nitrate, magnesium nitrate, cadmium nitrate, ferric nitrate and aluminum nitrate, the concentration of the mixed metal ion solutions is 0.1mol/L, the solution on the permeation side is deionized water, the solution on both sides is 200mL, and the two sides are always in a stirring state. After different permeation times, solutions on the permeation side were subjected to ICP-OES (inductively coupled plasma spectroscopy) testing and processing. As can be seen from fig. 19, the 24-hour modified graphene oxide membrane has a higher permeation flux for monovalent metal ions and a lower permeation flux for polyvalent metal ions, and the separation factor between potassium and sodium ions and most polyvalent metal ions can reach more than 100.

2. The membranes obtained in example 2, comparative example 2 and comparative example 3 were used to sandwich an H-type apparatus for test comparison. The solution on the supply side is 3 kinds of mixed ionic solutions, namely potassium nitrate, magnesium nitrate and chromium nitrate, the concentration of the mixed ionic solutions is 0.1mol/L, the solution on the permeation side is deionized water, the solution on both sides is 200mL, and both sides are always in a stirring state. After different permeation times, solutions on the permeation side were subjected to ICP-OES (inductively coupled plasma spectroscopy) testing and processing. As can be seen from fig. 20, the 24-hour modified graphene oxide film in example 2 has high separability of monovalent/polyvalent metal ions, whereas the 50% and 80% high-temperature reduced graphene oxide films obtained in comparative examples 2 and 3 have poor separability of monovalent/polyvalent metal ions. The reason is that the flatness of graphene oxide lamella is affected by high-temperature reduction, so that the two-dimensional nanostructure of the high-temperature reduction graphene oxide film is irregular and disordered compared with the 24-hour modified graphene oxide film, and the XRD peak value is not obvious, so that the separation of monovalent and polyvalent metal ions is not accurate enough.

(IV) Stable repeatability test of 24-hour modified graphene membrane on separation of monovalent and polyvalent metal ions

The ion isolation test was performed using the apparatus shown in fig. 18. The 24 hour modified graphene oxide film obtained in example 2 was inserted into an H-type apparatus for testing. The solution on the supply side was 9 kinds of mixed ionic solutions, i.e., potassium nitrate, sodium nitrate, lithium nitrate, lead nitrate, copper nitrate, magnesium nitrate, cadmium nitrate, ferric nitrate, and aluminum nitrate, each at a concentration of 0.1mol/L, the solution on the permeate side was deionized water, and the solutions on both sides were 200mL, and both sides were always stirred. After a permeation time of 1 day, the solution on the permeation side was subjected to an ICP-OES (inductively coupled plasma Spectroscopy) test and treated. Carefully rinsing the cup bodies on both sides, and soaking both sides of the device with deionized water while keeping the two sides in a stirring state. After 15 days, the above mixed solution was poured again, infiltrated for 1 day, and sampled for testing. The rinsing and soaking were repeated, and after 15 days, the test was repeated again. The ion separation performance of the same 24-hour modified graphene oxide membrane in the three monovalent/polyvalent metal ion separation tests is shown in fig. 21. The 24-hour modified graphene oxide membrane has stable separation performance of univalent and multivalent metal ions in 6 months, has good repeatability, and is beneficial to industrial large-scale production and application.

Claims

1. The modified graphene membrane for separating monovalent metal ions from polyvalent metal ions is characterized in that a membrane material is slightly reduced graphene oxide with the carbon-oxygen mass ratio of 1.2-1.4, slightly reduced graphene oxide sheets are stacked layer by layer through suction filtration, the slightly reduced graphene oxide is prepared by chemically reducing graphene oxide at normal temperature, and monovalent metal ions can pass through a two-dimensional nano mass transfer channel formed by the slightly reduced graphene oxide sheets in the membrane and the polyvalent metal ions are intercepted.

2. The modified graphene membrane for separation of monovalent and polyvalent metal ions according to claim 1, wherein the slightly reduced graphene oxide has a interplatelet distance of 0.765nm to 0.801 nm.

3. The modified graphene membrane for separation of monovalent and polyvalent metal ions according to claim 1, wherein the membrane thickness of the modified graphene membrane is 0.5 μm to 5.0 μm.

4. The modified graphene membrane for separation of monovalent and polyvalent metal ions according to claim 3, wherein the membrane thickness of the modified graphene membrane is 1.5 μm to 2.5 μm.

5. Method for preparing a modified graphene membrane for the separation of monovalent and polyvalent metal ions according to one of claims 1 to 4, characterized in that it comprises the following process steps:

(3) and drying the wet modified graphene film and the substrate film at normal temperature, and peeling the dried modified graphene film from the substrate film to obtain the modified graphene film.

6. The method of claim 5, wherein the substrate membrane is an organic membrane and the average pore size of the substrate membrane is from 200nm to 230 nm.

7. The method of claim 6, wherein the organic substrate membrane is a circular membrane having a diameter of 45mm to 50 mm.

8. The method for preparing a modified graphene membrane for separation of monovalent and polyvalent metal ions according to any one of claims 5 to 7, wherein the casting solution is used in an amount capable of forming a modified graphene membrane of a designed thickness.

9. The method for preparing the modified graphene membrane for the separation of monovalent and polyvalent metal ions according to claim 8, wherein the amount of the membrane casting solution used for filtering the membrane casting through the substrate membrane is 38mL to 42 mL.