Ultrathin magnesium-manganese hydrotalcite nanosheets and their preparation method and application

Abstract

The present invention provides ultrathin magnesium-manganese hydrotalcite nanosheets, as well as a preparation method and application thereof, belonging to the technical field of inorganic material synthesis. The present invention utilizes a coprecipitation reaction to prepare magnesium-manganese hydrotalcite, which is then calcined to obtain a composite metal oxide. The composite metal oxide is then mixed with a reducing agent and water for topological transformation to form ultrathin magnesium-manganese hydrotalcite nanosheets. The specific surface area is significantly increased, and the ultrathin structure provides more active sites for adsorption. The manganese in the ultrathin magnesium-manganese hydrotalcite nanosheets can activate hydroxyl groups on the surface of the hydrotalcite sheets, inducing the detachment of hydrogen from the hydroxyl groups to form unsaturated oxygen sites, which facilitate the capture of uranium. Furthermore, the ultrathin magnesium-manganese hydrotalcite nanosheets significantly increase the dissolution of magnesium in a uranium-containing solution, resulting in metal defects that also facilitate the formation of unsaturated oxygen sites and enhance uranium adsorption.

Description

Technical Field

[0001] The invention belongs to the technical field of inorganic material synthesis, and particularly relates to an ultra-thin magnesium-manganese hydrotalcite nanosheet, a preparation method and an application thereof. Background Art

[0002] Hydrotalcite, scientifically known as layered composite metal hydroxide, shows broad application prospects in the adsorption of uranyl ions and other substances due to its rich tunable properties (composition, ratio, particle size, morphology, etc.). Existing technologies typically require surface modification to improve hydrotalcite's adsorption performance, such as through mercaptopropylalkoxysilane modification, amidoximation, and PEI modification. This increases costs, and the maximum adsorption capacity of modified hydrotalcite for uranyl ions is approximately 200 mg / g, indicating that its adsorption performance still needs to be further improved. Summary of the Invention

[0003] The present invention aims to provide an ultrathin magnesium-manganese hydrotalcite nanosheet and its preparation method and application. The magnesium-manganese hydrotalcite nanosheet prepared by the present invention has an ultrathin structure and has better adsorption performance without surface modification.

[0004] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0005] The present invention provides a method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets, comprising the following steps:

[0006] (1) Mixing magnesium salt, manganese salt, water and alkali to obtain a mixed solution;

[0007] (2) subjecting the mixed solution obtained in step (1) to a coprecipitation reaction to obtain magnesium-manganese hydrotalcite;

[0008] (3) calcining the magnesium-manganese hydrotalcite obtained in step (2) to obtain a composite metal oxide;

[0009] (4) The composite metal oxide obtained in step (3) is mixed with a reducing agent and water to undergo a topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

[0010] Preferably, in step (1), the molar ratio of the magnesium ions in the magnesium salt to the manganese ions in the manganese salt is (3-8):1.

[0011] Preferably, the ratio of the amount of the base in step (1) to the total amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt is (1.5-2.5):1.

[0012] Preferably, the temperature of the coprecipitation reaction in step (2) is 30-65° C., and the coprecipitation reaction time is 2-6 hours.

[0013] Preferably, the calcination temperature in step (3) is 300-450° C., and the calcination time is 3-6 hours.

[0014] Preferably, the reducing agent in step (4) comprises one or more of ascorbic acid, sodium thiosulfate, sodium metabisulfite, sodium sulfite, sodium hypophosphite and sodium borohydride.

[0015] Preferably, in step (4), the mass ratio of the reducing agent to the composite metal oxide is (0.2-2):1.

[0016] Preferably, the temperature of the topological transformation in step (4) is 25-45° C., and the time of the topological transformation is 2-6 hours.

[0017] The present invention also provides ultrathin magnesium-manganese hydrotalcite nanosheets prepared by the preparation method described in the above technical solution, wherein the thickness of the ultrathin magnesium-manganese hydrotalcite nanosheets is 5-10 nm.

[0018] The present invention also provides the use of the ultrathin magnesium-manganese hydrotalcite nanosheets described in the above technical solution in the adsorption of uranium, cadmium and arsenic.

[0019] The present invention provides a method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets, comprising the following steps: (1) mixing magnesium salt, manganese salt, water and alkali to obtain a mixed solution; (2) subjecting the mixed solution obtained in step (1) to a coprecipitation reaction to obtain magnesium-manganese hydrotalcite; (3) calcining the magnesium-manganese hydrotalcite obtained in step (2) to obtain a composite metal oxide; (4) mixing the composite metal oxide obtained in step (3) with a reducing agent and water, and subjecting the composite metal oxide to a topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets. In the present invention, magnesium salt, manganese salt, water and alkali are mixed to obtain a mixed solution, and then a coprecipitation reaction is performed to prepare magnesium-manganese hydrotalcite, which is then calcined. During the calcination process, the layered structure is maintained, and most of the manganese element can be converted to a +4 valence during calcination, while Mn 4+ The ionic radius is smaller than that of Mn 2+ and Mn 3+ , which results in the formation of a large number of defects in the composite metal oxide after calcination, providing more active sites for the subsequent hydroxylation transformation of the topological transformation. The composite metal oxide is then mixed with a reducing agent and water, and the reducing agent converts part of the Mn 4+ Reduction to Mn 3+, reaching the trivalent metal state required to form a hydrotalcite structure, while simultaneously achieving bulk hydroxylation under the action of water, thereby undergoing a topological transformation and restoring to a magnesium-manganese hydrotalcite. During the restoration process, due to structural reconstruction, it becomes thinner in the c-axis direction, forming an ultrathin structure. The ultrathin structure provides more active sites for adsorption. The manganese in the ultrathin magnesium-manganese hydrotalcite nanosheets can activate the hydroxyl groups on the surface of the hydrotalcite layer, inducing the detachment of hydrogen from the hydroxyl groups, forming unsaturated oxygen sites, which is conducive to the capture of uranium. At the same time, the ultrathin magnesium-manganese hydrotalcite nanosheets significantly increase the dissolution of magnesium in a uranium-containing solution, resulting in metal defects, which also facilitate the formation of unsaturated oxygen sites and improve the adsorption capacity for uranium. In addition, it also has good adsorption performance for cadmium and arsenic. The results of the examples show that the ultrathin magnesium-manganese hydrotalcite nanosheets prepared by the present invention have a thickness of 5-10 nm and a maximum adsorption capacity of uranium of 2890 mg / g. It also has good adsorption performance for cadmium and arsenic. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the XRD pattern of the magnesium-manganese hydrotalcite prepared in step (2) of Example 1;

[0021] Figure 2 This is the SEM image of the magnesium-manganese hydrotalcite prepared in step (2) of Example 1;

[0022] Figure 3 The XRD pattern of the composite metal oxide prepared in step (3) of Example 1;

[0023] Figure 4 TEM image of the composite metal oxide prepared in step (3) of Example 1;

[0024] Figure 5 This is the XPS graph of Mn in the composite metal oxide prepared in step (3) of Example 1;

[0025] Figure 6 This is the XRD pattern of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1;

[0026] Figure 7 TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1;

[0027] Figure 8 This is the XRD pattern of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2;

[0028] Figure 9 TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2;

[0029] Figure 10 This is the XRD pattern of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 3;

[0030] Figure 11 TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 3;

[0031] Figure 12 The XRD pattern of material F prepared in Comparative Example 1;

[0032] Figure 13 This is the XRD pattern of material G prepared in Comparative Example 2;

[0033] Figure 14 This is a curve diagram of the adsorption of uranium at different concentrations by the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1. DETAILED DESCRIPTION

[0034] The present invention provides a method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets, comprising the following steps:

[0035] (1) Mixing magnesium salt, manganese salt, water and alkali to obtain a mixed solution;

[0036] (2) subjecting the mixed solution obtained in step (1) to a coprecipitation reaction to obtain magnesium-manganese hydrotalcite;

[0037] (3) calcining the magnesium-manganese hydrotalcite obtained in step (2) to obtain a composite metal oxide;

[0038] (4) The composite metal oxide obtained in step (3) is mixed with a reducing agent and water to undergo a topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

[0039] Unless otherwise specified, the present invention has no particular limitation on the sources of the raw materials, and commercially available products known to those skilled in the art may be used.

[0040] The invention mixes magnesium salt, manganese salt, water and alkali to obtain a mixed solution.

[0041] In the present invention, the magnesium salt preferably includes at least one of magnesium chloride, magnesium sulfate, magnesium acetate and magnesium nitrate.

[0042] In the present invention, the manganese salt preferably includes at least one of manganese chloride, manganese sulfate, manganese acetate and manganese nitrate.

[0043] In the present invention, the molar ratio of the magnesium ions in the magnesium salt to the manganese ions in the manganese salt is preferably (3-8):1. In one embodiment, the molar ratio of the magnesium ions in the magnesium salt to the manganese ions in the manganese salt can be specifically 3:1, 4:1, 5:1, 6:1, 7:1, or 8:1. By controlling the molar ratio of the magnesium ions in the magnesium salt to the manganese ions in the manganese salt within the above range, the two can fully react to produce magnesium-manganese hydrotalcite.

[0044] In the present invention, the water is preferably deionized water.

[0045] In the present invention, the base preferably includes at least one of sodium hydroxide and potassium hydroxide.

[0046] In the present invention, the ratio of the amount of the base to the total amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt is preferably (1.5-2.5):1. In one embodiment, the ratio of the amount of the base to the total amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt can be specifically 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, or 2.5:1. By controlling the ratio of the amount of the base to the total amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt within the above range, the magnesium salt and manganese salt can fully undergo a coprecipitation reaction to obtain magnesium-manganese hydrotalcite.

[0047] In the present invention, the mixing of the magnesium salt, manganese salt, water and alkali is preferably as follows: mixing the magnesium salt, manganese salt and part of the water to obtain a mixed salt solution; mixing the alkali and the remaining water to obtain an alkali solution; and mixing the mixed salt solution and the alkali solution to obtain a mixed solution.

[0048] In the present invention, the total concentration of magnesium ions and manganese ions in the mixed salt solution is preferably ≤1.2 mol / L. As an embodiment, the total concentration of magnesium ions and manganese ions in the mixed salt solution can be specifically 1.2 mol / L, 1.1 mol / L, 1.0 mol / L, 0.9 mol / L, 0.8 mol / L, 0.7 mol / L, 0.6 mol / L or 0.5 mol / L.

[0049] In the present invention, the concentration of the alkali solution is preferably 1-2 mol / L.

[0050] In the present invention, the mixing of the magnesium salt, manganese salt, water and base is preferably carried out under stirring conditions. The present invention has no particular limitation on the manner, rate and time of the stirring, and a stirring technique well known to those skilled in the art can be used to uniformly mix the raw materials.

[0051] The mixing method of the present invention can avoid the formation of impurities caused by the direct addition of alkali resulting in excessive local concentration.

[0052] After obtaining the mixed solution, the present invention performs a coprecipitation reaction on the mixed solution to obtain magnesium-manganese hydrotalcite.

[0053] In the present invention, the temperature of the coprecipitation reaction is preferably 30-65°C; the coprecipitation reaction time is preferably 2-6 hours. As an embodiment, the temperature of the coprecipitation reaction can be specifically 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, or 65°C; the coprecipitation reaction time can be specifically 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or 6 hours. In the present invention, the coprecipitation reaction is preferably carried out under stirring conditions. The present invention does not specifically limit the stirring method and rate; stirring technical solutions familiar to those skilled in the art can be used. The present invention controls the temperature and time of the coprecipitation reaction within the above ranges to ensure that the coprecipitation reaction proceeds fully.

[0054] After the coprecipitation reaction is completed, the product of the coprecipitation reaction is preferably subjected to solid-liquid separation, washing and drying in sequence to obtain magnesium-manganese hydrotalcite.

[0055] The present invention has no particular limitation on the solid-liquid separation operation, and a solid can be obtained by using a solid-liquid separation technique well known to those skilled in the art. As an embodiment, the solid-liquid separation is performed by centrifugation.

[0056] The present invention has no particular limitation on the washing operation, and the washing process can be performed by using a washing technique known to those skilled in the art until the mixture is neutral. As an embodiment, the washing process is performed by washing with deionized water.

[0057] The present invention has no particular limitation on the drying operation, and drying to a constant weight may be performed using a drying technique well known to those skilled in the art.

[0058] As an embodiment, the drying temperature is 60°C.

[0059] After obtaining the magnesium-manganese hydrotalcite, the present invention calcines the magnesium-manganese hydrotalcite to obtain a composite metal oxide.

[0060] In the present invention, the calcination temperature is preferably 300-450°C; the calcination time is preferably 3-6 hours; and the rate of heating to the calcination temperature is preferably 4-6°C / min. As an embodiment, the calcination temperature can be specifically 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C or 450°C; the calcination time can be specifically 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours; and the rate of heating to the calcination temperature can be specifically 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min or 6°C / min. The present invention controls the calcination temperature, time and heating rate within the above ranges, which can maintain the layered structure of magnesium-manganese hydrotalcite. At the same time, most of the manganese is converted to +4 valence during calcination, while Mn 4+ The ionic radius is smaller than that of Mn 2+ and Mn 3+ , resulting in the formation of a large number of defects in the composite metal oxide after calcination, providing more active sites for the subsequent hydroxylation transformation of the topological transformation.

[0061] After the calcination is completed, the calcined product is preferably cooled to obtain a composite metal oxide.

[0062] The present invention has no special limitation on the cooling operation, and cooling to room temperature can be performed using cooling technical solutions well known to those skilled in the art.

[0063] After obtaining the composite metal oxide, the present invention mixes the composite metal oxide with a reducing agent and water to perform topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

[0064] In the present invention, the reducing agent preferably includes one or more of ascorbic acid, sodium thiosulfate, sodium metabisulfite, sodium sulfite, sodium hypophosphite and sodium borohydride.

[0065] In the present invention, the mass ratio of the reducing agent to the composite metal oxide is preferably (0.2-2):1. As an embodiment, the mass ratio of the reducing agent to the composite metal oxide can be specifically 0.2:1, 0.5:1, 0.6:1, 0.8:1, 1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1 or 2:1. In the present invention, the reducing agent reduces part of the Mn in the composite metal oxide. 4+ Reduction to Mn 3+ The present invention controls the mass ratio of the reducing agent to the composite metal oxide within the above range, which can fully reduce the composite metal oxide.

[0066] In the present invention, the water is preferably deionized water. In the present invention, in addition to dissolving or dispersing the raw materials, the water also reacts with the composite metal oxide to form hydroxides, thereby forming magnesium-manganese hydrotalcite.

[0067] In the present invention, the mass ratio of the reducing agent to the volume of water is preferably (0.3-1) g:100 mL. In one embodiment, the mass ratio of the reducing agent to the volume of water can be specifically 0.3 g:100 mL, 0.4 g:100 mL, 0.45 g:100 mL, 0.5 g:100 mL, 0.55 g:100 mL, 0.6 g:100 mL, 0.7 g:100 mL, 0.8 g:100 mL, 0.9 g:100 mL, or 1 g:100 mL. Controlling the mass ratio of the reducing agent to the volume of water within this range ensures that the reducing agent is fully dissolved and the composite metal oxide fully forms a hydroxide.

[0068] In the present invention, the mixing of the composite metal oxide with the reducing agent and water is preferably carried out by mixing the reducing agent and water to obtain a reducing agent solution, and then adding the composite metal oxide.

[0069] In the present invention, the mixing of the composite metal oxide, the reducing agent and the water is preferably carried out under stirring conditions. The present invention has no particular limitation on the manner, rate and time of the stirring, and a stirring technique well known to those skilled in the art can be used to fully dissolve or disperse the components.

[0070] In the present invention, the temperature of the topological transformation is preferably 25~45℃; the time of the topological transformation is preferably 2~6h. As an embodiment, the temperature of the topological transformation can be specifically 25℃, 26℃, 28℃, 30℃, 32℃, 34℃, 36℃, 38℃, 40℃, 42℃, 44℃ or 45℃; the time of the topological transformation can be specifically 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h. In the present invention, the topological transformation is preferably carried out under stirring conditions. The present invention has no special restrictions on the stirring mode and rate, and the stirring technical scheme familiar to those skilled in the art can be adopted. In the present invention, during the topological transformation, the reducing agent oxidizes part of the Mn in the composite metal 4+ Reduction to Mn 3+ , reaching the trivalent metal state required to form a hydrotalcite structure. Simultaneously, under the action of water, the bulk phase undergoes hydroxylation, leading to a topological transformation back to magnesium-manganese hydrotalcite. During this process of restoration, structural reconstruction occurs, causing the structure to become thinner along the c-axis, forming an ultrathin structure. By controlling the temperature and time of the topological transformation within the aforementioned ranges, the present invention can further enhance the adsorption performance of ultrathin magnesium-manganese hydrotalcite nanosheets.

[0071] After the topological transformation is completed, the present invention preferably performs solid-liquid separation and washing on the product of the topological transformation in sequence to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

[0072] The present invention has no particular limitation on the solid-liquid separation operation, and a solid can be obtained by using a solid-liquid separation technique well known to those skilled in the art. As an embodiment, the solid-liquid separation is performed by centrifugation.

[0073] The present invention has no particular limitation on the washing operation, and unreacted raw materials and the like can be removed using washing techniques well known to those skilled in the art.

[0074] The present invention first prepares magnesium-manganese hydrotalcite by a coprecipitation method, then calcines the composite metal oxide, and then performs a topological transformation in a reducing agent solution to form ultrathin magnesium-manganese hydrotalcite nanosheets. The prepared ultrathin magnesium-manganese hydrotalcite nanosheets have excellent adsorption properties without the need for surface modification, and are more cost-effective.

[0075] The present invention also provides ultrathin magnesium-manganese hydrotalcite nanosheets prepared by the preparation method described in the above technical solution.

[0076] In the present invention, the thickness of the ultra-thin magnesium-manganese hydrotalcite nanosheets is 5-10 nm.

[0077] The magnesium-manganese hydrotalcite prepared by the present invention has an ultrathin nanosheet structure. The ultrathin structure significantly increases its specific surface area, providing more active sites for adsorption. Uranium ions are easily combined with oxygen. The manganese in the ultrathin magnesium-manganese hydrotalcite nanosheets can activate hydroxyl groups on the surface of the hydrotalcite layer, induce the separation of hydrogen from the hydroxyl groups, and form unsaturated oxygen sites, which are beneficial to the capture of uranium. In addition, the dissolution of magnesium in the ultrathin magnesium-manganese hydrotalcite nanosheets in a uranium-containing solution is significantly increased, resulting in metal defects, which is also beneficial to the formation of unsaturated oxygen sites, thereby improving the adsorption capacity of uranium.

[0078] The ultrathin magnesium-manganese hydrotalcite nanosheets prepared by the present invention have excellent adsorption performance for uranium, with a maximum adsorption capacity of 2890 mg / g, and also have good adsorption performance for cadmium and arsenic, with a maximum adsorption capacity for cadmium of 200 mg / g and a maximum adsorption capacity for arsenic of 197.4 mg / g.

[0079] The present invention also provides the use of the ultrathin magnesium-manganese hydrotalcite nanosheets described in the above technical solution in the adsorption of uranium, cadmium and arsenic.

[0080] The ultrathin magnesium-manganese hydrotalcite nanosheets prepared by the invention have excellent uranium adsorption performance and good cadmium and arsenic adsorption performance, and can be used for the adsorption of uranium, cadmium and arsenic, especially for extracting uranium from seawater.

[0081] The present invention has no special limitation on the operation of the application, and the application technical solutions well known to those skilled in the art can be adopted.

[0082] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0083] Example 1

[0084] A method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets is as follows:

[0085] (1) 12.20 g (0.06 mol) of magnesium chloride hexahydrate, 2.03 g (0.01 mol) of manganese chloride tetrahydrate and 100 mL of deionized water were mixed and stirred until the mixture was clear to obtain a mixed salt solution (the molar ratio of magnesium ions in magnesium chloride hexahydrate to manganese ions in manganese chloride tetrahydrate was 6:1, and the total concentration of magnesium ions and manganese ions in the mixed salt solution was 0.7 mol / L); 0.14 mol of sodium hydroxide was mixed and stirred with 100 mL of deionized water until the mixture was clear to obtain an alkaline solution (the concentration of the alkaline solution was 1.4 mol / L); the mixed salt solution and the alkaline solution were mixed and stirred for 2 min to obtain a mixed solution (the molar ratio of sodium hydroxide to the total molar ratio of magnesium ions in magnesium chloride hexahydrate and manganese ions in manganese chloride tetrahydrate was 2:1);

[0086] (2) The mixed solution obtained in step (1) was subjected to a coprecipitation reaction at 45° C. under stirring conditions for 4 h, and then centrifuged, washed with deionized water, and dried at 60° C. to obtain magnesium-manganese hydrotalcite;

[0087] (3) placing the magnesium-manganese hydrotalcite obtained in step (2) in a muffle furnace, heating to 350°C at a rate of 5°C / min and calcining for 3 hours, and cooling to obtain a composite metal oxide;

[0088] (4) 0.5 g of ascorbic acid was mixed with 100 mL of deionized water (the mass ratio of ascorbic acid to deionized water was 0.5 g:100 mL) and stirred until the mixture was clear. Then, 1.0 g of the composite metal oxide obtained in step (3) was added (the mass ratio of ascorbic acid to the composite metal oxide was 0.5:1). The mixture was subjected to topological transformation at 25 °C under stirring for 3 h. After centrifugation and washing, ultrathin magnesium manganese hydrotalcite nanosheets were obtained.

[0089] The XRD pattern of the magnesium-manganese hydrotalcite prepared in step (2) of Example 1 is as follows: Figure 1 As shown. Figure 1It can be seen that the product prepared in step (2) of Example 1 has a typical hydrotalcite structure, indicating that the co-precipitation method can be used to synthesize magnesium-manganese hydrotalcite.

[0090] The SEM image of the magnesium-manganese hydrotalcite prepared in step (2) of Example 1 is as follows: Figure 2 As shown. Figure 2 It can be seen from the figure that magnesium-manganese hydrotalcite is a two-dimensional sheet structure with a thickness of 40~60nm.

[0091] The XRD pattern of the composite metal oxide prepared in step (3) of Example 1 is as follows: Figure 3 As shown. Figure 3 It can be seen that the product prepared in step (3) has a typical structure of a composite metal oxide, indicating that the structure of the magnesium-manganese hydrotalcite undergoes structural changes during calcination.

[0092] The TEM image of the composite metal oxide prepared in step (3) of Example 1 is as follows: Figure 4 As shown. Figure 4 It can be seen from the figure that there are a large number of void structures on the surface of the composite metal oxide. The reason is that during the roasting process, the valence of manganese changes to +4, the ion radius becomes smaller, and the lattice shrinks, resulting in defects.

[0093] The XPS pattern of Mn in the composite metal oxide prepared in step (3) of Example 1 is as follows: Figure 5 As shown. Figure 5 It can be seen that the valence of Mn in the composite metal oxide is mainly +4, Mn 4+ The mass content is 64.1%.

[0094] The XRD pattern of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 1 is as follows: Figure 6 As shown. Figure 6 It can be seen that the ultrathin magnesium-manganese hydrotalcite nanosheets are typical hydrotalcite structures, indicating that the composite metal oxides are transformed into hydrotalcite structures through topological transformation.

[0095] The TEM image of the ultrathin magnesium manganese hydrotalcite nanosheet prepared in Example 1 is as follows: Figure 7 As shown. Figure 7 It can be seen that the ultrathin magnesium-manganese hydrotalcite nanosheets are in a twisted state with a thickness of 5~10nm, showing an ultrathin structure.

[0096] Example 2

[0097] A method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets is as follows:

[0098] (1) 14.79 g (0.06 mol) of magnesium nitrate hexahydrate, 2.51 g (0.01 mol) of manganese nitrate tetrahydrate and 100 mL of deionized water were mixed and stirred until the mixture was clear to obtain a mixed salt solution (the molar ratio of magnesium ions in magnesium nitrate hexahydrate to manganese ions in manganese nitrate tetrahydrate was 6:1, and the total concentration of magnesium ions and manganese ions in the mixed salt solution was 0.7 mol / L); 0.14 mol of sodium hydroxide was mixed and stirred with 100 mL of deionized water until the mixture was clear to obtain an alkaline solution (the concentration of the alkaline solution was 1.4 mol / L); the mixed salt solution and the alkaline solution were mixed and stirred for 2 min to obtain a mixed solution (the molar ratio of sodium hydroxide to the total molar ratio of magnesium ions in magnesium nitrate hexahydrate and manganese ions in manganese nitrate tetrahydrate was 2:1);

[0099] (2) The mixed solution obtained in step (1) was subjected to a coprecipitation reaction at 45° C. under stirring conditions for 4 h, and then centrifuged, washed with deionized water, and dried at 60° C. to obtain magnesium-manganese hydrotalcite;

[0100] (3) placing the magnesium-manganese hydrotalcite obtained in step (2) in a muffle furnace, heating to 350°C at a rate of 5°C / min and calcining for 3 hours, and cooling to obtain a composite metal oxide;

[0101] (4) 1.0 g of sodium metabisulfite was mixed with 100 mL of deionized water (the mass ratio of sodium metabisulfite to deionized water was 1.0 g:100 mL) and stirred until clear. Then, 1.0 g of the composite metal oxide obtained in step (3) was added (the mass ratio of sodium metabisulfite to the composite metal oxide was 1:1). The topological transformation was carried out at 25 ° C under stirring for 3 h. After centrifugation and washing, ultrathin magnesium manganese hydrotalcite nanosheets were obtained.

[0102] The XRD pattern of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 is as follows: Figure 8 As shown. Figure 8 It can be seen that the ultrathin Mg-Mn hydrotalcite nanosheets have a typical hydrotalcite structure.

[0103] The TEM image of the ultrathin magnesium manganese hydrotalcite nanosheet prepared in Example 2 is as follows: Figure 9 As shown. Figure 9 It can be seen that the ultrathin MgMn hydrotalcite nanosheets exhibit an ultrathin structure.

[0104] Example 3

[0105] A method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets is as follows:

[0106] (1) 8.30 g (0.06 mol) of magnesium sulfate monohydrate, 2.23 g (0.01 mol) of manganese sulfate tetrahydrate and 100 mL of deionized water were mixed and stirred until the mixture was clear to obtain a mixed salt solution (the molar ratio of magnesium ions in magnesium sulfate monohydrate to manganese ions in manganese sulfate tetrahydrate was 6:1, and the total concentration of magnesium ions and manganese ions in the mixed salt solution was 0.7 mol / L); 0.14 mol of sodium hydroxide was mixed and stirred with 100 mL of deionized water until the mixture was clear to obtain an alkaline solution (the concentration of the alkaline solution was 1.4 mol / L); the mixed salt solution and the alkaline solution were mixed and stirred for 2 min to obtain a mixed solution (the molar ratio of sodium hydroxide to the total molar ratio of magnesium ions in magnesium sulfate monohydrate and manganese ions in manganese sulfate tetrahydrate was 2:1);

[0107] (2) The mixed solution obtained in step (1) was subjected to a coprecipitation reaction at 35° C. under stirring conditions for 3 h, and then centrifuged, washed with deionized water, and dried at 60° C. to obtain magnesium-manganese hydrotalcite;

[0108] (3) placing the magnesium-manganese hydrotalcite obtained in step (2) in a muffle furnace, heating to 350°C at a rate of 5°C / min and calcining for 3 hours, and cooling to obtain a composite metal oxide;

[0109] (4) 0.3 g of sodium borohydride was mixed with 100 mL of deionized water (the mass ratio of sodium borohydride to deionized water was 0.3 g:100 mL) and stirred until clear. Then, 1.0 g of the composite metal oxide obtained in step (3) was added (the mass ratio of sodium borohydride to the composite metal oxide was 0.3:1). The topological transformation was carried out at 25 ° C under stirring for 3 h. After centrifugation and washing, ultrathin magnesium manganese hydrotalcite nanosheets were obtained.

[0110] The XRD pattern of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 3 is as follows: Figure 10 As shown. Figure 10 It can be seen that the ultrathin Mg-Mn hydrotalcite nanosheets have a typical hydrotalcite structure.

[0111] The TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheet prepared in Example 3 is as follows: Figure 11 As shown. Figure 11 It can be seen that the ultrathin MgMn hydrotalcite nanosheets exhibit an ultrathin structure.

[0112] Comparative Example 1

[0113] (1) to (3) are the same as in Example 1 to obtain a composite metal oxide;

[0114] (4) 1.0 g of the composite metal oxide obtained in step (3) was mixed with 100 mL of deionized water, stirred at 25° C. for 3 h, and centrifuged and washed to obtain substance F.

[0115] The XRD pattern of the substance F prepared in Comparative Example 1 is as follows: Figure 12 As shown. Figure 12 It can be seen that substance F does not show the characteristic diffraction peak of the hydrotalcite structure, indicating that the composite metal oxide cannot topologically transform into the hydrotalcite structure in deionized water.

[0116] Comparative Example 2

[0117] (1) to (3) are the same as in Example 1 to obtain a composite metal oxide;

[0118] (4) 0.5 g of sodium carbonate and 100 mL of deionized water were mixed and stirred until the mixture was clear. Then, 1.0 g of the composite metal oxide obtained in step (3) was added and stirred at 25° C. for 3 h. After centrifugation and washing, substance G was obtained.

[0119] The XRD pattern of the substance G prepared in Comparative Example 2 is as follows: Figure 13 As shown. Figure 13 It can be seen that substance G does not show the characteristic diffraction peak of the hydrotalcite structure, indicating that the composite metal oxide cannot topologically transform into the hydrotalcite structure in the sodium carbonate solution.

[0120] Application Example 1

[0121] 0.01 g of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1 was placed in a 100 mL beaker, and 50 mL of uranyl nitrate solution was added, wherein the uranium concentrations were 50 mg / L, 200 mg / L, 400 mg / L and 600 mg / L, respectively. After magnetic stirring at room temperature for 4 h to reach adsorption equilibrium, the upper layer solution was taken and filtered with a 0.22 μm filter membrane, and the uranium concentration was measured by ICP. The adsorption amount of uranium at different concentrations is shown as follows: Figure 14 shown.

[0122] from Figure 14 According to calculations, when the initial concentration is 600 mg / L, the equilibrium concentration is 27 mg / L, the removal rate is 95.5%, and the maximum adsorption capacity is 2865 mg / g.

[0123] Application Example 2

[0124] 0.4 g of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1 were placed in a 500 mL beaker and 200 mL of natural seawater (collected from the Huangdao District near Lingshan Island, Qingdao, at 35.82°N, 120.18°E) containing 3.2 μg / L of uranium was added. After magnetic stirring at room temperature for 4 hours, the supernatant was filtered through a 0.22 μm filter membrane and the uranium concentration was measured by ICP. The ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1 achieved a 97.2% uranium removal rate from seawater, demonstrating excellent selectivity for uranium in seawater.

[0125] Application Example 3

[0126] 0.4 g of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2 were placed in a 500 mL beaker and 200 mL of natural seawater (collected from the Huangdao District near Lingshan Island, Qingdao, at 35.82°N, 120.18°E) with a uranium concentration of 3.2 μg / L was added. After magnetic stirring at room temperature for 4 hours, the supernatant was filtered through a 0.22 μm filter membrane and the uranium concentration was measured by ICP. The ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2 achieved a 96.4% uranium removal rate from seawater.

[0127] Application Example 4

[0128] 0.01 g of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 3 was placed in a 100 mL beaker, and 50 mL of uranyl nitrate solution (600 mg / L uranium concentration) was added. After magnetic stirring at room temperature for 4 hours, the supernatant solution was filtered through a 0.22 μm filter membrane and measured by ICP. The uranium concentration was 22 mg / L. The ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 3 achieved a 96.3% uranium removal rate, with a maximum adsorption capacity of 2890 mg / g.

[0129] Application Example 5

[0130] 0.1 g of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1 was placed in a 200 mL beaker, and 100 mL of sodium arsenite solution with an arsenic concentration of 200 mg / L was added. After magnetic stirring at room temperature for 4 hours to reach adsorption equilibrium, the upper layer of the solution was filtered through a 0.22 μm filter membrane and the arsenic concentration was measured by ICP. The equilibrium concentration was 2.6 mg / L. The ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1 had an arsenic removal efficiency of 98.7%, and a maximum adsorption capacity of 197.4 mg / g.

[0131] Application Example 6

[0132] 0.1 g of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2 was placed in a 200 mL beaker, and 100 mL of a cadmium nitrate solution (200 mg / L) was added. The solution was magnetically stirred at room temperature for 30 minutes to reach adsorption equilibrium. The upper layer of the solution was filtered through a 0.22 μm filter membrane and the cadmium concentration was measured by ICP. The equilibrium concentration was 0 mg / L. The ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2 had a 100% cadmium removal rate and a maximum adsorption capacity of 200 mg / g.

[0133] In summary, the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in the present invention have excellent adsorption properties for uranium without the need for surface modification, and also have good adsorption properties for cadmium and arsenic.

[0134] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing ultrathin magnesium-manganese hydrotalcite nanosheets, characterized in that: The steps are: (1) Mixing magnesium salt, manganese salt, water and alkali to obtain a mixed solution; (2) subjecting the mixed solution obtained in step (1) to a coprecipitation reaction to obtain magnesium-manganese hydrotalcite; the coprecipitation reaction temperature is 30-65° C., and the coprecipitation reaction time is 2-6 hours; (3) calcining the magnesium-manganese hydrotalcite obtained in step (2) to obtain a composite metal oxide; (4) The composite metal oxide obtained in step (3) is mixed with a reducing agent and water to undergo a topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

2. The preparation method according to claim 1, characterized in that In the step (1), the molar ratio of the magnesium ions in the magnesium salt to the manganese ions in the manganese salt is (3-8):

1.

3. The preparation method according to claim 1, characterized in that The ratio of the amount of the base in step (1) to the total amount of the magnesium ions in the magnesium salt and the manganese ions in the manganese salt is (1.5-2.5):

1.

4. The preparation method according to claim 1, characterized in that The calcination temperature in step (3) is 300-450° C., and the calcination time is 3-6 hours.

5. The preparation method according to claim 1, characterized in that The reducing agent in step (4) includes one or more of ascorbic acid, sodium thiosulfate, sodium metabisulfite, sodium sulfite, sodium hypophosphite and sodium borohydride.

6. The preparation method according to claim 1, characterized in that The mass ratio of the reducing agent to the composite metal oxide in step (4) is (0.2-2):

1.

7. The preparation method according to claim 1, characterized in that The temperature of the topological transformation in step (4) is 25-45° C., and the time of the topological transformation is 2-6 hours.

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