Ultrathin magnesium-manganese hydrotalcite nanosheet as well as preparation method and application thereof

Abstract

The invention provides an ultrathin magnesium-manganese hydrotalcite nanosheet as well as a preparation method and application thereof, and belongs to the technical field of inorganic material synthesis. Magnesium-manganese hydrotalcite is prepared through a co-precipitation reaction, the magnesium-manganese hydrotalcite is roasted to obtain a composite metal oxide, then the composite metal oxide is mixed with a reducing agent and water for topological transformation, the ultrathin magnesium-manganese hydrotalcite nanosheet is formed, the specific surface area is remarkably increased, the ultrathin structure provides more active sites for adsorption, and the adsorption efficiency is improved. Manganese in the ultrathin magnesium-manganese hydrotalcite nanosheet can activate hydroxyl on the surface of a hydrotalcite laminate and induce hydrogen in the hydroxyl to be separated, an unsaturated oxygen site is formed, uranium capture is facilitated, meanwhile, dissolution of magnesium in a uranium-containing solution of the ultrathin magnesium-manganese hydrotalcite nanosheet can be obviously increased, metal defects are caused, and the uranium-containing solution can be used as a uranium-containing solution. And the formation of unsaturated oxygen sites is facilitated, so that the uranium adsorption capacity is improved.

Description

Technical Field

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

[0002] Hydrotalcite, scientifically named layered double metal hydroxides, shows broad application prospects in the adsorption field of uranyl ions due to its rich adjustable space (composition, ratio, particle size, morphology, etc.). In the prior art, in order to improve the adsorption performance of hydrotalcite, it is usually necessary to perform surface modification on hydrotalcite, such as modifying hydrotalcite with mercaptopropylalkoxysilane, amidoximation of hydrotalcite, PEI modification of hydrotalcite, etc., which increases the cost. Moreover, the maximum adsorption capacity of the modified hydrotalcite for uranyl ions is about 200 mg / g, and its adsorption performance still needs to be further improved. Summary of the Invention

[0003] The purpose of the present invention is to provide an ultrathin magnesium manganese hydrotalcite nanosheet, a preparation method thereof, and an application thereof. 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 invention purpose, the present invention provides the following technical solutions: The present invention provides a preparation method of an ultrathin magnesium manganese hydrotalcite nanosheet, comprising the following steps: (1) Mixing a magnesium salt, a manganese salt, water, and an alkali to obtain a mixed solution; (2) Performing a coprecipitation reaction on the mixed solution obtained in step (1) 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 performing a topological transformation to obtain an ultrathin magnesium manganese hydrotalcite nanosheet.

[0005] Preferably, the molar ratio of magnesium ions in the magnesium salt to manganese ions in the manganese salt in step (1) is (3 - 8):1.

[0006] Preferably, the molar ratio of the alkali in step (1) to the total molar amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt is (1.5 - 2.5):1.

[0007] Preferably, the temperature of the coprecipitation reaction in step (2) is 30 - 65°C, and the time of the coprecipitation reaction is 2 - 6 h.

[0008] Preferably, the temperature of the calcination in step (3) is 300 - 450°C, and the time of the calcination is 3 - 6 h.

[0009] Preferably, the reducing agent in the step (4) includes one or more of ascorbic acid, sodium thiosulfate, sodium metabisulfite, sodium sulfite, sodium hypophosphite, and sodium borohydride.

[0010] Preferably, the mass ratio of the reducing agent to the composite metal oxide in the step (4) is (0.2~2):1.

[0011] Preferably, the temperature of the topological transformation in the step (4) is 25~45°C, and the time of the topological transformation is 2~6 h.

[0012] The present invention also provides an ultrathin magnesium manganese hydrotalcite nanosheet prepared by the preparation method described in the above technical solution, and the thickness of the ultrathin magnesium manganese hydrotalcite nanosheet is 5~10 nm.

[0013] The present invention also provides the application of the ultrathin magnesium manganese hydrotalcite nanosheet described in the above technical solution in the adsorption of uranium, cadmium, and arsenic.

[0014] The present invention provides a preparation method of an ultrathin magnesium manganese hydrotalcite nanosheet, which includes the following steps: (1) mixing a magnesium salt, a manganese salt, water, and an alkali to obtain a mixed solution; (2) performing a coprecipitation reaction on the mixed solution obtained in the step (1) to obtain magnesium manganese hydrotalcite; (3) calcining the magnesium manganese hydrotalcite obtained in the step (2) to obtain a composite metal oxide; (4) mixing the composite metal oxide obtained in the step (3) with a reducing agent and water, and performing a topological transformation to obtain an ultrathin magnesium manganese hydrotalcite nanosheet. In the present invention, a magnesium salt, a manganese salt, water, and an alkali are mixed to obtain a mixed solution, and then coprecipitation reaction is performed to prepare magnesium manganese hydrotalcite, and then it is calcined. During the calcination process, its layered structure is maintained, and at the same time, most of the manganese element can be transformed to +4 valence during calcination, while the ionic radius of Mn 4+ is smaller than that of Mn 2+ and Mn 3+ , resulting in a large number of defects in the calcined composite metal oxide, providing more active sites for the hydroxylation transformation of the subsequent topological transformation. Then, the composite metal oxide is mixed with a reducing agent and water, and the reducing agent reduces part of Mn 4+ to Mn 3+, reaching the trivalent metal state required for the formation of the hydrotalcite structure. Meanwhile, under the action of water, bulk hydroxylation occurs, and then topological transformation takes place to restore to magnesium-manganese hydrotalcite. During the restoration process, due to the reconstruction of the structure, it becomes thinner in the c-axis direction, forming an ultrathin structure. The ultrathin structure provides more active sites for adsorption. Manganese in the ultrathin magnesium-manganese hydrotalcite nanosheets can activate the hydroxyl groups on the surface of the hydrotalcite layer, induce the detachment of hydrogen in the hydroxyl groups, and form unsaturated oxygen sites, which is beneficial to the capture of uranium. At the same time, the dissolution of magnesium in the ultrathin magnesium-manganese hydrotalcite nanosheets in the uranium-containing solution will increase significantly, resulting in metal defects, which is also beneficial to the formation of unsaturated oxygen sites and improves 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 thickness of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in the present invention is 5 - 10 nm, the maximum adsorption capacity for uranium reaches 2890 mg / g, and it also has good adsorption performance for cadmium and arsenic. Description of the Drawings

[0015] Figure 1 XRD pattern of the magnesium-manganese hydrotalcite prepared in step (2) of Example 1; Figure 2 SEM image of the magnesium-manganese hydrotalcite prepared in step (2) of Example 1; Figure 3 XRD pattern of the composite metal oxide prepared in step (3) of Example 1; Figure 4 TEM image of the composite metal oxide prepared in step (3) of Example 1; Figure 5 XPS spectrum of Mn in the composite metal oxide prepared in step (3) of Example 1; Figure 6 XRD pattern of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1; Figure 7 TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 1; Figure 8 XRD pattern of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2; Figure 9 TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 2; Figure 10 XRD pattern of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 3; Figure 11 TEM image of the ultrathin magnesium-manganese hydrotalcite nanosheets prepared in Example 3; Figure 12 XRD pattern of the substance F prepared in Comparative Example 1; Figure 13 XRD pattern of the substance G prepared in Comparative Example 2; Figure 14 Adsorption capacity curve of ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 1 for uranium at different concentrations. Detailed implementation manners

[0016] The present invention provides a method for preparing ultrathin magnesium manganese hydrotalcite nanosheets, comprising the following steps: (1) Mixing a magnesium salt, a manganese salt, water and an alkali to obtain a mixed solution; (2) Performing a coprecipitation reaction on the mixed solution obtained in step (1) 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 performing a topological transformation to obtain ultrathin magnesium manganese hydrotalcite nanosheets.

[0017] Unless otherwise specified, the present invention does not have special limitations on the sources of each raw material, and commercially available products well-known to those skilled in the art can be used.

[0018] The present invention mixes a magnesium salt, a manganese salt, water and an alkali to obtain a mixed solution.

[0019] In the present invention, the magnesium salt preferably includes at least one of magnesium chloride, magnesium sulfate, magnesium acetate and magnesium nitrate.

[0020] In the present invention, the manganese salt preferably includes at least one of manganese chloride, manganese sulfate, manganese acetate and manganese nitrate.

[0021] In the present invention, the molar ratio of magnesium ions in the magnesium salt to manganese ions in the manganese salt is preferably (3 - 8):1. As an implementation manner, the molar ratio of magnesium ions in the magnesium salt to manganese ions in the manganese salt can specifically be 3:1, 4:1, 5:1, 6:1, 7:1 or 8:1. By controlling the molar ratio of magnesium ions in the magnesium salt to manganese ions in the manganese salt within the above range, the present invention can enable the two to react fully to obtain magnesium manganese hydrotalcite.

[0022] In the present invention, the water is preferably deionized water.

[0023] In the present invention, the alkali preferably includes at least one of sodium hydroxide and potassium hydroxide.

[0024] In the present invention, the molar ratio of the base to the total molar amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt is preferably (1.5 to 2.5):1. As an implementation manner, the molar ratio of the base to the total molar amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt can specifically be 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 molar ratio of the base to the total molar amount of magnesium ions in the magnesium salt and manganese ions in the manganese salt within the above range in the present invention, the magnesium salt and the manganese salt can fully undergo a coprecipitation reaction to obtain magnesium-manganese hydrotalcite.

[0025] In the present invention, the mixing of the magnesium salt, the manganese salt, water and the base is preferably as follows: the magnesium salt, the manganese salt and part of the water are mixed to obtain a mixed salt solution; the base and the remaining water are mixed to obtain an alkali solution; and the mixed salt solution and the alkali solution are mixed to obtain a mixed solution.

[0026] 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 implementation manner, the total concentration of magnesium ions and manganese ions in the mixed salt solution can specifically be 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.

[0027] In the present invention, the concentration of the alkali solution is preferably 1 to 2 mol / L.

[0028] In the present invention, the mixing of the magnesium salt, the manganese salt, water and the base is preferably carried out under stirring conditions. The present invention has no special limitations on the manner, rate and time of the stirring. Adopting the stirring technical solutions well-known to those skilled in the art, the respective raw materials can be mixed evenly.

[0029] Adopting the mixing method of the present invention can avoid the formation of impurities due to the direct addition of the base resulting in too high a local concentration.

[0030] After obtaining the mixed solution, the present invention carries out a coprecipitation reaction on the mixed solution to obtain magnesium-manganese hydrotalcite.

[0031] In the present invention, the temperature of the coprecipitation reaction is preferably 30 - 65 °C; the time of the coprecipitation reaction is preferably 2 - 6 h. As an embodiment, the temperature of the coprecipitation reaction can specifically be 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C or 65 °C; the time of the coprecipitation reaction can specifically be 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h. In the present invention, the coprecipitation reaction is preferably carried out under stirring conditions. The present invention has no special limitation on the mode and rate of the stirring, and the technical solutions of stirring well-known to those skilled in the art can be adopted. By controlling the temperature and time of the coprecipitation reaction within the above ranges, the coprecipitation reaction can proceed sufficiently in the present invention.

[0032] After the coprecipitation reaction is completed, the present invention preferably separates the solid and liquid, washes and dries the product of the coprecipitation reaction in sequence to obtain magnesium - manganese hydrotalcite.

[0033] The present invention has no special limitation on the operation of the solid - liquid separation, and the solid can be obtained by adopting the technical solutions of solid - liquid separation well-known to those skilled in the art. As an embodiment, the solid - liquid separation is centrifugation.

[0034] The present invention has no special limitation on the operation of the washing, and the washing can be carried out until neutral by adopting the technical solutions of washing well-known to those skilled in the art. As an embodiment, the washing is carried out with deionized water.

[0035] The present invention has no special limitation on the operation of the drying, and the drying can be carried out until constant weight by adopting the technical solutions of drying well-known to those skilled in the art.

[0036] As an embodiment, the drying temperature is 60 °C.

[0037] After obtaining the magnesium - manganese hydrotalcite, the present invention calcines the magnesium - manganese hydrotalcite to obtain a composite metal oxide.

[0038] In the present invention, the calcination temperature is preferably 300-450 °C; the calcination time is preferably 3-6 h; the heating rate to the calcination temperature is preferably 4-6 °C / min. As an embodiment, the calcination temperature can specifically be 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 specifically be 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h; the heating rate to the calcination temperature can specifically be 4 °C / min, 4.5 °C / min, 5 °C / min, 5.5 °C / min or 6 °C / min. By controlling the calcination temperature, time and heating rate within the above ranges in the present invention, the layered structure of magnesium-manganese hydrotalcite can be maintained, and at the same time, most of the manganese is transformed into +4 valence during calcination, while the ionic radius of Mn 4+ is smaller than that of Mn 2+ and Mn 3+ , resulting in a large number of defects in the calcined composite metal oxide, providing more active sites for the subsequent hydroxylation transformation of the topological transformation.

[0039] After the calcination is completed, the present invention preferably cools the calcined product to obtain a composite metal oxide.

[0040] The present invention has no special limitation on the cooling operation, and it can be cooled to room temperature by using the cooling technical solutions well-known to those skilled in the art.

[0041] After obtaining the composite metal oxide, the present invention mixes the composite metal oxide with a reducing agent and water to carry out a topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

[0042] 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.

[0043] 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 specifically be 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 a part of Mn 4+ in the composite metal oxide to Mn 3+ , reaching the trivalent metal state required for forming the hydrotalcite structure. By controlling the mass ratio of the reducing agent to the composite metal oxide within the above ranges in the present invention, the composite metal oxide can be fully reduced.

[0044] 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 will also react with the composite metal oxide to form a hydroxide, and then form a magnesium-manganese hydrotalcite.

[0045] In the present invention, the mass ratio of the reducing agent to the volume of water is preferably (0.3 - 1) g: 100 mL. As an embodiment, the mass ratio of the reducing agent to the volume of water can specifically be 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. By controlling the mass ratio of the reducing agent to the volume of water within the above range in the present invention, the reducing agent can be fully dissolved and the composite metal oxide can fully form a hydroxide.

[0046] In the present invention, the mixing of the composite metal oxide with the reducing agent and water is preferably as follows: the reducing agent and water are mixed to obtain a reducing agent solution, and then the composite metal oxide is added.

[0047] In the present invention, the mixing of the composite metal oxide with the reducing agent and water is preferably carried out under stirring conditions. The present invention has no special limitations on the manner, rate and time of the stirring. The technical solutions of stirring well-known to those skilled in the art can be adopted to fully dissolve or disperse each component.

[0048] In the present invention, the temperature of the topological transformation is preferably 25 - 45 °C; the time of the topological transformation is preferably 2 - 6 h. As an embodiment, the temperature of the topological transformation can specifically be 25 °C, 26 °C, 28 °C, 30 °C, 32 °C, 34 °C, 36 °C, 38 °C, 40 °C, 42 °C, 44 °C or 45 °C; the time of the topological transformation can specifically be 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h. In the present invention, the topological transformation is preferably carried out under stirring conditions. The present invention has no special limitations on the manner and rate of the stirring. The technical solutions of stirring well-known to those skilled in the art can be adopted. In the present invention, during the topological transformation process, the reducing agent reduces part of the Mn in the composite metal oxide 4+ to Mn 3+ , reaching the trivalent metal state required for forming the hydrotalcite structure. At the same time, under the action of water, bulk hydroxylation is realized and then topological transformation occurs to restore to magnesium-manganese hydrotalcite. During the restoration process, due to the reconstruction of the structure, it becomes thinner in the c-axis direction, forming an ultrathin structure. By controlling the temperature and time of the topological transformation within the above range in the present invention, the adsorption performance of the ultrathin magnesium-manganese hydrotalcite nanosheets can be further improved.

[0049] 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.

[0050] The present invention has no special limitation on the operation of the solid-liquid separation, and a solid can be obtained by adopting a technical solution of solid-liquid separation well-known to those skilled in the art. As an implementation manner, the solid-liquid separation is centrifugation.

[0051] The present invention has no special limitation on the operation of the washing, and unreacted raw materials, etc. can be removed by adopting a technical solution of washing well-known to those skilled in the art.

[0052] The present invention first prepares magnesium manganese hydrotalcite by the coprecipitation method, then calcines it to obtain a 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 performance without surface modification and lower cost.

[0053] The present invention also provides ultrathin magnesium manganese hydrotalcite nanosheets prepared by the preparation method described in the above technical solution.

[0054] In the present invention, the thickness of the ultrathin magnesium manganese hydrotalcite nanosheets is 5-10 nm.

[0055] 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 extremely easy to combine with oxygen. Manganese in the ultrathin magnesium manganese hydrotalcite nanosheets can activate the hydroxyl groups on the surface of the hydrotalcite layer, induce the detachment of hydrogen in the hydroxyl groups, and form unsaturated oxygen sites, which is beneficial to the capture of uranium. In addition, the dissolution of magnesium in the ultrathin magnesium manganese hydrotalcite nanosheets in a uranium-containing solution will significantly increase, resulting in metal defects, which is also beneficial to the formation of unsaturated oxygen sites, thereby improving the adsorption capacity for uranium.

[0056] The ultrathin magnesium manganese hydrotalcite nanosheets prepared by the present invention have excellent adsorption performance for uranium, and the maximum adsorption capacity can reach 2890 mg / g. They also have good adsorption performance for cadmium and arsenic. The maximum adsorption capacity for cadmium can reach 200 mg / g, and the maximum adsorption capacity for arsenic can reach 197.4 mg / g.

[0057] The present invention also provides the application of the ultrathin magnesium manganese hydrotalcite nanosheets described in the above technical solution in the adsorption of uranium, cadmium, and arsenic.

[0058] The ultrathin magnesium manganese hydrotalcite nanosheets prepared by the present 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 uranium extraction from seawater.

[0059] The present invention has no special limitation on the operation of the application, and the technical solutions of the application well-known to those skilled in the art can be adopted.

[0060] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0061] Example 1 A preparation method of ultrathin magnesium manganese hydrotalcite nanosheets is as follows: (1) Mix 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 and stir until clear to obtain a mixed salt solution (the molar ratio of magnesium ions in magnesium chloride hexahydrate to manganese ions in manganese chloride tetrahydrate is 6:1, and the total concentration of magnesium ions and manganese ions in the mixed salt solution is 0.7 mol / L); mix 0.14 mol of sodium hydroxide and 100 mL of deionized water and stir until clear to obtain an alkali solution (the concentration of the alkali solution is 1.4 mol / L); mix the mixed salt solution and the alkali solution and stir for 2 min to obtain a mixed solution (the molar ratio of sodium hydroxide to the total amount of magnesium ions in magnesium chloride hexahydrate and manganese ions in manganese chloride tetrahydrate is 2:1); (2) Carry out a coprecipitation reaction on the mixed solution obtained in step (1) at 45 °C for 4 h under stirring conditions, and after centrifugation and washing with deionized water, dry at 60 °C to obtain magnesium manganese hydrotalcite; (3) Place the magnesium manganese hydrotalcite obtained in step (2) in a muffle furnace, heat it to 350 °C at a rate of 5 °C / min and calcine for 3 h, and then cool to obtain a composite metal oxide; (4) Mix 0.5 g of ascorbic acid and 100 mL of deionized water (the mass ratio of ascorbic acid to the volume of deionized water is 0.5 g:100 mL) and stir until clear, then add 1.0 g of the composite metal oxide obtained in step (3) (the mass ratio of ascorbic acid to the composite metal oxide is 0.5:1), and carry out a topological transformation at 25 °C for 3 h under stirring conditions. After centrifugation and washing, ultrathin magnesium manganese hydrotalcite nanosheets are obtained.

[0062] The XRD pattern of the magnesium manganese hydrotalcite prepared in step (2) of Example 1 is as Figure 1 shown. It can be seen from Figure 1 that the product prepared in step (2) of Example 1 is a typical hydrotalcite structure, indicating that the coprecipitation method can synthesize magnesium manganese hydrotalcite.

[0063] The SEM image of the magnesium manganese hydrotalcite prepared in step (2) of Example 1 is as Figure 2 shown. It can be seen from Figure 2 that the magnesium manganese hydrotalcite has a two-dimensional sheet structure with a thickness of 40 - 60 nm.

[0064] The XRD pattern of the composite metal oxide prepared in step (3) of Example 1 is as Figure 3 shown. It can be seen from Figure 3 that the product prepared in step (3) has a typical structure of the composite metal oxide, indicating that the structure of the magnesium manganese hydrotalcite has changed during calcination.

[0065] The TEM image of the composite metal oxide prepared in step (3) of Example 1 is as Figure 4 shown. It can be seen from Figure 4 that there are a large number of void structures on the surface of the composite metal oxide. The reason is that during the calcination process, the valence of manganese changes to +4, and the ionic radius becomes smaller, resulting in lattice contraction and defects.

[0066] The XPS pattern of Mn in the composite metal oxide prepared in step (3) of Example 1 is as Figure 5 shown. It can be seen from Figure 5 that the valence of Mn in the composite metal oxide is mainly +4, and the mass content of Mn 4+ is 64.1%.

[0067] The XRD pattern of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 1 is as Figure 6 shown. It can be seen from Figure 6 that the ultrathin magnesium manganese hydrotalcite nanosheets have a typical hydrotalcite structure, indicating that the composite metal oxide has been transformed back into the hydrotalcite structure through topological transformation.

[0068] The TEM image of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 1 is as Figure 7 shown. It can be seen from Figure 7 that the ultrathin magnesium manganese hydrotalcite nanosheets are in a twisted state with a thickness of 5 - 10 nm, showing an ultrathin structure.

[0069] Example 2 A method for preparing ultrathin magnesium manganese hydrotalcite nanosheets is as follows: (1) Mix 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, and stir until clear to obtain a mixed salt solution (the molar ratio of magnesium ions in magnesium nitrate hexahydrate to manganese ions in manganese nitrate tetrahydrate is 6:1, and the total concentration of magnesium ions and manganese ions in the mixed salt solution is 0.7 mol / L); Mix 0.14 mol of sodium hydroxide and 100 mL of deionized water, and stir until clear to obtain an alkali solution (the concentration of the alkali solution is 1.4 mol / L); Mix the mixed salt solution and the alkali solution, and stir for 2 min to obtain a mixed solution (the molar ratio of sodium hydroxide to the total amount of magnesium ions in magnesium nitrate hexahydrate and manganese ions in manganese nitrate tetrahydrate is 2:1); (2) Carry out a coprecipitation reaction on the mixed solution obtained in step (1) at 45 °C for 4 h under stirring conditions, centrifuge, wash with deionized water, and then dry at 60 °C to obtain magnesium manganese hydrotalcite; (3) Place the magnesium manganese hydrotalcite obtained in step (2) in a muffle furnace, heat it to 350 °C at a rate of 5 °C / min, calcine for 3 h, and then cool to obtain a composite metal oxide; (4) Mix 1.0 g of sodium metabisulfite and 100 mL of deionized water (the mass ratio of sodium metabisulfite to the volume of deionized water is 1.0 g:100 mL), and stir until clear. Then add 1.0 g of the composite metal oxide obtained in step (3) (the mass ratio of sodium metabisulfite to the composite metal oxide is 1:1), and carry out a topological transformation at 25 °C for 3 h under stirring conditions. After centrifugation and washing, obtain ultrathin magnesium manganese hydrotalcite nanosheets.

[0070] The XRD pattern of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 is as Figure 8 shown. It can be seen from Figure 8 that the ultrathin magnesium manganese hydrotalcite nanosheets have a typical hydrotalcite structure.

[0071] The TEM image of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 is as Figure 9 shown. It can be seen from Figure 9 that the ultrathin magnesium manganese hydrotalcite nanosheets exhibit an ultrathin structure.

[0072] Example 3 A method for preparing ultrathin magnesium manganese hydrotalcite nanosheets is as follows: (1) Mix 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 and stir until clear to obtain a mixed salt solution (the molar ratio of magnesium ions in magnesium sulfate monohydrate to manganese ions in manganese sulfate tetrahydrate is 6:1, and the total concentration of magnesium ions and manganese ions in the mixed salt solution is 0.7 mol / L); mix 0.14 mol of sodium hydroxide and 100 mL of deionized water and stir until clear to obtain an alkali solution (the concentration of the alkali solution is 1.4 mol / L); mix the mixed salt solution and the alkali solution, and stir for 2 min to obtain a mixed solution (the molar ratio of sodium hydroxide to the total amount of magnesium ions in magnesium sulfate monohydrate and manganese ions in manganese sulfate tetrahydrate is 2:1). (2) Carry out a coprecipitation reaction on the mixed solution obtained in step (1) at 35 °C for 3 h under stirring conditions, centrifuge, wash with deionized water, and then dry at 60 °C to obtain magnesium manganese hydrotalcite. (3) Place the magnesium manganese hydrotalcite obtained in step (2) in a muffle furnace, heat it to 350 °C at a rate of 5 °C / min, calcine for 3 h, and then cool to obtain a composite metal oxide. (4) Mix 0.3 g of sodium borohydride and 100 mL of deionized water (the mass ratio of sodium borohydride to the volume of deionized water is 0.3 g:100 mL) and stir until clear, then add 1.0 g of the composite metal oxide obtained in step (3) (the mass ratio of sodium borohydride to the composite metal oxide is 0.3:1), carry out a topological transformation at 25 °C for 3 h under stirring conditions, centrifuge and wash to obtain ultrathin magnesium manganese hydrotalcite nanosheets.

[0073] The XRD pattern of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 3 is as Figure 10 shown. It can be seen from Figure 10 that the ultrathin magnesium manganese hydrotalcite nanosheets have a typical hydrotalcite structure.

[0074] The TEM image of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 3 is as Figure 11 shown. It can be seen from Figure 11 that the ultrathin magnesium manganese hydrotalcite nanosheets exhibit an ultrathin structure.

[0075] Comparative Example 1 (1)-(3) are the same as in Example 1 to obtain a composite metal oxide; (4) Mix 1.0 g of the composite metal oxide obtained in step (3) with 100 mL of deionized water, stir at 25 °C for 3 h, centrifuge and wash to obtain Substance F.

[0076] The XRD pattern of Substance F prepared in Comparative Example 1 is as Figure 12 shown. It can be seen from Figure 12It can be seen that Substance F does not exhibit the characteristic diffraction peaks of the hydrotalcite structure, indicating that the composite metal oxide cannot undergo a topological transformation into the hydrotalcite structure in deionized water.

[0077] Comparative Example 2 (1) - (3) are the same as in Example 1 to obtain the composite metal oxide; (4) Mix 0.5 g of sodium carbonate and 100 mL of deionized water and stir until clear, then add 1.0 g of the composite metal oxide obtained in step (3), stir at 25 °C for 3 h, and after centrifugation and washing, obtain Substance G.

[0078] The XRD pattern of Substance G prepared in Comparative Example 2 is as Figure 13 shown. It can be seen from Figure 13 that Substance G does not exhibit the characteristic diffraction peaks of the hydrotalcite structure, indicating that the composite metal oxide cannot undergo a topological transformation into the hydrotalcite structure in the sodium carbonate solution.

[0079] Application Example 1 Place 0.01 g of the ultrathin magnesium - manganese hydrotalcite nanosheets prepared in Example 1 in a 100 - mL beaker, add 50 mL of uranyl nitrate solutions with uranium concentrations of 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 the adsorption equilibrium, take the upper - layer solution, filter it through a 0.22 - μm filter membrane, and measure the uranium concentration by ICP. The adsorption amounts of uranium at different concentrations are as Figure 14 shown.

[0080] Calculated from Figure 14 it, 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 amount is 2865 mg / g.

[0081] Application Example 2 Place 0.4 g of the ultrathin magnesium - manganese hydrotalcite nanosheets prepared in Example 1 in a 500 - mL beaker, add 200 mL of natural seawater (collected from the sea area near Lingdao Island, Huangdao District, Qingdao, with the specific geographical location of 35.82°N, 120.18°E), where the uranium concentration is 3.2 μg / L. After magnetic stirring at room temperature for 4 h, take the upper - layer solution, filter it through a 0.22 - μm filter membrane, and measure the uranium concentration by ICP. The removal rate of uranium in seawater by the ultrathin magnesium - manganese hydrotalcite nanosheets prepared in Example 1 is 97.2%, and it has excellent selectivity for uranium in seawater.

[0082] Application Example 3 Put 0.4 g of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 into a 500 mL beaker, add 200 mL of natural seawater (collected from the sea area of Huangdao District near Lingshan Island, Qingdao, with the specific geographical location of 35.82°N and 120.18°E), where the uranium concentration is 3.2 μg / L. After magnetic stirring at room temperature for 4 h, take the upper layer solution, filter it through a 0.22 μm filter membrane, and measure the uranium concentration by ICP. The removal rate of uranium in seawater by the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 is 96.4%.

[0083] Application Example 4 Put 0.01 g of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 3 into a 100 mL beaker, add 50 mL of uranyl nitrate solution, where the uranium concentration is 600 mg / L. After magnetic stirring at room temperature for 4 h, take the upper layer solution, filter it through a 0.22 μm filter membrane, and measure the uranium concentration by ICP, which is 22 mg / L. The removal rate of uranium by the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 3 is 96.3%, and the maximum adsorption capacity is 2890 mg / g.

[0084] Application Example 5 Put 0.1 g of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 1 into a 200 mL beaker, add 100 mL of sodium arsenite solution, where the arsenic concentration is 200 mg / L. After magnetic stirring at room temperature for 4 h to reach the adsorption equilibrium, take the upper layer solution, filter it through a 0.22 μm filter membrane, and measure the arsenic concentration by ICP. The equilibrium concentration is 2.6 mg / L. The removal rate of arsenic by the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 1 is 98.7%, and the maximum adsorption capacity is 197.4 mg / g.

[0085] Application Example 6 Put 0.1 g of the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 into a 200 mL beaker, add 100 mL of cadmium nitrate solution, where the cadmium concentration is 200 mg / L. After magnetic stirring at room temperature for 30 min to reach the adsorption equilibrium, take the upper layer solution, filter it through a 0.22 μm filter membrane, and measure the cadmium concentration by ICP. The equilibrium concentration is 0 mg / L. The removal rate of cadmium by the ultrathin magnesium manganese hydrotalcite nanosheets prepared in Example 2 is 100%, and the maximum adsorption capacity is 200 mg / g.

[0086] In summary, the ultrathin magnesium manganese hydrotalcite nanosheets prepared by the present invention have excellent adsorption performance for uranium without surface modification, and also have good adsorption performance for cadmium and arsenic.

[0087] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of ultrathin magnesium manganese hydrotalcite nanosheets, characterized in that It includes the following steps: (1) Mix a magnesium salt, a manganese salt, water and an alkali to obtain a mixed solution; (2) Conduct a coprecipitation reaction on the mixed solution obtained in the step (1) to obtain a magnesium-manganese hydrotalcite; (3) Calcinate the magnesium-manganese hydrotalcite obtained in the step (2) to obtain a composite metal oxide; (4) Mix the composite metal oxide obtained in the step (3) with a reducing agent and water, and conduct a topological transformation to obtain ultrathin magnesium-manganese hydrotalcite nanosheets.

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

1.

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

1.

4. The preparation method according to claim 1, characterized in that, In the step (2), the temperature of the coprecipitation reaction is 30-65°C, and the time of the coprecipitation reaction is 2-6 h.

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

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

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

1.

8. The preparation method according to claim 1, wherein, In the step (4), the temperature of the topological transformation is 25-45°C, and the time of the topological transformation is 2-6 h.

9. The ultrathin magnesium manganese hydrotalcite nanosheets prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The thickness of the ultrathin magnesium-manganese hydrotalcite nanosheets is 5-10 nm.

10. Application of the ultrathin magnesium-manganese hydrotalcite nanosheets according to claim 9 in the adsorption of uranium, cadmium and arsenic.

Images