A layered double metal oxide composite and its preparation method and application

Abstract

The present invention belongs to the technical field of adsorption materials, and particularly relates to a layered double metal oxide and a preparation method and application thereof. The preparation method of the layered double metal oxide provided by the present invention comprises the following steps: mixing magnesium oxide and a hydrate of a magnesium salt and then ball-milling to obtain a ball-milled material; mixing the ball-milled material and a hydrate of a manganese salt and then wet-milling to obtain a precursor of the double metal oxide; calcining the precursor of the double metal oxide to obtain the layered double metal oxide; the molar ratio of the magnesium element in the magnesium oxide to the magnesium element in the magnesium salt is 1 to 6:1. The preparation method provided by the present invention is not only convenient and produces no waste water, and the prepared composite metal oxide exhibits excellent performance in the synchronous remediation of cadmium-arsenic combined pollution, that is, the material can achieve the treatment of cadmium while remediating arsenic pollution, providing an effective solution to the problem that it is difficult to simultaneously treat cadmium-arsenic pollution at present.

Description

Technical Field

[0001] The present invention belongs to the technical field of adsorption materials, and particularly relates to a layered composite double metal oxide, a preparation method thereof, and an application thereof. Background Art

[0002] Heavy metal ions in aqueous solutions exist in the form of cations or metal oxyanions. Among them, cadmium has strong toxicity, and excessive intake of cadmium may cause cadmium poisoning in humans, muscle atrophy, and joint deformation. In addition, arsenic, a heavy metal, has been designated as a "known human carcinogen", and long-term intake of drinking water with excessive arsenic content will cause chronic poisoning of the body. Moreover, compared with the cationic Cd form that is easily removed by adsorption or precipitation, the oxyanion form AsO2 - has higher solubility and environmental mobility and can migrate freely in the environment.

[0003] Currently, there are various materials for heavy metal adsorption and solidification in water / soil, such as metal-organic framework materials (MOFs), activated carbon, layered double metal hydroxides (LDHs), and iron-manganese-based oxides. Among them, LDHs are a type of clay compound assembled by host lamellar and interlayer anion intercalation. Due to their special two-dimensional layered structure, they can remove heavy metals through various action modes, such as interlayer anion exchange, memory effect, isomorphous substitution, etc., and have shown good application prospects in the field of heavy metal contaminated water body remediation.

[0004] However, the remediation of heavy metal arsenic-cadmium coexistence systems is very difficult and is a bottleneck problem in the field of heavy metal pollution control. This is mainly due to the differences in the pH value and redox potential (Eh) dependence of the two heavy metals, arsenic and cadmium: the effects of arsenic and cadmium on pH value and Eh are completely opposite. Arsenic is more stable under acidic conditions, while cadmium is more stable under alkaline conditions. This makes it difficult to prepare a suitable remediation agent to jointly treat composite heavy metals, thereby reducing the harm to the human body. At present, there are great problems in the preparation of materials for adsorbing heavy metals. For example, the preparation process of MOFs or covalent organic framework (COF) materials with porous structures is complex, and the raw materials used are mainly expensive organic substances, resulting in high material costs and easy generation of waste water containing organic substances, leading to pollution. For common inorganic materials, co-precipitation reactions or hydrothermal reactions of salts and alkalis are mainly used, and a large amount of saline waste water will be generated during the preparation process. With the increasingly strict environmental protection requirements, the discharge of saline waste water is also an urgent problem to be solved. More importantly, the adsorption sites of the currently prepared adsorption materials are single, that is, they can only selectively adsorb cadmium or only have good adsorption ability for arsenic, but it is difficult to have good adsorption performance for both cadmium and arsenic at the same time. Therefore, it is urgent to develop a green and convenient synthesis method and an adsorbent material with the function of composite heavy metal remediation. Summary of the Invention

[0005] The object of the present invention is to provide a layered double metal oxide and its preparation method and application. The method for preparing the layered double metal oxide provided by the present invention is simple, and the prepared layered double metal oxide can remove arsenic and cadmium synchronously.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a preparation method of a layered double metal oxide, comprising the following steps:

[0008] Mix magnesium oxide and a hydrate of a magnesium salt and then ball-mill to obtain a ball-milled material;

[0009] Mix the ball-milled material and a hydrate of a manganese salt and then wet-mill to obtain a precursor of the double metal oxide;

[0010] Calcine the precursor of the double metal oxide to obtain a layered double metal oxide;

[0011] The molar ratio of magnesium element in the magnesium oxide to magnesium element in the magnesium salt is 1-6:1.

[0012] Preferably, the magnesium salt includes at least one of magnesium chloride, magnesium sulfate, magnesium carbonate and magnesium nitrate.

[0013] Preferably, the manganese salt includes at least one of manganese chloride, manganese nitrate, manganese sulfate and manganese carbonate.

[0014] Preferably, the molar ratio of manganese element in the manganese salt to magnesium element in the ball-milled material is 1:10-1:5.

[0015] Preferably, the ball-to-material ratio of the ball-milling and the wet-milling is independently 5-20:1; the rotation speed of the ball-milling and the wet-milling is independently 400-800 rpm; the material of the grinding balls in the ball-milling and the wet-milling is zirconia; the diameter of the grinding balls is independently 1-10 mm.

[0016] Preferably, the temperature of the calcination is 300-500 °C, and the heat preservation time of the calcination is 2-6 h.

[0017] Preferably, the heating rate for heating to the temperature of the calcination is 3-10 °C / min.

[0018] The present invention provides a layered double metal oxide prepared by the preparation method described in the above technical solutions. The layered double metal oxide is a magnesium-manganese layered double metal oxide, and the molar ratio of manganese element to magnesium element in the magnesium-manganese layered double metal oxide is 1:10-1:5.

[0019] Preferably, the BET specific surface area of the layered double metal oxide is 50~200 m 2 / g.

[0020] The present invention provides the application of the layered double metal oxide prepared by the preparation method described in the above technical solution or the layered double metal oxide described in the above technical solution in the synchronous removal of arsenic and cadmium.

[0021] The present invention provides a preparation method of a layered double metal oxide, comprising the following steps: mixing magnesium oxide and a hydrate of a magnesium salt and then ball-milling to obtain a ball-milled material; mixing the ball-milled material and a hydrate of a manganese salt and then wet-milling to obtain a precursor of a double metal oxide; calcining the precursor of the double metal oxide to obtain a layered double metal oxide; the molar ratio of magnesium element in the magnesium oxide to magnesium element in the magnesium salt is 1~6:1. During the synthesis of the layered double metal oxide, at the above molar ratio, the magnesium oxide and the hydrate of the magnesium salt can activate the magnesium oxide by utilizing the acidic characteristics of the magnesium salt, obtaining a large number of active sites on its surface, forming abundant magnesium defect sites, and then being able to undergo a chemical reaction under the action of the crystal water in the magnesium salt and the manganese salt to form a pure-phase composite metal hydroxide without over-oxidizing it to manganese in the +4 valence state to form a manganese oxide heterophase (MnO2). In addition, during the reaction process, manganese ions are first adsorbed on the surface of magnesium oxide and react with the free magnesium ions to form a double metal hydroxide, and the double metal hydroxide can be obtained as a layered double metal oxide through simple calcination. Compared with conventional strong bases such as sodium hydroxide or potassium hydroxide, the appropriate alkalinity of magnesium oxide will not cause the formation of a manganese oxide heterophase, and a large amount of alkali is saved. At the same time, the application of mechanical grinding methods (ball milling or wet milling) avoids the generation of wastewater, reduces the preparation cost, and reduces the environmental protection pressure. In summary, the preparation method provided by the present invention is not only convenient but also produces no wastewater. In addition, the data of the examples show that: the composite metal oxide prepared by the present invention exhibits excellent performance in the synchronous remediation of cadmium and arsenic composite pollution, that is, the material can achieve the treatment of cadmium while remediating arsenic pollution, providing an effective solution to the problem that it is difficult to simultaneously treat cadmium and arsenic pollution at present. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0023] Figure 1 XRD pattern of the magnesium-manganese double metal oxide obtained in Example 1;

[0024] Figure 2 TEM image of the magnesium-manganese bimetallic oxide obtained in Example 1;

[0025] Figure 3 BET image of the magnesium-manganese bimetallic oxide obtained in Example 1;

[0026] Figure 4 Kinetic curve of arsenic adsorption by the magnesium-manganese bimetallic oxide obtained in Application Example 1;

[0027] Figure 5 Comparison chart of the adsorption capacity of arsenic adsorption by the magnesium-manganese bimetallic oxide in Application Example 2 and the adsorption of arsenic by other adsorbents;

[0028] Figure 6 Kinetic curve of the composite adsorption of arsenic and cadmium by the magnesium-manganese bimetallic oxide in Application Example 2;

[0029] Figure 7 XRD image of the magnesium-manganese bimetallic oxide after cadmium and arsenic adsorption in Application Example 2. Detailed implementation mode

[0030] The present invention provides a preparation method of a layered composite bimetallic oxide, comprising the following steps:

[0031] Mix magnesium oxide and a hydrate of a magnesium salt and then ball-mill to obtain a ball-milled material;

[0032] Mix the ball-milled material and a hydrate of a manganese salt and then wet-mill to obtain a precursor of the composite bimetallic oxide;

[0033] Calcine the precursor of the composite bimetallic oxide to obtain a layered composite bimetallic oxide;

[0034] The molar ratio of magnesium element in the magnesium oxide to magnesium element in the magnesium salt is 1-6:1.

[0035] As an implementation mode of the present invention, the magnesium salt may include at least one of magnesium chloride, magnesium sulfate, magnesium carbonate and magnesium nitrate; in the embodiments of the present invention, the hydrate of the magnesium salt is specifically described by taking magnesium chloride hexahydrate or magnesium nitrate hexahydrate as an example. As an implementation mode of the present invention, the average particle size of the magnesium oxide may be 20-500 nm; in the embodiments of the present invention, magnesium oxide with a particle size of 50 nm is specifically taken as an example for description. As an implementation mode of the present invention, the molar ratio of magnesium element in the magnesium oxide to magnesium element in the magnesium salt may be 1-6:1, specifically 1:1, 2:1, 3:1, 4:1, 5:1 or 6:1.

[0036] As an embodiment of the present invention, the ball-to-material ratio of the ball milling can be 5 to 20:1, specifically 5:1, 10:1, 15:1, 20:1; the rotation speed of the ball milling can be 400 to 800 rpm, specifically 400 rpm, 500 rpm, 600 rpm, 700 rpm or 800 rpm; the time of the ball milling can be 4 to 6 h, specifically 4 h, 5 h or 6 h; the material of the grinding balls for the ball milling is zirconia; the diameter of the grinding balls can be 1 to 10 mm, specifically 1 mm, 5 mm or 10 mm; in the embodiments of the present invention, an example is given where the mass ratio of the grinding balls with a diameter of 1 mm in the zirconia balls is 15%, the mass ratio of the grinding balls with a diameter of 5 mm is 25%, and the mass ratio of the grinding balls with a diameter of 10 mm is 60%. Ball milling under the above conditions in the present invention helps to form a large number of magnesium defects on the surface of magnesium oxide, enhance its chemical activity, and thus make it easier to chemically react with manganese ions to form a pure-phase composite metal hydroxide.

[0037] After obtaining the ball-milled material, the present invention mixes the ball-milled material and a manganese salt and then performs wet milling to obtain a composite double-metal oxide precursor.

[0038] As an embodiment of the present invention, the manganese salt can include at least one of manganese chloride, manganese nitrate, manganese sulfate and manganese carbonate; in the embodiments of the present invention, the hydrate of the manganese salt is specifically illustrated by taking manganese nitrate tetrahydrate or manganese chloride tetrahydrate as an example. As an embodiment of the present invention, the molar ratio of manganese element in the manganese salt to magnesium element in the ball-milled material can be 1:10 to 1:5, specifically 1:9, 1:8, 1:7, 1:6 or 1:5.

[0039] As an embodiment of the present invention, the ball-to-material ratio of the wet milling can be 5 to 20:1, specifically 5:1, 10:1, 15:1, 20:1; the medium of the wet milling can include absolute ethanol; the rotation speed of the wet milling can be 400 to 800 rpm, specifically 400 rpm, 500 rpm, 600 rpm, 700 rpm or 800 rpm; the time of the wet milling can be 3 to 5 h, specifically 3 h, 4 h or 5 h; the material of the grinding balls for the wet milling is zirconia; the diameter of the grinding balls can be 1 to 10 mm, specifically 1 mm, 5 mm or 10 mm; in the embodiments of the present invention, an example is given where the mass ratio of the grinding balls with a diameter of 1 mm in the zirconia balls is 15%, the mass ratio of the grinding balls with a diameter of 5 mm is 25%, and the mass ratio of the grinding balls with a diameter of 10 mm is 60%. Wet milling under the above conditions in the present invention helps to continue to form a large number of magnesium defects on the surface of magnesium oxide, enhance its chemical activity, and thus make it easier to chemically react with manganese ions to form a pure-phase composite metal hydroxide.

[0040] As an embodiment of the present invention, after the wet grinding, it further includes: centrifuging, washing and then drying the product after the wet grinding treatment to obtain the composite double metal oxide precursor; the reagents used for the washing can be deionized water and ethanol in sequence; the drying method can be drying in an oven; the drying temperature can be 40-90 °C, specifically 60 °C; the drying time can be 6-24 h, specifically 12 h.

[0041] After obtaining the composite double metal oxide precursor, the present invention calcines the composite double metal oxide precursor to obtain a layered composite double metal oxide.

[0042] As an embodiment of the present invention, the calcination temperature can be 300-500 °C, specifically 300 °C, 400 °C or 500 °C; the heat preservation time of the calcination can be 2-6 h, specifically 2 h, 3 h, 4 h, 5 h or 6 h; the heating rate to the calcination can be 3-10 °C / min, specifically 5 °C / min. The present invention calcines under the above conditions and can successfully prepare the layered composite double metal oxide on the basis of lower energy consumption. In addition, the above calcination temperature range can avoid the appearance of manganese oxide impurity phases in the composite material. At the same time, the above heating rate can effectively avoid the sintering phenomenon of the material caused by too fast heating rate (the sintering phenomenon will reduce the specific surface area of the material), and at the same time, the above heating rate can improve the preparation efficiency.

[0043] As an embodiment of the present invention, after the calcination, it further includes cooling; the present invention does not make specific limitations on the cooling method, and natural cooling can be adopted.

[0044] The present invention provides a layered composite double metal oxide prepared by the preparation method described in the above technical solution. As an embodiment of the present invention, the molar ratio of manganese element to magnesium element in the magnesium-manganese layered double metal oxide can be 1:10-1:5, specifically 1:5, 1:7 or 1:10.

[0045] As an embodiment of the present invention, the BET specific surface area of the layered composite double metal oxide magnesium-manganese double metal oxide can be 50-200 m 2 / g, specifically 185 m 2 / g.

[0046] The composite double metal oxide obtained by the present invention has a larger specific surface area and rich active sites (including oxidation sites and adsorption sites) compared with magnesium-manganese layered double hydroxide (MgMn-LDH), which is beneficial to the treatment of the combined pollution of heavy metals arsenic and cadmium.

[0047] The present invention provides an application of the layered double metal oxide prepared by the preparation method described in the above technical solution in synchronously removing arsenic and cadmium in water. The present invention has no special limitation on the specific application mode of the layered double metal oxide, and the methods well-known to those skilled in the art can be adopted.

[0048] Due to its high catalytic activity in aqueous solution, the magnesium-manganese composite oxide obtained in the present invention can undergo a hydration reaction with the water molecules on the surface, hydroxylating the magnesium-manganese composite oxide. During the hydroxylation process, arsenite, arsenate, etc. are very likely to undergo complexation reactions with hydroxyl groups, thereby adsorbing arsenite and arsenate ions in the magnesium-manganese composite oxide. At the same time, during the hydration process of the magnesium-manganese composite oxide, arsenic pollutants in anionic state can also enter the interlayer. That is to say, the removal of arsenic by the magnesium-manganese composite oxide presents multiple channels. Therefore, its adsorption capacity is much greater than that of existing adsorbents for arsenic removal. On the other hand, magnesium ions and cadmium ions have relatively similar ionic radii and similar coordination numbers, and the solubility of magnesium hydroxide is greater than that of cadmium hydroxide. Therefore, cadmium ions are prone to undergo substitution reactions with active magnesium ions, that is, cadmium ions enter the lattice of the magnesium-manganese composite oxide, and a cadmium-containing composite hydroxide is formed during the hydroxylation process, thereby achieving the simultaneous removal of cadmium and arsenic.

[0049] In summary, the magnesium-manganese composite oxide obtained in the present invention can achieve the removal of arsenic through hydroxyl complexation and interlayer confinement, and achieve the removal of cadmium through isomorphous substitution of equivalent classes, and has good application prospects in synchronously removing arsenic and cadmium in water.

[0050] To further illustrate the present invention, the technical solution of the present invention will be described in detail below with reference to the drawings and embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0051] Example 1

[0052] 1.21 g (0.03 mol) of nano-magnesium oxide (particle size of 50 nm) and 2.03 g (0.01 mol) of magnesium chloride hexahydrate were placed in a 316L stainless steel grinding jar. Among them, the total mass of zirconia balls was 20 g. The mass ratio of zirconia balls with a diameter of 1 mm was 15%, the mass ratio of those with a diameter of 5 mm was 25%, and the mass ratio of those with a diameter of 10 mm was 60%. After ball milling at a speed of 600 rpm for 4 h, the ball milling jar was opened. Then, 1.58 g (0.008 mol) of manganese chloride tetrahydrate was placed in the opened ball milling jar, and 3 mL of absolute ethanol was added. After mixing evenly, it was continued to be ball milled at a speed of 600 rpm for 6 h. Then, it was centrifuged. The obtained precipitate was washed 3 times with deionized water and ethanol respectively, and then the precipitate was placed in an oven at 60 °C and dried for 12 h to obtain a magnesium-manganese bimetallic oxide precursor (magnesium-manganese layered double hydroxide MgMn-LDH). In the obtained magnesium-manganese bimetallic oxide precursor, the molar ratio of Mg 2+ :Mn 2+ was 5:1. The magnesium-manganese bimetallic oxide precursor was placed in a muffle furnace and heated to 350 °C at a rate of 5 °C / min. After calcination at 350 °C for 3 h, it was naturally cooled to obtain a layered magnesium-manganese bimetallic oxide (in this layered magnesium-manganese bimetallic oxide, the molar ratio of manganese element to magnesium element was 1:5).

[0053] Figure 1 XRD pattern of the magnesium-manganese bimetallic oxide obtained in Example 1. It can be seen from Figure 1 that when the calcination temperature was 350 °C, layered MgMn-LDO was formed (the spectrum information was: 2θ = 18.7°, 29.8°, 32.9°, 36.9°, 43.3° and 62.8°).

[0054] Figure 2 TEM image of the magnesium-manganese bimetallic oxide obtained in Example 1. It clearly shows that there is no obvious difference in the irregular hexagonal nanosheet morphology between the magnesium-manganese bimetallic oxide obtained in Example 1 and MgMn-LDH. However, different from the smooth surface of MgMn-LDH nanosheets, the magnesium-manganese bimetallic oxide nanosheets obtained in Example 1 were formed by the aggregation of a large number of nanoparticles and showed a porous structure.

[0055] Figure 3 BET image of the magnesium-manganese bimetallic oxide obtained in Example 1. It can be seen from Figure 3 that the BET specific surface area of the magnesium-manganese bimetallic oxide was 185 m 2 / g, which was about 3 times that of the magnesium-manganese bimetallic oxide precursor (MgMn-LDH).

[0056] Example 2

[0057] 2.42 g (0.06 mol) of nano-magnesium oxide (particle size of 50 nm) and 2.56 g (0.01 mol) of magnesium nitrate hexahydrate were placed in a 316 L stainless steel grinding jar. Among them, the total mass of zirconia balls was 20 g, and the total mass of zirconia balls was 20 g. The mass ratio of zirconia balls with a diameter of 1 mm was 15%, the mass ratio of zirconia balls with a diameter of 5 mm was 25%, and the mass ratio of zirconia balls with a diameter of 10 mm was 60%. After ball milling at a speed of 600 rpm for 5 h, the ball milling jar was opened, and 2.51 g (0.01 mol) of manganese nitrate tetrahydrate was placed in the opened ball milling jar. 5 mL of absolute ethanol was added, and after mixing evenly, it was continued to ball mill at a speed of 700 rpm for 6 h, centrifuged, and the obtained precipitate was washed 3 times with deionized water and ethanol respectively. The precipitate was placed in an oven at 60 °C and dried for 12 h to obtain a magnesium-manganese bimetallic oxide precursor. In the magnesium-manganese bimetallic oxide precursor, Mg 2+ : Mn 2+ The molar ratio was 7:1. The magnesium-manganese bimetallic oxide precursor was placed in a muffle furnace and heated to 350 °C at a rate of 5 °C / min. After calcining at 350 °C for 3 h, it was naturally cooled to obtain a layered magnesium-manganese bimetallic oxide (the molar ratio of manganese element to magnesium element in this layered magnesium-manganese bimetallic oxide was 1:7).

[0058] Application Example 1

[0059] Adsorption of arsenic by the magnesium-manganese bimetallic oxide obtained in Example 1:

[0060] 0.1 g of the magnesium-manganese bimetallic oxide obtained in Example 1 was placed in a 200 mL beaker, and 100 mL of sodium arsenite solution with an arsenic concentration of 10 - 2000 mg / L was added. After magnetic stirring at room temperature for 12 h to reach the adsorption equilibrium, the upper layer solution was taken, filtered through a 0.22 µm filter membrane, and the concentration of As in it was measured by inductively coupled plasma technology (ICP-OES). The adsorption amounts of arsenic at different concentrations are as Figure 4 shown. After calculation, its maximum adsorption amount was 662.4 mg / g, and this value was much larger than other adsorbents reported currently (see Figure 5 ).

[0061] Application Example 2

[0062] Adsorption of arsenic-cadmium composite heavy metals by the magnesium-manganese bimetallic oxide obtained in Example 2:

[0063] Put 0.1 g of the magnesium-manganese bimetallic oxide obtained in Example 2 into a 200 mL beaker, add 100 mL of a sodium arsenite-cadmium nitrate mixed solution, where the concentration of arsenic is 100 mg / L and the concentration of cadmium is 200 mg / L. Adjust the pH value of the mixed solution to about 5. After magnetic stirring at room temperature for 12 h to reach adsorption equilibrium, take the upper layer solution, filter it through a 0.22 µm filter membrane, and measure the concentrations of As and Cd in it by ICP-OES. The removal rate results at different time intervals are as Figure 6 shown. It can be seen from Figure 6 that the magnesium-manganese bimetallic oxide in Example 2 can reach adsorption equilibrium for arsenic-cadmium composite heavy metals within 10 min. The addition of Cd greatly increases the adsorption rate of the magnesium-manganese bimetallic oxide for arsenic, indicating that the magnesium-manganese bimetallic oxide has a strong affinity for arsenic and cadmium composite heavy metals.

[0064] Figure 7 Figure Figure 7 Figure 6 is the XRD pattern of the magnesium-manganese bimetallic oxide after adsorbing cadmium and arsenic in Example 2. It can be seen from

[0065] that after adsorbing arsenic, the XRD diffraction pattern of the magnesium-manganese bimetallic oxide shows typical characteristic diffraction peaks of hydrotalcite, indicating that the adsorption mechanism of the prepared magnesium-manganese bimetallic oxide for cadmium-arsenic composite pollution is significantly different from that of existing materials. By analyzing the XRD pattern, it can be seen that during the process of adsorbing cadmium-arsenic composite pollutants, the magnesium-manganese bimetallic oxide undergoes a phase topology transformation process. Cadmium ions in the solution replace magnesium in the magnesium-manganese bimetallic oxide to form a magnesium-cadmium-manganese ternary hydrotalcite, which is consistent with isomorphous substitution in geology, rather than the conventional material adsorbing cadmium ions on the surface. This special adsorption method fixes cadmium in the crystal lattice. Due to the rich coordination bonds and host-guest interactions between the lattices, it is extremely difficult for cadmium ions to be desorbed again. At the same time, it can also be seen from the XRD pattern that there is an obvious increase in the layer spacing, indicating that interlayer guest molecules with larger sizes enter the interlayer. By knowing the types of anions in the reaction system, it can be known that the guest anion is arsenate ion or arsenite ion, indicating that the pollutant arsenic is confined in the limited space between the layers. The above results show that the magnesium-manganese bimetallic oxide realizes simultaneous adsorption through different adsorption paths when treating cadmium-arsenic composite pollution, solving the problem that it is difficult for the original materials to effectively adsorb cadmium and arsenic synchronously. It can be seen from Application Examples 1 to 2 that the magnesium-manganese bimetallic oxide shows excellent performance both when treating arsenic pollutants alone and when treating cadmium-arsenic composite pollutants simultaneously, further indicating that the simultaneous adsorption of cadmium-arsenic pollutants by the material provided by the present invention is carried out through different adsorption mechanisms.

[0066] Application Example 3

[0067] Remediation of cadmium-arsenic composite heavy metal contaminated soil with the magnesium-manganese bimetallic oxide obtained in Example 2:

[0068] Take 500 g of cadmium-arsenic contaminated soil sample and 10 g of the magnesium-manganese bimetallic oxide obtained in Example 2 and mix them thoroughly in a beaker. Then add 300 mL of deionized water and continue stirring until it reaches a muddy paste state, and then let it stand naturally. After natural drying, use this soil sample to grow Maclura cochinchinensis. After the Maclura cochinchinensis seedlings emerge, continue to grow for 20 days. Carefully pull out the Maclura cochinchinensis seedlings with their roots, wash the soil off, air-dry them naturally, weigh them, and then use the national standard method to determine the cadmium and arsenic content in the Maclura cochinchinensis seedlings. Among them, the cadmium content is 0.021 mg / kg, and the arsenic content is 0.11 mg / kg, which meets the limit values that the cadmium and arsenic content in vegetables shall not exceed as stipulated in the national standard (the cadmium content shall not exceed 0.1 mg / kg, and the arsenic shall not exceed 0.5 mg / kg).

[0069] Comparative Example

[0070] To further illustrate the effect of the magnesium-manganese bimetallic oxide in the remediation of cadmium-arsenic contaminated soil, a verification of the comparative example was carried out. The difference between this comparative example and Application Example 3 is that no magnesium-manganese bimetallic oxide is added to the contaminated soil, and other operations are the same.

[0071] Take 500 g of cadmium-arsenic composite heavy metal contaminated soil, mix it thoroughly in a beaker, then add 300 mL of deionized water and continue stirring until it reaches a muddy paste state, and then let it stand naturally. After natural drying, use this soil sample to grow Maclura cochinchinensis. After the Maclura cochinchinensis seedlings emerge, continue to grow for 20 days. Carefully pull out the Maclura cochinchinensis seedlings with their roots, wash the soil off, air-dry them naturally, weigh them, and then use the national standard method to determine the cadmium and arsenic content in the Maclura cochinchinensis seedlings. Among them, the cadmium content is 0.283 mg / kg, and the arsenic content is 0.741 mg / kg. Compared with the national standard, it exceeds the upper limit, and it belongs to vegetables that do not meet the national requirements. It is very likely to cause harm to human health after consumption.

[0072] By comparing Application Example 3 and the comparative example, it can be proved that the magnesium-manganese bimetallic oxide provided by the present invention has a very good effect in the remediation of cadmium-arsenic contaminated soil.

[0073] Although the above embodiments have made a detailed description of the present invention, they are only a part of the embodiments of the present invention, not all embodiments. Other embodiments can also be obtained according to these embodiments without creative work, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a layered composite bimetallic oxide, comprising the following steps: The magnesium oxide and the hydrate of the magnesium salt are mixed and then ball-milled to obtain a ball-milled material; The ball mill material and the hydrate of the manganese salt are mixed and wet-milled to obtain a composite bimetallic oxide precursor; calcining the composite bimetallic oxide precursor to obtain a layered composite bimetallic oxide; The molar ratio of the magnesium element in the magnesium oxide to the magnesium element in the magnesium salt is 1 to 6:1; The molar ratio of the manganese element in the manganese salt to the magnesium element in the ball-milled material is 1:10 to 1:

5.

2. The preparation method according to claim 1, characterized in that The magnesium salt includes at least one of magnesium chloride, magnesium sulfate, magnesium carbonate and magnesium nitrate.

3. The preparation method according to claim 1, characterized in that: The manganese salt includes at least one of manganese chloride, manganese nitrate, manganese sulfate and manganese carbonate.

4. The preparation method according to any one of claims 1 to 3, characterized in that The ball-to-material ratio of the ball mill and the wet mill is independently 5-20:1; the rotation speed of the ball mill and the wet mill is independently 400-800 rpm; the material of the grinding balls in the ball mill and the wet mill is zirconium oxide; the diameter of the grinding balls is independently 1-10 mm.

5. The preparation method according to claim 1, characterized in that: The calcination temperature is 300-500°C, and the calcination holding time is 2-6 h.

6. The preparation method according to claim 5, characterized in that: The heating rate to the calcination temperature is 3-10°C / min.

7. The layered composite bimetallic oxide prepared by the preparation method according to any one of claims 1 to 6, wherein the layered composite bimetallic oxide is a magnesium-manganese layered bimetallic oxide, characterized in that: The molar ratio of manganese element to magnesium element in the magnesium-manganese layered bimetallic oxide is 1:10-1:

5.

8. The layered composite bimetallic oxide according to claim 7, characterized in that: The BET specific surface area of ​​the layered composite bimetallic oxide is 50-200 m 2 / g.

9. Use of the layered composite bimetallic oxide prepared by the preparation method according to any one of claims 1 to 6 or the layered composite bimetallic oxide according to claim 7 or 8 in the simultaneous removal of arsenic and cadmium.

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