Preparation method and application of manganese-iron hydrotalcite

Abstract

The present invention belongs to the field of material technology, and discloses a preparation method and application of manganese-iron hydrotalcite. The preparation method of the manganese-iron hydrotalcite comprises the following steps: (1) introducing water into an inert gas, then adding manganese salt and iron salt, and dissolving to obtain a metal solution; (2) dissolving an alkali in water treated with an inert gas to obtain an alkali solution; (3) under the protection of an inert gas, simultaneously dropping the metal solution and the alkali solution into water treated with an inert gas, and stirring during the dropping process to obtain a mixed solution; (4) under the protection of an inert gas, aging the mixed solution in a water bath, filtering, taking the precipitate, washing, and freeze-drying to obtain manganese-iron hydrotalcite. The manganese-iron hydrotalcite has high purity and high crystallinity, and the adsorption capacity for As(III) exceeds 130 mg / g, and the adsorption capacity for As(V) exceeds 70 mg / g.

Description

Technical Field

[0001] The present invention belongs to the technical field of materials, and particularly relates to a preparation method and application of manganese-iron hydrotalcite. Background Art

[0002] Arsenic and its compounds are highly toxic pollutants in the environment, which can cause serious harm to water quality, ecology and human health. Arsenic in the water environment exists in two valence states, As(Ⅲ) and As(Ⅴ), mainly in the form of inorganic anions. The treatment methods for arsenic pollution mainly include precipitation method, adsorption method, biological method and membrane separation method, etc. Among them, the adsorption method is a method that uses an adsorbent with a large specific surface area and affinity to adsorb and fix arsenic on the surface or inside of the adsorbent, and the operation is simple. The adsorbents in the prior art have a certain arsenic removal effect.

[0003] Hydrotalcite, also called layered double hydroxide (LDH), is a general term for hydrotalcite and hydrotalcite-like compounds, which are assembled by a positively charged main layer board and interlayer anions through non-covalent interactions, and have excellent adsorption performance for anionic pollutants. The chemical composition of the main layer board of LDH can be adjusted, and has M 2+ with an ionic radius close to that of Mg 2+ , M 3+ or M 4+ , such as Mn 2+ , Zn 2+ , Ni 2+ , Al 3+ and Fe 3+ etc. can all form the LDH layer board. Iron oxides and manganese oxides both have a strong affinity for arsenic, and their main arsenic adsorption sites are Mn-O(H) or Fe-O(H), and they are commonly used as adsorption materials for arsenic pollution. Manganese-iron hydrotalcite (Mn-Fe LDH) has a high density of Mn-OH and Fe-OH sites on the layer board, and the interlayer can exchange ions with anionic pollutants, and has a larger arsenic adsorption capacity compared with other iron-manganese oxides.

[0004] In the prior art, coprecipitation method and hydrothermal method are commonly used to synthesize manganese-iron hydrotalcite. However, the synthesized manganese-iron hydrotalcite generally contains impurities or has low crystallinity. According to the reported preparation methods of manganese-iron hydrotalcite, the following attempts were made: 1. When prepared by coprecipitation method without oxygen isolation, Mn-Fe LDH cannot be obtained, and a large amount of Mn2CO3 impurities are contained in the sample; 2. When prepared by coprecipitation method under oxygen isolation conditions, Mn-Fe LDH with relatively low crystallinity and a large amount of Mn2CO3 impurities will be obtained; 3. When hydrothermal method capable of improving the crystallinity of hydrotalcite is used to synthesize Mn-Fe LDH, the oxidation of Mn salt will be accelerated, forming iron-manganese spinel and various minerals of Mn. Therefore, for the reported preparation methods, whether oxygen is isolated or not, it is difficult to obtain manganese-iron hydrotalcite with high purity and high crystallinity. Manganese-iron hydrotalcite with impurities or low crystallinity will result in general adsorption effect on arsenic ions. For example, the adsorption capacity of manganese-iron hydrotalcite prepared in the prior art for arsenic ions is difficult to exceed 6.8 mg / g.

[0005] Therefore, there is an urgent need to provide a new preparation method of manganese-iron hydrotalcite, so that the prepared manganese-iron hydrotalcite not only has high purity, but also has high crystallinity, thereby improving the adsorption effect on arsenic. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the above prior art. For this reason, the present invention provides a preparation method and application of manganese-iron hydrotalcite.

[0007] The object of the present invention is to solve the technical problem that it is difficult to obtain manganese-iron hydrotalcite with high purity and high crystallinity by the existing preparation methods. According to the synthesis characteristics of manganese-iron hydrotalcite, the present invention provides a preparation method of manganese-iron hydrotalcite by selecting a specific mixing method and reaction conditions (i.e., dropping metal solution and alkali solution under specific conditions and controlling the temperature of the mixed solution). The preparation method has simple steps, short time consumption, high success rate, and the prepared Mn-Fe LDH (manganese-iron hydrotalcite) has high purity and high crystallinity, and has excellent arsenic adsorption performance. For example, the adsorption amount of As(Ⅲ) exceeds 130 mg / g, and the adsorption amount of As(Ⅴ) exceeds 70 mg / g, and the adsorption effect is far better than that of the prior art.

[0008] The first aspect of the present invention provides a preparation method of manganese-iron hydrotalcite.

[0009] Specifically, a preparation method of manganese-iron hydrotalcite includes the following steps:

[0010] (1) Introduce inert gas into water, then add manganese salt and iron salt, and dissolve to obtain a metal solution;

[0011] (2) Dissolve alkali in water treated with inert gas to obtain an alkali solution;

[0012] (3) Under the protection of inert gas, the metal solution and the alkali solution are simultaneously dropped into the water that has been treated with inert gas, and the dropping process is stirred to obtain a mixed solution;

[0013] (4) Under the protection of inert gas, the mixed solution is aged in a water bath, filtered, the precipitate is taken, washed, and freeze-dried to obtain the manganese-iron hydrotalcite.

[0014] Preferably, the inert gas is selected from nitrogen or noble gases. The purpose of the inert gas is to remove oxygen and carbon dioxide in the water, or isolate oxygen or carbon dioxide, thereby reducing or preventing the generation of impurities.

[0015] Preferably, the water is deionized water.

[0016] Preferably, in step (1), the manganese salt is selected from at least one of manganese chloride, manganese sulfate, manganese nitrate or their hydrates; further preferably, the manganese salt is manganese chloride or manganese chloride tetrahydrate.

[0017] Preferably, in step (1), the iron salt is selected from at least one of ferric chloride, ferric sulfate, ferric nitrate or their hydrates; further preferably, the iron salt is ferric chloride or ferric chloride hexahydrate.

[0018] Preferably, in step (1), the molar ratio of Mn to Fe in the manganese salt and the iron salt is (1-6):1, preferably (2-4):1.

[0019] Preferably, in step (2), the alkali includes sodium hydroxide and / or sodium carbonate; further preferably, the alkali includes sodium hydroxide and sodium carbonate.

[0020] Preferably, in step (2), the alkali solution contains sodium hydroxide and sodium carbonate, the concentration of sodium hydroxide is the same as the concentration of the manganese salt in the metal solution, and the concentration of sodium carbonate is the same as the concentration of the iron salt in the metal solution.

[0021] Preferably, in step (3), the dropping rates of the metal solution and the alkali solution are both 0.5-2 mL / min, and the temperature of the mixed solution is 60-70 °C.

[0022] Preferably, in step (3), the pH value of the water treated with inert gas is adjusted to 9-12.5 with sodium hydroxide, and the temperature is maintained at 60-70 °C, preferably the pH value is 10-11, and the temperature is maintained at 60-62 °C.

[0023] Preferably, in step (3), after the dropping is completed, stirring is continued for 8-20 minutes, preferably 10-12 minutes.

[0024] Preferably, in step (4), the temperature of the water bath aging is 60 - 70 °C, and the aging time is 10 - 12 hours.

[0025] Preferably, in step (4), the washing is performed with water treated by inert gas until the pH of the washing liquid is stable at 7 - 7.1, preferably stable at pH 7.

[0026] Preferably, in step (4), after the washing, the precipitate is centrifuged, then freeze - dried at a temperature below - 80 °C for 40 - 48 hours, and ground through a 100 - 200 mesh sieve to obtain the manganese - iron hydrotalcite.

[0027] The second aspect of the present invention provides a manganese - iron hydrotalcite.

[0028] A manganese - iron hydrotalcite, prepared by the above - mentioned preparation method, and having a hexagonal petal - like structure, with a diameter of 8 - 310 nm and a width of 8 - 22 nm.

[0029] Preferably, the manganese - iron hydrotalcite has a mutually interlaced and stacked hexagonal petal - like structure.

[0030] Preferably, the manganese - iron hydrotalcite has a diameter of 10 - 300 nm and a width of 10 - 20 nm.

[0031] Preferably, the adsorption capacity of the manganese - iron hydrotalcite for As(Ⅲ) exceeds 130 mg / g, and the adsorption capacity for As(Ⅴ) exceeds 70 mg / g.

[0032] More preferably, the adsorption capacity of the manganese - iron hydrotalcite for As(Ⅲ) is 140 - 150 mg / g, and the adsorption capacity for As(Ⅴ) is 80 - 90 mg / g.

[0033] The third aspect of the present invention provides an application of a manganese - iron hydrotalcite.

[0034] The application of the above - mentioned manganese - iron hydrotalcite in sewage treatment.

[0035] Preferably, the sewage is As - containing sewage.

[0036] More preferably, the sewage is As(Ⅲ) - and / or As(Ⅴ) - containing sewage.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] The present invention provides a preparation method of manganese-iron layered double hydroxide by selecting a specific mixing method and reaction conditions (i.e., dropping a metal solution and an alkali solution under specific conditions and controlling the temperature of the mixed solution). The preparation method has simple steps, short time consumption, high success rate, and the prepared Mn-Fe LDH (manganese-iron layered double hydroxide) has high purity and high crystallinity, and has excellent arsenic adsorption performance. For example, the adsorption capacity for As(Ⅲ) exceeds 130 mg / g, and the adsorption capacity for As(Ⅴ) exceeds 70 mg / g, and the adsorption effect is far better than that of the prior art. Description of the Drawings

[0039] Figure 1 Schematic diagram of the preparation method of Mn-Fe LDH in Example 1;

[0040] Figure 2 XRD pattern of Mn-Fe LDH in Example 1;

[0041] Figure 3 SEM pattern of Mn-Fe LDH in Example 1;

[0042] Figure 4 XRD pattern of the product prepared in Comparative Example 1;

[0043] Figure 5 XRD pattern of the product prepared in Comparative Example 2;

[0044] Figure 6 XRD pattern of the product prepared in Comparative Example 3;

[0045] Figure 7 Langmuir adsorption isotherm of the products prepared in Example 1, Comparative Example 2, and Comparative Example 3 for As(Ⅲ);

[0046] Figure 8 Langmuir adsorption isotherm of the products prepared in Example 1, Comparative Example 2, and Comparative Example 3 for As(Ⅴ). Detailed Description of the Invention

[0047] In order to make those skilled in the art more clearly understand the technical solutions of the present invention, the following examples are listed for illustration. It should be noted that the following examples do not limit the scope of protection required by the present invention.

[0048] The raw materials, reagents or devices used in the following examples can be obtained from conventional commercial channels or can be obtained by existing known methods without special instructions.

[0049] Example 1

[0050] A preparation method of manganese-iron layered double hydroxide (Mn-Fe LDH) is as follows:

[0051] (1) Bubble N2 gas into deionized water for 20 minutes to remove O2 and CO2 in the deionized water (the deionized water used in the following steps is deionized water that has been bubbled with N2 gas for 20 minutes);

[0052] (2) Dissolve manganese chloride tetrahydrate (0.67 mol) and ferric chloride hexahydrate (0.33 mol) in 50 mL of deionized water to obtain a metal solution;

[0053] (3) Dissolve NaOH (0.67 mol) and Na2CO3 (0.33 mol) in 50 mL of deionized water to obtain an alkali solution;

[0054] (4) Prepare a conical flask filled with 100 mL of deionized water, stir magnetically and heat to 60 °C, and add the alkali solution to the deionized water to adjust the pH to 10;

[0055] (5) Under a N2 atmosphere, use a peristaltic pump to simultaneously drip the metal solution and the alkali solution into the conical flask at a drip rate of 1 mL / min. During the dripping process, vigorously stir the mixture in the conical flask with a magnetic stirrer to keep the pH of the mixture at 10 and the temperature at 60 °C. After the dripping is completed, continue stirring for 10 min;

[0056] (6) Seal the conical flask containing the mixture to isolate air and place it in a water bath at 60 °C for aging for 12 h;

[0057] (7) Filter to remove the supernatant, take the precipitate, and repeatedly centrifuge and wash the precipitate with deionized water until the pH value of the washing solution stabilizes at 7;

[0058] (8) After centrifuging the precipitate, vacuum freeze-dry it at -80 °C for 48 h, grind it through a 100-mesh sieve to obtain manganese-iron layered double hydroxide (Mn-Fe LDH).

[0059] Figure 1 It is a schematic diagram of the preparation method of Mn-Fe LDH in Example 1.

[0060] Figure 2 It is the XRD pattern of Mn-Fe LDH in Example 1. From Figure 2 ( Figure 2"Intensity" represents intensity, and "2-Thate (degree)" represents 2θ (degree). It can be seen that the XRD pattern of the Mn-Fe LDH prepared in Example 1 is close to the PDF#46-0098 card (green rust, hydrotalcite-like structure), and has characteristic peaks related to hydrotalcite such as (003), (006), (012), (015), (018), (110), (113), and (116), showing an obvious layered structure. The XRD pattern of the prepared Mn-Fe LDH is clear without impurity peaks, proving that the Mn-Fe LDH prepared by this method has high purity and high crystallinity.

[0061] Figure 3 Figure 4 is the SEM pattern of the Mn-Fe LDH in Example 1; from Figure 3 it can be seen that the Mn-Fe LDH flakes prepared in Example 1 have an interlaced and stacked hexagonal petal-like structure, with a diameter of about 10 - 300 nm and a width of about 10 - 20 nm.

[0062] Comparative Example 1

[0063] Compared with Example 1, the difference in Comparative Example 1 is only that in Comparative Example 1, Mn-Fe LDH was prepared under non-air-blocking conditions (that is, the deionized water used was not aerated with N2, and the dropping operation was not carried out in an N2 atmosphere), and a product with Mn2CO3 impurities was obtained.

[0064] Figure 4 Figure 16 is the XRD pattern of the product prepared in Comparative Example 1; from Figure 4 ( Figure 4 "Intensity" represents intensity, and "2-Thate (degree)" represents 2θ (degree)) it can be seen that the XRD pattern of the product prepared in Comparative Example 1 contains, in addition to the Mn-Fe LDH peaks close to the PDF#46-0098 card (green rust, hydrotalcite-like structure), a large number of spectral peaks consistent with the characteristic peaks of Mn2CO3, indicating that the formation of a large amount of Mn2CO3 impurities will occur if oxygen is not isolated during the preparation process of Comparative Example 1.

[0065] Comparative Example 2

[0066] In Comparative Example 2, the product was prepared by a conventional coprecipitation method to obtain a Mn2CO3 product without Mn-Fe LDH.

[0067] The specific process of Comparative Example 2 is as follows:

[0068] (1) Dissolve manganese chloride tetrahydrate (0.67 mol) and ferric chloride hexahydrate (0.33 mol) in 50 mL of deionized water as the metal solution;

[0069] (2) Dissolve NaOH (0.67 mol) and Na2CO3 (0.33 mol) in 50 mL of deionized water to serve as the alkali solution;

[0070] (3) Under vigorous stirring of a magnetic stirrer, use a peristaltic pump to drip the alkali solution into the metal solution at a dropping rate of 1 mL / min until the pH of the resulting mixed solution reaches 10. After the dropping is completed, continue stirring for 10 min;

[0071] (4) Seal the conical flask containing the mixed solution and place it in a water bath at 60 °C for aging for 12 h;

[0072] (5) Filter to remove the supernatant, take the precipitate, and repeatedly centrifuge and wash the precipitate with deionized water until the pH value of the washing solution stabilizes at 7;

[0073] (6) After centrifuging the precipitate, vacuum freeze-dry it at -80 °C for 48 h, and mill it through a 100-mesh sieve to obtain the product.

[0074] Figure 5 XRD pattern of the product prepared in Comparative Example 2; It can be seen from Figure 5 ( Figure 5 "Intensity" represents intensity, and "2-Thate (degree)" represents 2θ (degree)) that the XRD pattern of the prepared product is consistent with the characteristic peaks of Mn2CO3, and no crystal phase other than Mn2CO3 is found, indicating that the conventional coprecipitation method will lead to the preferential formation of Mn2CO3.

[0075] Comparative Example 3

[0076] In Comparative Example 3, a hydrothermal method was used to prepare the product, and a magnetic iron-manganese spinel product without Mn-Fe LDH and various minerals of Mn were obtained.

[0077] The specific process of Comparative Example 3 is as follows:

[0078] (1) Dissolve manganese chloride tetrahydrate (0.67 mol) and iron chloride hexahydrate (0.33 mol) in 50 mL of deionized water to serve as the metal solution;

[0079] (2) Dissolve NaOH (0.67 mol) and Na2CO3 (0.33 mol) in 50 mL of deionized water to serve as the alkali solution;

[0080] (3) Prepare a conical flask containing 100 mL of deionized water, stir magnetically and heat to 60 °C, and drip the alkali solution into the deionized water and adjust the pH to about 10;

[0081] (4) Use a peristaltic pump to simultaneously drip the metal solution and the alkali solution into a conical flask at a dripping rate of 1 mL / min. During the dripping process, use a magnetic stirrer to vigorously stir the mixed solution in the conical flask, maintaining the pH of the mixed solution at 10 and the temperature at 60 °C. After the dripping is completed, continue stirring for 10 min;

[0082] (5) Transfer the mixed solution to a 200 mL polytetrafluoroethylene high-pressure hydrothermal reactor. After sealing, place the hydrothermal reactor in an oil bath at 120 °C for crystallization for 24 h. After crystallization, naturally cool it to room temperature;

[0083] (6) Filter to remove the supernatant. Take the precipitate and repeatedly centrifuge and wash the precipitate with deionized water until the pH value of the washing solution stabilizes at 7;

[0084] (7) After centrifuging the precipitate, vacuum freeze-dry it at -80 °C for 48 h, and mill it through a 100-mesh sieve to obtain the product.

[0085] Figure 6 is the XRD pattern of the product prepared in Comparative Example 3; from Figure 6 ( Figure 6 "Intensity" represents intensity, and "2-Thate (degree)" represents 2θ (degree)). It can be seen that the XRD pattern of the product prepared in Comparative Example 3 has peaks consistent with the characteristic peaks of Mn2CO3, Mn3O4, and MnFeO4 (ferromanganese spinel), indicating that the hydrothermal method will exacerbate the oxidation of Mn salts and form ferromanganese spinel with Fe salts.

[0086] Take the products prepared in Example 1 and Comparative Examples 2-3, and under the same conditions, adsorb sewage containing different concentrations of As(Ⅲ) and As(Ⅴ) respectively. The adsorption effects of the products on As(Ⅲ) and As(Ⅴ) are as Figure 7 and Figure 8 shown ( Figure 7 、 Figure 8 in the "q e " represents the adsorption capacity, and "C e " represents the concentration).

[0087] Figure 7 is the Langmuir adsorption isotherm of the products prepared in Example 1, Comparative Example 2, and Comparative Example 3 for As(Ⅲ); Figure 8 is the Langmuir adsorption isotherm of the products prepared in Example 1, Comparative Example 2, and Comparative Example 3 for As(Ⅴ).

[0088] From Figure 7 、 Figure 8It can be seen that, by using the preparation method of Example 1 of the present invention, the adsorption amounts of the prepared manganese-iron hydrotalcite for As(Ⅲ) and As(Ⅴ) in water are significantly greater than those of the products prepared in Comparative Examples 1-3. The maximum adsorption amount of the Mn-Fe LDH product prepared by the preparation method of Example 1 of the present invention for As(Ⅲ) is 146.0 mg / g, and the adsorption amount for As(Ⅴ) is 89.1 mg / g, indicating that the Mn-Fe LDH prepared by the present invention has excellent performance in treating arsenic pollution in the environment.

[0089] In addition, if the types of manganese salts and iron salts are changed, or the dropping rate is adjusted in the range of 0.5-2 mL / min, the adsorption amount of the prepared Mn-Fe LDH for As(Ⅲ) exceeds 130 mg / g, and the adsorption amount for As(Ⅴ) exceeds 70 mg / g, but the effect is not as good as that of Example 1.

Claims

1. A preparation method of manganese-iron hydrotalcite, characterized in that, It includes the following steps: (1) Pass water into an inert gas, then add manganese salts and iron salts and dissolve them to obtain a metal solution; (2) Dissolve an alkali in water treated with an inert gas to obtain an alkali solution; (3) Under the protection of an inert gas, simultaneously drip the metal solution and the alkali solution into water treated with an inert gas, and stir during the dripping process to obtain a mixed solution; (4) Under the protection of an inert gas, age the mixed solution in a water bath, filter it, take the precipitate, wash it, and freeze-dry it to obtain the manganese-iron hydrotalcite; In step (3), the pH value of the water treated with an inert gas is adjusted to 9 - 12.5 with sodium hydroxide, and the temperature is maintained at 60 - 70 °C; In step (3), after the dripping is completed, continue to stir for 8 - 20 minutes.

2. The preparation method according to claim 1, characterized in that, The inert gas is selected from nitrogen or noble gases; in step (3), the dripping rates of the metal solution and the alkali solution are both 0.5 - 2 mL / min, and the temperature of the mixed solution is 60 - 70 °C.

3. The preparation method according to claim 1, characterized in that, In step (1), the manganese salt is selected from at least one of manganese chloride, manganese sulfate, manganese nitrate or their hydrates.

4. The preparation method according to claim 1, wherein In step (1), the iron salt is selected from at least one of iron chloride, iron sulfate, iron nitrate or their hydrates.

5. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of Mn to Fe in the manganese salt and the iron salt is (1 - 6):

1.

6. The preparation method according to claim 1, characterized in that, In step (2), the alkali includes sodium hydroxide and / or sodium carbonate.

7. The preparation method according to claim 1, characterized in that, In step (4), the temperature of the water bath aging is 60 - 70 °C, and the aging time is 10 - 12 hours.

8. A manganese iron hydrotalcite, characterized in that, Prepared by the preparation method according to any one of claims 1 - 7, and having a hexagonal petal-like structure, with a diameter of 8 - 310 nm and a width of 8 - 22 nm.

9. Application of the manganese-iron hydrotalcite according to claim 8 in sewage treatment.

10. The application according to claim 9, characterized in that, The adsorption capacity of the manganese-iron hydrotalcite for As(Ⅲ) exceeds 130 mg / g, and the adsorption capacity for As(Ⅴ) exceeds 70 mg / g.

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