A method for preparing and applying a MnO2-WO3-CNT composite material for treating chromium-containing wastewater.

Abstract

This invention discloses a method for preparing MnO2-WO3-CNT composite material for treating chromium-containing wastewater, comprising the following steps: (1) Preparation of MnO2-CNT solution: carbon nanotubes are dispersed in KMnO4 solution and mixed, Mn(Ac)2 solution is added and mixed thoroughly to obtain MnO2-CNT solution; (2) Preparation of powdered MnO2-CNT composite material: after removing the supernatant from the prepared MnO2-CNT solution, centrifuge and wash, dry and grind to obtain powdered MnO2-CNT composite material; (3) Preparation of MnO2-WO3-CNT composite material: the MnO2-CNT composite material powder is dispersed in tungsten hexachloride solution, stirred for 1-4 hours, centrifuged, washed and dried, the dried product is ground and calcined, and after calcination, the product is taken out to obtain MnO2-WO3-CNT composite material. The preparation method provided by this invention is simple, does not cause serious pollution, and is inexpensive. It achieves the conversion of hexavalent chromium to trivalent chromium by introducing tungsten oxide and manganese dioxide into carbon nanotubes to form MnO2-WO3-CNT composite material, providing a new approach for treating chromium-containing wastewater.

Description

Technical Field

[0001] This invention relates to the field of materials preparation, specifically to a method for preparing and applying a MnO2-WO3-CNT composite material for treating chromium-containing wastewater. Background Technology

[0002] Hexavalent chromium (Cr(VI)) is a persistent environmental hazard and a carcinogen. It is easily absorbed by the human body and can damage the body through the respiratory tract, skin, and mucous membranes, causing numerous health problems such as gastrointestinal, liver, and kidney dysfunction, posing a serious threat to human health. Therefore, we need to treat chromium-containing wastewater to reduce chromium pollution and harm to organisms.

[0003] Currently, there are various methods for treating chromium-containing wastewater, and capacitive deionization (CDI) is one of them. This is a technology based on the concept of an electric double-layer capacitor, involving adsorption / desorption. For CDI, the choice of electrode material is particularly important, as it is a major factor affecting CDI performance. Previous studies have reported that carbon materials with high specific surface areas, such as activated carbon (AC), mesoporous carbon (MC), carbon nanotubes (CNTs), carbon aerogels (CAs), and graphene, are widely used as electrode materials for CDI. Among these carbon materials, carbon nanotubes are considered ideal candidates for electro-adsorption treatment of chromium-containing wastewater due to their high mechanical strength, high adsorption capacity, good conductivity, and good chemical stability. However, in practical applications, carbon nanotubes suffer from limited effective adsorption space and low surface utilization due to their structural shape. Furthermore, carbon materials alone can only adsorb hexavalent chromium, and cannot effectively solve the problem of chromium pollution.

[0004] MnO2 is a transition metal oxide with high theoretical specific capacitance, low cost, abundant reserves, stable electrochemical performance, and environmental friendliness. Combining it with carbon nanotubes can modify the carbon nanotubes, increasing their effective surface area and adsorption space. Furthermore, Mn in MnO2 is a variable-valence metal, capable of converting hexavalent chromium to trivalent chromium during electroadsorption. WO3 is also a transition metal oxide with high theoretical specific capacitance and low environmental pollution. WO3 has high conductivity, good electrochemical performance, good stability, and electron transfer capabilities; however, it is relatively expensive, while MnO2 is cheaper. Combining WO3 and MnO2 with carbon nanotubes simultaneously not only reduces cost but also further improves the electrochemical performance of the material and its adsorption and conversion capacity for hexavalent chromium. Therefore, how to combine MnO2, WO3 and carbon nanotubes and realize the conversion of hexavalent chromium to trivalent chromium through electroadsorption is a research direction. Based on this, this application provides a method for preparing MnO2-WO3-CNT composite material for treating chromium-containing wastewater and its application. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a MnO2-WO3-CNT composite material for treating chromium-containing wastewater and its application in this treatment. The preparation method described in this invention is simple, does not cause serious pollution, and is low in cost. Furthermore, by introducing tungsten oxide and manganese dioxide into carbon nanotubes to form the MnO2-WO3-CNT composite material, the conversion of hexavalent chromium to trivalent chromium is achieved, providing a new approach for treating chromium-containing wastewater.

[0006] In a first aspect, the present invention provides a method for preparing a MnO2-WO3-CNT composite material for treating chromium-containing wastewater, comprising the following steps:

[0007] (1) Preparation of MnO2-CNT solution: Carbon nanotubes were dispersed in KMnO4 solution and mixed, Mn(Ac)2 solution was added and mixed thoroughly to obtain MnO2-CNT solution;

[0008] (2) Preparation method of powdered MnO2-CNT composite material: After removing the supernatant from the prepared MnO2-CNT solution, centrifuge and wash, dry and grind to obtain powdered MnO2-CNT composite material.

[0009] (3) Preparation method of MnO2-WO3-CNT composite material: The MnO2-CNT composite material powder is dispersed in tungsten hexachloride solution, stirred for 1-4 hours, centrifuged, washed and dried, the dried product is ground and calcined, and the MnO2-WO3-CNT composite material is obtained after calcination.

[0010] Furthermore, in step (1), the concentration of KMnO4 solution is 0.05-0.2 mol / L, and the amount of carbon nanotubes and KMnO4 solution used is 0.4 g / (100-300) mL.

[0011] Furthermore, in step (2), the washing solution is deionized water, and the washing is performed 2-4 times.

[0012] Furthermore, in step (2), the drying is carried out under vacuum at 50-70°C for 10-14 hours.

[0013] Furthermore, the solvent for the tungsten hexachloride solution in step (3) is an ethanol solution.

[0014] Furthermore, in step (3), the concentration of the tungsten hexachloride solution is 0.01-0.02 g / ml.

[0015] Furthermore, in step (3), the washing solution is anhydrous ethanol, and the washing is performed 2-4 times.

[0016] Furthermore, in step (3), the drying is carried out under vacuum at 50-70°C for 6-10 hours.

[0017] Further, the calcination step in step (3) is as follows: calcination is carried out in a tube furnace under air atmosphere, and the temperature is raised to 400-500℃ at a heating rate of 1-3℃ / min, and held at this temperature for 1-3 hours.

[0018] Secondly, the MnO2-WO3-CNT composite material prepared by the aforementioned method is used to treat chromium-containing wastewater.

[0019] The beneficial effects of this invention are as follows:

[0020] (1) The WO3, MnO2 and carbon nanotubes used in this invention are all environmentally friendly substances that will not cause serious pollution. Moreover, the preparation method of MnO2-WO3-CNT composite material is simple, low-cost and practically applicable.

[0021] (2) The MnO2-WO3-CNT composite material prepared by this invention and the capacitor deionization technology used can not only adsorb chromium in wastewater, but also realize the conversion of hexavalent chromium to trivalent chromium, which can more effectively solve the problem of metallic chromium pollution.

[0022] (3) In this invention, firstly, MnO2-CNT composite material is prepared by coating MnO2 onto the surface of CNTs using KMnO4 and Mn(Ac)2 as raw materials and employing a redox method. Then, MnO2-WO3-CNT composite material is prepared using MnO2-CNT and tungsten hexachloride as raw materials by impregnation method, which improves the specific capacitance and electron transfer ability of the material, thereby improving its ability to electro-adsorb and remove and convert hexavalent chromium.

[0023] (4) In this invention, the MnO2-WO3-CNT composite material is coated on the anode of the CDI device, and during the electro-adsorption process, it can adsorb hexavalent chromium anions CrO4. 2- Furthermore, during this process, since Mn in MnO2 is a variable-valence metal, it can simultaneously transfer electrons to CrO4 under energized conditions. 2- CrO4 2- In the reaction, hexavalent chromium can be reduced to trivalent chromium after receiving electrons from Mn, thus achieving the conversion of hexavalent chromium to trivalent chromium. WO3, a metal oxide with excellent electron transfer capabilities, can further facilitate the transfer of electrons from Mn to chromium during the reaction, allowing chromium to gain electrons and be reduced, thereby further improving the conversion rate of hexavalent chromium to trivalent chromium. Attached Figure Description

[0024] Figure 1 The image shows a scanning electron microscope image of the MnO2-WO3-CNT composite material prepared in Example 1.

[0025] Figure 2 The images show the treatment effect of the MnO2-WO3-CNT composite materials prepared in Examples 1, 2, and 3 on chromium-containing wastewater.

[0026] Figure 3 This is an experimental diagram illustrating the electro-adsorption method for treating chromium-containing wastewater in the examples. Detailed Implementation

[0027] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1

[0029] 1. Preparation of electrode materials

[0030] 1) Disperse 0.4 g of carbon nanotubes (CNTs) in 200 mL of 0.1 mol / L KMnO4 solution and stir with a magnetic stirrer at 600 rpm for 2 h to ensure thorough mixing; add 200 mL of 0.15 mol / L Mn(Ac)2 solution to the above solution and continue stirring with a magnetic stirrer for 12 h to ensure thorough mixing, thus obtaining MnO2 / CNT solution;

[0031] 2) Transfer the solution obtained in step 1) to a centrifuge tube, centrifuge at 8000 rpm for 5 min, remove the supernatant, add 30 mL of deionized water for washing, centrifuge for 5 min under the same conditions, and repeat the washing step twice.

[0032] 3) The MnO2 / CNT material obtained in step 2) was dried in a vacuum drying oven at 60℃ for 12 hours;

[0033] 4) After drying, grind the solid obtained in step 3) to obtain a black powdery MnO2 / CNT composite material.

[0034] 5) Using ethanol as a solvent, dissolve 2g of tungsten hexachloride in 200ml of ethanol to prepare a tungsten hexachloride-ethanol solution with a concentration of 0.01g / ml. Disperse 1g of MnO2 / CNT powder in 200ml of tungsten hexachloride-ethanol solution by ultrasonication, and then stir with a magnetic stirrer at room temperature for 2h.

[0035] 6) Transfer the solution obtained in step (5) to a centrifuge tube. Centrifuge at 8000 rpm for 5 min, discard the supernatant, wash the precipitate with anhydrous ethanol, centrifuge for 5 min under the same conditions, and repeat the washing step twice;

[0036] 7) Dry the product obtained in step (6) in a vacuum drying oven at 60°C for 8 hours;

[0037] 8) After grinding the product dried in step (7), calcine it in a tube furnace under air atmosphere, raise the temperature to 450°C at a heating rate of 2°C / min, and keep it at this temperature for 2 hours. After it cools naturally to room temperature, take it out to obtain the MnO2 / WO3 / CNT composite material.

[0038] 2. Electroadsorption method for treating chromium-containing wastewater

[0039] like Figure 3As shown, a MnO2 / WO3 / CNT composite material was coated onto a graphite carbon electrode, which served as the anode. Another graphite carbon electrode was used as the cathode, forming a capacitive adsorption device. A DC regulated power supply was used for power. 100 mL of 100 mg / L Na2CrO4 solution and 100 mL of 100 mg / L NaCl solution were added to the storage bottle. The capacitive adsorption device, DC regulated power supply, storage tube, and peristaltic pump were connected by tubing to achieve solution circulation. During the adsorption phase, the DC regulated power supply voltage was adjusted to 1.5V, and the total adsorption phase duration was 60 min. A 2 mL sample was taken every 10 min, with an initial sample taken at the beginning. During the desorption phase, the DC regulated power supply voltage was adjusted to -1.2V, and the total adsorption phase duration was 30 min. A 2 mL sample was taken every 5 min, with an initial sample taken at the beginning. Each 2 mL sample was diluted with deionized water to 10 mL to obtain the treated chromium-containing solution sample. The hexavalent chromium content of chromium-containing wastewater samples was determined and analyzed.

[0040] Example 2

[0041] The only difference between this embodiment and Embodiment 1 is that the voltage of the DC regulated power supply during the desorption stage is -2.5V. All other matters are the same as in Embodiment 1.

[0042] Example 3

[0043] The only difference between this embodiment and Embodiment 1 is that the voltage of the DC regulated power supply during the desorption stage is -4V; all other matters are the same as in Embodiment 1.

[0044] Table 1. Conversion rate of hexavalent chromium to trivalent chromium by MnO2-WO3-CNT composite material at different voltages during the desorption stage

[0045]

[0046] The prepared MnO2-WO3-CNT composite material was scanned by electron microscopy, and the results are as follows: Figure 1 As shown. From Figure 1 It can be seen that the combination of WO3, MnO2 and carbon nanotubes increases the effective surface area of ​​carbon nanotubes to a certain extent. The pores between carbon nanotubes are smaller and the surface is rougher, which is beneficial to improving the performance of the electrode material and thus further improving the treatment effect on chromium-containing wastewater.

[0047] Adsorption and conversion experiments of hexavalent chromium were conducted on Na₂CrO₄ solution by varying the voltage during the desorption stage as shown in Examples 1-3. The experimental results are shown in Table 1 and... Figure 2 As shown in Table 1 and Figure 2As can be seen, at -1.2V, the conversion rate of hexavalent chromium to trivalent chromium by the MnO2-WO3-CNT composite material is 10.8%; at -2.5V, the conversion rate is 21.7%; and at -4V, the conversion rate is 33.6%. The MnO2-WO3-CNT composite material prepared in this invention has the ability to convert hexavalent chromium to trivalent chromium, and has broad prospects for development in the treatment of chromium-containing wastewater.

[0048] In addition to the foregoing embodiments, the present invention also provides the following embodiments for comparison and illustration:

[0049] Example 4

[0050] Preparation of electrode materials

[0051] 1) Disperse 0.3 g of carbon nanotubes (CNTs) in 200 mL of 0.1 mol / L KMnO4 solution and stir with a magnetic stirrer at 500 rpm for 3 h to ensure thorough mixing; add 150 mL of 0.15 mol / L Mn(Ac)2 solution to the above solution and continue stirring with a magnetic stirrer for 12 h to ensure thorough mixing, thus obtaining MnO2 / CNT solution;

[0052] 2) Transfer the solution obtained in step 1) to a centrifuge tube, centrifuge at 7000 rpm for 5 min, remove the supernatant, add 30 mL of deionized water for washing, centrifuge for 5 min under the same conditions, and repeat the washing step twice.

[0053] 3) Dry the MnO2 / CNT material obtained in step 2) in a vacuum drying oven at 50℃ for 10 h;

[0054] 4) After drying, grind the solid obtained in step 3) to obtain a black powdery MnO2 / CNT composite material.

[0055] 5) Using ethanol as a solvent, dissolve 2g of tungsten hexachloride in 100ml of ethanol to prepare a tungsten hexachloride-ethanol solution with a concentration of 0.02g / ml. Disperse 1g of MnO2 / CNT powder in 200ml of tungsten hexachloride-ethanol solution by ultrasonication, and then stir with a magnetic stirrer at room temperature for 2h.

[0056] 6) Transfer the solution obtained in step (5) to a centrifuge tube. Centrifuge at 7000 rpm for 5 min, discard the supernatant, wash the precipitate with anhydrous ethanol, centrifuge for 5 min under the same conditions, and repeat the washing step twice;

[0057] 7) Dry the product obtained in step (6) in a vacuum drying oven at 50°C for 6 hours;

[0058] 8) After grinding the product dried in step (7), calcine it in a tube furnace under air atmosphere, raise the temperature to 400°C at a heating rate of 2°C / min, and keep it at this temperature for 2 hours. After it cools naturally to room temperature, take it out to obtain the MnO2 / WO3 / CNT composite material.

[0059] Example 5

[0060] Preparation of electrode materials

[0061] 1) Disperse 0.4 g of carbon nanotubes (CNTs) in 300 mL of 0.1 mol / L KMnO4 solution and stir with a magnetic stirrer at 700 rpm for 2 h to ensure thorough mixing; add 300 mL of 0.15 mol / L Mn(Ac)2 solution to the above solution and continue stirring with a magnetic stirrer for 10 h to ensure thorough mixing, thus obtaining MnO2 / CNT solution;

[0062] 2) Transfer the solution obtained in step 1) to a centrifuge tube, centrifuge at 9000 rpm for 5 min, remove the supernatant, add 30 mL of deionized water for washing, centrifuge for 5 min under the same conditions, and repeat the washing step twice.

[0063] 3) Dry the MnO2 / CNT material obtained in step 2) in a vacuum drying oven at 70℃ for 10 h;

[0064] 4) After drying, grind the solid obtained in step 3) to obtain a black powdery MnO2 / CNT composite material.

[0065] 5) Using ethanol as a solvent, dissolve 2g of tungsten hexachloride in 200ml of ethanol to prepare a tungsten hexachloride-ethanol solution with a concentration of 0.01g / ml. Disperse 2g of MnO2 / CNT powder in 200ml of tungsten hexachloride-ethanol solution by ultrasonication, and then stir with a magnetic stirrer at room temperature for 2h.

[0066] 6) Transfer the solution obtained in step (5) to a centrifuge tube. Centrifuge at 9000 rpm for 5 min, discard the supernatant, wash the precipitate with anhydrous ethanol, centrifuge for 5 min under the same conditions, and repeat the washing step twice;

[0067] 7) Dry the product obtained in step (6) in a vacuum drying oven at 50°C for 10 hours;

[0068] 8) After grinding the dried product from step (7), calcine it in a tube furnace under air atmosphere, raise the temperature to 450°C at a heating rate of 3°C / min, and keep it at this temperature for 2 hours. After it cools naturally to room temperature, take it out to obtain the MnO2 / WO3 / CNT composite material.

[0069] Example 6

[0070] Preparation of electrode materials

[0071] 1) Disperse 0.5 g of carbon nanotubes (CNTs) in 200 mL of 0.1 mol / L KMnO4 solution and stir with a magnetic stirrer at 600 rpm for 2 h to ensure thorough mixing; add 300 mL of 0.15 mol / L Mn(Ac)2 solution to the above solution and continue stirring with a magnetic stirrer for 12 h to ensure thorough mixing, thus obtaining MnO2 / CNT solution;

[0072] 2) Transfer the solution obtained in step 1) to a centrifuge tube, centrifuge at 9000 rpm for 5 min, remove the supernatant, add 30 mL of deionized water for washing, centrifuge for 5 min under the same conditions, and repeat the washing step twice.

[0073] 3) Dry the MnO2 / CNT material obtained in step 2) in a vacuum drying oven at 70℃ for 14 hours;

[0074] 4) After drying, grind the solid obtained in step 3) to obtain a black powdery MnO2 / CNT composite material.

[0075] 5) Using ethanol as a solvent, dissolve 2g of tungsten hexachloride in 200ml of ethanol to prepare a tungsten hexachloride-ethanol solution with a concentration of 0.01g / ml. Disperse 1g of MnO2 / CNT powder in 200ml of tungsten hexachloride-ethanol solution by ultrasonication, and then stir with a magnetic stirrer at room temperature for 2h.

[0076] 6) Transfer the solution obtained in step (5) to a centrifuge tube. Centrifuge at 9000 rpm for 5 min, discard the supernatant, wash the precipitate with anhydrous ethanol, centrifuge for 5 min under the same conditions, and repeat the washing step twice;

[0077] 7) Dry the product obtained in step (6) in a vacuum drying oven at 70°C for 10 hours;

[0078] 8) After grinding the product dried in step (7), calcine it in a tube furnace under air atmosphere, raise the temperature to 500°C at a heating rate of 2°C / min, and keep it at this temperature for 2 hours. After it cools naturally to room temperature, take it out to obtain the MnO2 / WO3 / CNT composite material.

[0079] Comparative Example 1

[0080] Unlike Example 1, the calcination temperature in step (8) of Comparative Example 1 was 300°C. The prepared composite material was used to treat chromium-containing wastewater by the electroadsorption method in Example 1. The voltages of the DC regulated power supply during the desorption stage were adjusted to -1.2V, -2.5V, and -4V, respectively. The final conversion rates of hexavalent chromium to trivalent chromium were 2.7%, 3.2%, and 3.8%, respectively.

[0081] Comparative Example 2

[0082] Unlike Example 1, the dried product in Comparative Example 2 was not calcined in step (8). The prepared composite material was used to treat chromium-containing wastewater using the electroadsorption method described in Example 1. The voltages of the DC regulated power supply during the desorption stage were adjusted to -1.2V, -2.5V, and -4V, respectively. The final conversion rates of hexavalent chromium to trivalent chromium were 1.8%, 2.2%, and 2.3%, respectively.

[0083] Comparative Example 3

[0084] Unlike Example 1, Comparative Example 3 prepared a black powdered MnO2 / CNT composite material. The prepared MnO2 / CNT composite material was used to treat chromium-containing wastewater using the electroadsorption method described in Example 1. The voltages of the DC regulated power supply during the desorption stage were adjusted to -1.2V, -2.5V, and -4V, respectively. The final conversion rates of hexavalent chromium to trivalent chromium were 1.1%, 1.6%, and 2.3%, respectively.

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention; those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention; and all technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. Use of MnO2-WO3-CNT composite material in treating wastewater containing chromium, characterized in that, The preparation steps of the MnO2-WO3-CNT composite material include: (1) Preparation of MnO2-CNT solution: carbon nanotubes are dispersed in KMnO4 solution, Mn(Ac)2 solution is added and mixed thoroughly to prepare MnO2-CNT solution; (2) Preparation method of powder-like MnO2-CNT composite material: after removing the supernatant of the prepared MnO2-CNT solution, centrifugation and washing are performed, and after drying and grinding, the powder-like MnO2-CNT composite material is obtained; (3) Preparation method of MnO2-WO3-CNT composite material: the MnO2-CNT composite material powder is dispersed in tungsten hexachloride solution, centrifugation, washing and drying are performed after stirring for 1-4 h, the dried product is ground and calcined, and after calcination, the MnO2-WO3-CNT composite material is obtained.

2. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (1), the concentration of KMnO4 solution is 0.05-0.2 mol / L, and the use amount of carbon nanotubes and KMnO4 solution is 0.4 g / (100-300) mL.

3. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (2), the washing liquid is deionized water, and the washing frequency is 2-4 times.

4. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (2), the drying is vacuum drying at 50-70℃, and the drying time is 10-14 h.

5. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (3), the solvent of tungsten hexachloride solution is ethanol solution.

6. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (3), the concentration of tungsten hexachloride solution is 0.01-0.02 g / mL.

7. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (3), the washing liquid is anhydrous ethanol, and the washing frequency is 2-4 times.

8. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (3), the drying is vacuum drying at 50-70℃, and the drying time is 6-10 h.

9. Use of the MnO2-WO3-CNT composite material according to claim 1 for treating wastewater containing chromium, characterized in that, In step (3), the calcination step is: calcination in a tube furnace under air atmosphere, heating at a rate of 1-3 ℃ / min to 400-500℃, and holding at this temperature for 1-3 h.

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