A method for separating and enriching hexavalent chromium by using capacitive deionization technology

Abstract

The application discloses a method for separating and enriching hexavalent chromium by using a capacitive deionization technology. The method comprises the following steps: mixing ZIF-8 derived porous carbon material with an extractant and a dispersant, oscillating and adsorbing, performing solid-liquid separation, washing with water, and drying to obtain ZIF-8 characteristic adsorption carbon material; uniformly mixing the ZIF-8 characteristic adsorption carbon material, a binder and an organic solvent, then coating on both sides of a current collector, and drying to obtain an electrode plate; parallelly arranging the electrode plate in a capacitive deionization module, connecting to positive and negative poles of a direct current power supply respectively, then passing a hexavalent chromium-containing solution into the capacitive deionization module, and circulating the hexavalent chromium-containing solution in the device. The capacitive deionization technology based on the ZIF-8 characteristic adsorption carbon material electrode has the characteristics of short process and simple operation, and the adsorption difference of different ions in the chromium-containing solution can achieve the purpose of separating and enriching hexavalent chromium and other impurity ions.

Description

Technical Field

[0001] This invention relates to the field of separation and removal technology of heavy metal chromium in water, and in particular to a method for separating and enriching hexavalent chromium using capacitive deionization technology. Background Technology

[0002] Water resources are essential for the survival and reproduction of plants, animals, and humans. With the development of industrial and agricultural modernization, heavy metal chromium often enters water bodies through mining, smelting, processing, and agricultural production, causing serious heavy metal pollution. Chromium cannot be decomposed and easily accumulates in organisms through the food chain, causing significant harm to animals and humans. The resulting food safety and human health risks have become major societal issues. Therefore, the treatment of chromium-polluted water bodies has been a hot research topic both domestically and internationally. Traditional chromium removal technologies often suffer from drawbacks such as complex operation, cumbersome processes, and poor treatment efficiency. Therefore, there is an urgent need to develop a new, simple, and effective method for removing heavy metal chromium.

[0003] Capacitive deionization (CDI) is an electro-adsorption desalination technology based on modern electrochemical double-layer capacitance. Compared with other technologies in the same field, CDI has advantages such as low energy consumption, simple equipment, convenient operation, long service life, and environmental friendliness. It is widely used in seawater desalination, desalination of industrial and agricultural water, and desalination of domestic water. However, capacitive deionization technology lacks application in the separation and enrichment of chromium ions. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned technical deficiencies and propose a method for separating and enriching hexavalent chromium using capacitive deionization technology, thereby solving the technical problems of cumbersome process and poor treatment effect in the removal of heavy metal chromium in the prior art.

[0005] A method for separating and enriching hexavalent chromium using capacitive deionization technology includes the following steps:

[0006] Preparation of ZIF-8 characteristic adsorption carbon material: ZIF-8 derived porous carbon material was mixed with extractant and dispersant and then subjected to oscillation adsorption. Subsequently, it was subjected to solid-liquid separation, water washing and drying to obtain ZIF-8 characteristic adsorption carbon material.

[0007] Preparation of electrode plate: ZIF-8 characteristic adsorption carbon material, binder and organic solvent are mixed evenly, and then the resulting mixed slurry is evenly coated on both sides of the current collector. After drying, the electrode plate is obtained.

[0008] Adsorption of hexavalent chromium in water: Several sets of electrode plates are placed in parallel in a capacitor deionization module. The parallel electrode plates are connected to the positive and negative terminals of a DC power supply. The hexavalent chromium-containing solution to be treated is then passed into the capacitor deionization module for treatment. The hexavalent chromium-containing solution is circulated in the device to adsorb hexavalent chromium in the water.

[0009] Compared with the prior art, the beneficial effects of the present invention include:

[0010] This invention utilizes capacitive deionization technology based on ZIF-8 characteristic adsorption carbon material electrodes to separate and enrich hexavalent chromium and other impurity ions in chromium-containing solutions by exploiting the adsorption differences of different ions. It features a short process and simple operation. The ZIF-8 derived porous carbon material, used as the electrode material in a CDI device, exhibits reversibility and double-layer properties, along with excellent electrochemical performance. Further loading with an extractant enhances the separation effect on hexavalent chromium and other impurity ions. Simultaneously, the voltage applied during the process is only 0.6–1.5V, and the adsorption process involves no redox reactions, resulting in low energy consumption. Furthermore, the adsorption process does not require the use of chemical reagents, avoiding secondary pollution of water bodies caused by the large-scale use of chemical reagents, making it highly environmentally friendly. Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0012] This invention provides a method for separating and enriching hexavalent chromium using capacitive deionization technology, comprising the following steps:

[0013] Preparation of S1 and ZIF-8 characteristic adsorption carbon materials: ZIF-8 derived porous carbon materials were mixed with extractant and dispersant and then subjected to oscillation adsorption. Subsequently, solid-liquid separation, water washing and drying were performed to obtain ZIF-8 characteristic adsorption carbon materials.

[0014] S2. Preparation of electrode plate: ZIF-8 characteristic adsorption carbon material, binder and organic solvent are mixed evenly, and then the resulting mixed slurry is evenly coated on both sides of the current collector. After drying, the electrode plate is obtained.

[0015] S3. Adsorption of hexavalent chromium in water: Several sets of electrode plates are placed in parallel in the capacitor deionization module. The parallel electrode plates are connected to the positive and negative terminals of a DC power supply. The hexavalent chromium-containing solution to be treated is then passed into the capacitor deionization module for treatment. The hexavalent chromium-containing solution is circulated in the device to adsorb hexavalent chromium in the water.

[0016] S4. Desorption of hexavalent chromium on the electrode plate: After adsorption, drain the solution in the capacitor deionization module, introduce dilute sulfuric acid to remove impurity ions, drain the dilute sulfuric acid again, introduce a back-extraction agent, then short-circuit or reverse the electrode plates connected to the positive and negative terminals of the DC power supply respectively, and circulate the back-extraction agent in the device. After the desorption process is completed, a chromium-rich solution is obtained.

[0017] In this embodiment, the ZIF-8-derived porous carbon material is prepared through the following steps in the preparation of the ZIF-8 characteristic adsorbent carbon material:

[0018] Preparation of S11 and ZIF-8 crystals: Zinc nitrate hexahydrate is mixed evenly with anhydrous methanol to obtain solution A; 2-methylimidazole is mixed evenly with anhydrous methanol to obtain solution B; solutions A and B are mixed evenly, and then the mixture is sequentially filtered, washed with anhydrous methanol, and dried to obtain ZIF-8 crystals; wherein, the molar ratio of zinc nitrate hexahydrate to 2-methylimidazole in the mixture is 1:1.5-2.3. In some specific embodiments of the present invention, in solution A, the solid-liquid ratio of zinc nitrate hexahydrate to anhydrous methanol is 1g:20-50mL; the solid-liquid ratio of 2-methylimidazole to anhydrous methanol is 1g:20-80mL.

[0019] Preparation of S12 and ZIF-8 derived porous carbon materials: ZIF-8 crystals were carbonized under the protection of an inert gas, and after cooling, they were sequentially acid-washed, water-washed, and dried to obtain ZIF-8 derived porous carbon materials; wherein, during the carbonization process, the inert gas was nitrogen, the heating rate was 5-10℃ / min, the carbonization temperature was 850-950℃, and the carbonization time was 2-4h; during the acid washing process, the acid used was a 3-8M HCl solution.

[0020] In this embodiment, in the preparation steps of the ZIF-8 characteristic adsorbent carbon material, the extractant is tetrabutylammonium chloride (TBAC) with a volume fraction of 10% to 75%; the dispersant is one of ethanol, n-hexane, chloroform, and methyl isobutyl ketone; the volume ratio of extractant to dispersant is 1:(0.5 to 2); the solid-liquid ratio of the ZIF-8 derived porous carbon material to the mixed solution of extractant and dispersant is 1g:5 to 20mL; and the shaking adsorption time is 2 to 12h.

[0021] In this embodiment, in the preparation step of the electrode plate, the binder is polyvinylidene fluoride; the amount of binder added accounts for 5% to 20% of the mass of the ZIF-8 characteristic adsorption carbon material; the organic solvent is at least one of dimethylacetamide, dimethylformamide, and ethanol; the solid-liquid ratio of the ZIF-8 derived porous carbon material to the organic solution is 1g:5 to 20mL; and the current collector is a high-purity graphite sheet or a coated titanium electrode plate.

[0022] In this embodiment, during the adsorption step of hexavalent chromium in water, the spacing between the electrode plates is 0.2–1.0 cm; the DC power supply voltage is 0.6–1.5 V; a peristaltic pump is used to control the flow rate of the solution between the parallel electrode plates to 10–50 mL / min; the pH of the hexavalent chromium-containing solution is 0.5–4, preferably 0.5–2; hexavalent chromium mainly exists as HCr₂O₇. - Cr2O7 2- CrO4 2- HCrO4 - It exists in the form of H2CrO4; the concentration of chromium (as an element) is 100-3000 mg / L, the concentration of iron (as an element) is 10-10000 mg / L, the concentration of aluminum (as an element) is 10-10000 mg / L, the concentration of copper (as an element) is 10-10000 mg / L, and the concentration of sulfur (as an element) is 10-1000 mg / L.

[0023] In this embodiment, during the desorption step of hexavalent chromium on the electrode plate, the concentration of dilute sulfuric acid is 0.2–0.5 M; the back-extraction agent is sodium hydroxide solution with a concentration of 0.3–5 M.

[0024] Example 1

[0025] (1) Preparation of ZIF-8 crystals: Zinc nitrate hexahydrate and anhydrous methanol were mixed evenly at a solid-liquid ratio (g / mL) of 1:30 to obtain solution A; 2-methylimidazole and anhydrous methanol were mixed evenly at a solid-liquid ratio (g / mL) of 1:50 to obtain solution B. Solutions A and B were mixed so that the molar ratio of zinc nitrate hexahydrate to 2-methylimidazole in the mixture was 1:2.0. After stirring evenly, the solid fraction was obtained by vacuum filtration, washed four times with anhydrous methanol, and dried in an oven to constant weight to obtain ZIF-8 crystals.

[0026] (2) Preparation of ZIF-8 derived porous carbon material: ZIF-8 crystals were heated to 950℃ and held for 2h under N2 atmosphere to carbonize ZIF-8 crystals, and then cooled to room temperature naturally to obtain carbonized products; the carbonized products were then immersed in 5M HCl solution to remove metals and metal oxides, washed with deionized water, and dried to constant weight to obtain ZIF-8 derived porous carbon material.

[0027] (3) Preparation of ZIF-8 characteristic adsorption carbon material: ZIF-8 derived porous carbon material was mixed with a 1:1 mixture of 60% tetrabutylammonium chloride (TBAC) and ethanol at a solid-liquid ratio of 1:8. The mixture was shaken for 4 hours, and after solid-liquid separation, the obtained solid was washed with deionized water and then dried to constant weight to obtain ZIF-8 characteristic adsorption carbon material.

[0028] (4) Preparation of electrode plate: Take ZIF-8 characteristic adsorbent carbon material, add 12% of the mass of ZIF-8 characteristic adsorbent carbon material binder polyvinylidene fluoride, and add dimethylacetamide according to the solid-liquid ratio (g / mL) of ZIF-8 characteristic adsorbent carbon material to organic solvent 1:8. Stir evenly to obtain slurry, and then coat the slurry evenly on both sides of the high-purity graphite sheet of the current collector. Then dry the current collector to constant weight to obtain electrode plate.

[0029] (5) Adsorption of hexavalent chromium in water: Five sets of electrode plates (each set consisting of positive and negative electrodes) were placed in parallel in the capacitor deionization module with a plate spacing of 0.6 cm. The parallel electrode plates were connected to the positive and negative terminals of a DC power supply, and the DC power supply voltage was set to 1.5 V. Then, 1500 mL of hexavalent chromium-containing solution (pH = 1.2) was introduced into the capacitor deionization module for treatment. A peristaltic pump was used to control the flow rate of the solution between the parallel electrode plates to 10 mL / min, and the solution was circulated in the device.

[0030] (6) Desorption of hexavalent chromium on the electrode plate: After the electrode plate reaches adsorption saturation, the solution in the capacitor deionization module is drained, and 0.5M sulfuric acid is introduced to remove impurity ions. After draining the dilute sulfuric acid, 3M sodium hydroxide solution is introduced. Then, the electrode plates connected to the positive and negative terminals of the DC power supply are short-circuited or reversed respectively. A peristaltic pump is used to achieve the effect of circulating the solution in the device. After the desorption treatment is completed, a chromium-rich solution is obtained.

[0031] The treatment of hexavalent chromium solution in this embodiment is shown in Table 1.

[0032] Table 1

[0033]

[0034] (In Table 1, the concentrations are expressed as elements)

[0035] Comparative Example 1

[0036] Compared with Example 1, the only difference is that activated carbon is used instead of ZIF-8 derived porous carbon material as the electrode material. The specific steps are as follows:

[0037] (1) Preparation of supported activated carbon: Activated carbon was mixed with a 1:1 mixture of 60% tetrabutylammonium chloride (TBAC) and ethanol at a solid-liquid ratio of 1:8. The mixture was shaken for 4 hours. After solid-liquid separation, the obtained solid was washed with deionized water and then dried to constant weight to obtain supported activated carbon.

[0038] (2) Preparation of electrode plate: Take the loaded activated carbon, add 12% of the binder polyvinylidene fluoride according to the mass of the loaded activated carbon, and then add dimethylacetamide according to the solid-liquid ratio (g / mL) of the loaded activated carbon to the organic solvent 1:8. Stir evenly to obtain a slurry, and then coat the slurry evenly on both sides of the high-purity graphite sheet of the current collector. Then dry the current collector to constant weight to obtain the electrode plate.

[0039] The steps for adsorption of hexavalent chromium in water and desorption of hexavalent chromium on the electrode plate are the same as in Example 1, and will not be repeated here.

[0040] The treatment results of the hexavalent chromium solution in this comparative example are shown in Table 2.

[0041] Table 2

[0042]

[0043] (In Table 2, the concentrations are expressed as elements)

[0044] As can be seen from Example 1 and Comparative Example 1, compared with ordinary porous activated carbon materials, the capacitive deionization technology based on ZIF-8 characteristic adsorption carbon material electrode of the present invention has a better enrichment and separation effect on hexavalent chromium ions.

[0045] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for separating and enriching hexavalent chromium using capacitive deionization technology, characterized in that, Includes the following steps: Preparation of ZIF-8 characteristic adsorption carbon material: ZIF-8 derived porous carbon material was mixed with extractant and dispersant and then subjected to oscillation adsorption. Subsequently, it was subjected to solid-liquid separation, water washing and drying to obtain ZIF-8 characteristic adsorption carbon material. Preparation of electrode plate: ZIF-8 characteristic adsorption carbon material, binder and organic solvent are mixed evenly, and then the resulting mixed slurry is evenly coated on both sides of the current collector. After drying, the electrode plate is obtained. Adsorption of hexavalent chromium in water: Several sets of electrode plates are placed in parallel in the capacitor deionization module. The parallel electrode plates are connected to the positive and negative terminals of a DC power supply. The hexavalent chromium-containing solution to be treated is then passed into the capacitor deionization module for treatment. The hexavalent chromium-containing solution is circulated in the device to adsorb hexavalent chromium in the water. Desorption of hexavalent chromium from the electrode plates: After adsorption, the solution in the capacitor deionization module is drained, and dilute sulfuric acid is introduced to remove impurity ions. After draining the dilute sulfuric acid, a back-extraction agent is introduced. Then, the electrode plates connected to the positive and negative terminals of the DC power supply are short-circuited or reversed, and the back-extraction agent is circulated in the device. After the desorption process is completed, a chromium-rich solution is obtained. In the preparation steps of the ZIF-8 characteristic adsorbent carbon material, the ZIF-8 derived porous carbon material is prepared through the following steps: zinc nitrate hexahydrate is mixed evenly with anhydrous methanol to obtain solution A; 2-methylimidazole is mixed evenly with anhydrous methanol to obtain solution B; solutions A and B are mixed evenly, and then the mixture is sequentially filtered, washed with anhydrous methanol, and dried to obtain ZIF-8 crystals; the ZIF-8 crystals are carbonized under the protection of an inert gas, cooled, and then sequentially acid-washed, water-washed, and dried to obtain the ZIF-8 derived porous carbon material; the extractant is tetrabutylammonium chloride, and the dispersant is one of ethanol, n-hexane, chloroform, and methyl isobutyl ketone; The hexavalent chromium solution has a pH of 0.5–2; the chromium concentration is 100–3000 mg / L, the iron impurity ion concentration is 10–10000 mg / L, the aluminum impurity ion concentration is 10–10000 mg / L, the copper impurity ion concentration is 10–10000 mg / L, and the sulfur impurity ion concentration is 10–1000 mg / L.

2. The method for separating and enriching hexavalent chromium using capacitive deionization technology according to claim 1, characterized in that, In the mixture, the molar ratio of zinc nitrate hexahydrate to 2-methylimidazole is 1:1.5-2.3; during the carbonization process, the inert gas is nitrogen, the heating rate is 5-10℃ / min, the carbonization temperature is 850-950℃, and the carbonization time is 2-4h; during the pickling process, the acid used is a 3-8 M HCl solution.

3. The method for separating and enriching hexavalent chromium using capacitive deionization technology according to claim 1, characterized in that, The volume fraction of the extractant is 10%–75%; the volume ratio of extractant to dispersant is 1:(0.5–2); the solid-liquid ratio of the ZIF-8 derived porous carbon material to the mixed solution of extractant and dispersant is 1g:5–20mL; and the shaking adsorption time is 2–12h.

4. The method for separating and enriching hexavalent chromium using capacitive deionization technology according to claim 1, characterized in that, In the preparation steps of the electrode plate, the binder is polyvinylidene fluoride; the amount of binder added accounts for 5% to 20% of the mass of ZIF-8 characteristic adsorption carbon material; the organic solvent is at least one of dimethylacetamide, dimethylformamide, and ethanol; the solid-liquid ratio of ZIF-8 derived porous carbon material to organic solution is 1g:5 to 20mL.

5. The method for separating and enriching hexavalent chromium using capacitive deionization technology according to claim 1, characterized in that, In the preparation step of the electrode plate, the current collector is a high-purity graphite sheet or a coated titanium electrode plate.

6. The method for separating and enriching hexavalent chromium using capacitive deionization technology according to claim 1, characterized in that, In the adsorption step of hexavalent chromium in the water, the spacing between the electrode plates is 0.2–1.0 cm; the DC power supply voltage is 0.6–1.5 V; and the flow rate of the solution between the parallel electrode plates is controlled at 10–50 mL / min.

7. The method for separating and enriching hexavalent chromium using capacitive deionization technology according to claim 1, characterized in that, In the desorption step of hexavalent chromium on the electrode plate, the concentration of dilute sulfuric acid is 0.2-0.5 M; the back-extraction agent is sodium hydroxide solution with a concentration of 0.3-5 M.

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