Modified biochar electrode for electro-adsorption and preparation thereof

Abstract

The invention relates to a modified charcoal electrode for electro-adsorption and a preparation method thereof, and the preparation method comprises the following steps: taking rice husk as a biomass raw material, preparing rice husk charcoal (RHAC) through potassium hydroxide activation and high-temperature pyrolysis, mixing with an adhesive, a conductive agent and an organic solvent, and coating on foamed nickel to prepare the rice husk charcoal electrode. Compared with rice husk charcoal (RHC) which is not activated by potassium hydroxide, the RHAC has higher specific surface area and pore volume and shows more excellent adsorption performance. An electro-adsorption test result shows that the removal rate of the modified biochar electrode on methyl orange dye is obviously higher than that of conventional adsorption under the dual action of an electric field and biochar. The preparation method of the modified biochar electrode provided by the invention is simple, simple and convenient to operate and low in cost; the modified biochar electrode provided by the invention has a remarkable electro-adsorption effect on azo dyes, is high in reusability and has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment, and in particular relates to a modified biochar electrode for electrical adsorption and a preparation method thereof. Background Art

[0002] With the development of society and the economy, people's demand for industrial production is increasing. At the same time, the amount of wastewater generated is also increasing. The large amount of organic pollutants in the wastewater indirectly endangers human health. Azo dyes are widely used in industries such as dyeing, printing, and papermaking. Taking methyl orange (MO) as an example, it has characteristics such as chemical stability, difficulty in degradation, and toxicity, which seriously endangers environmental safety. Therefore, treating azo dyes in water has become one of the indispensable technologies in industrial production. Common treatment methods for azo dyes include membrane separation, flocculation, precipitation, and adsorption.

[0003] Among them, biochar adsorption has the advantages of no secondary pollution, simple operation, and low equipment requirements. In particular, biochar materials are abundant, such as crop straw, sugarcane bagasse, and rice husks. After modification or activation, biochar has a higher specific surface area and more surface functional groups, thus adsorbing more pollutants. However, biochar has problems such as limited adsorption capacity, complex regeneration and processing, and a single adsorption technology cannot completely degrade and remove pollutants, and is limited by the adsorption-desorption equilibrium.

[0004] How to make biochar have greater adsorption capacity and utilization rate has become one of the problems in the industrial treatment of azo dyes. Summary of the Invention

[0005] To address the aforementioned shortcomings of biochar in adsorbing azo dyes, the present invention provides a method for preparing an electrode based on modified biochar, which electrosorbs methyl orange using an applied voltage. Rice husk charcoal (RHAC) is prepared by high-temperature pyrolysis using rice husks as the biomass raw material and potassium hydroxide as the activator. The prepared RHAC is then mixed with a binder, a conductive agent, and an organic solvent in a uniform ratio and then coated onto a porous nickel foam to form the electrode.

[0006] The specific technical solutions are as follows:

[0007] The first aspect of the present invention is to provide a method for preparing a modified biochar electrode, comprising:

[0008] Step 1: Wash the rice husks with deionized water and air dry them under ventilation. Then, dry them in a 60°C forced-air drying oven to a constant weight. Immerse the dried rice husks in a potassium hydroxide (KOH) solution and stir them continuously for 12 hours using a constant-temperature magnetic stirrer. After immersion, drain the filtrate and dry the rice husks in a 60°C forced-air drying oven to a constant weight.

[0009] Step 2: The rice husk treated with potassium hydroxide is placed in a tube furnace for pyrolysis. The pyrolysis product is washed with HCl and deionized water until neutral, and then dried in a constant temperature forced air drying oven at 60°C to constant weight, ground and sieved to obtain rice husk charcoal (RHAC).

[0010] Step 3: Pre-treat the nickel foam, rinse it with deionized water, and dry it in a 60°C constant-temperature forced air drying oven. Mix the prepared rice husk charcoal with a binder, a conductive agent, and an organic solvent in a specific proportion to create an electrode slurry. Apply the slurry evenly to one surface of the nickel foam, ensuring a smooth coating with no visible holes or accumulation. Keep the biochar powder loading within a certain range. Dry the coated nickel foam electrode to a constant weight to obtain the modified biochar electrode.

[0011] Specifically, the pyrolysis temperature was raised to 600° C. at a heating rate of 5° C. / min, and carbonization was carried out at this temperature for 2 hours, with nitrogen as the protective gas.

[0012] Specifically, the pyrolysis product is washed, dried, and then ground through a 60-mesh sieve.

[0013] Specifically, polyvinylidene fluoride (PVDF) is used as a binder and carbon black is used as a conductive agent. According to the mass ratio of RHAC, carbon black and PVDF of 8:1:1, the three are placed in an agate mortar, and an appropriate amount of N-methylpyrrolidone (NMP) organic solvent is added dropwise to grind the three evenly to obtain an electrode slurry.

[0014] Specifically, the mass loading of biochar powder is controlled to be about 50 mg.

[0015] The second aspect of the present invention is to provide a modified biochar electrode prepared according to the above method.

[0016] The third aspect of the present invention is to provide the use of the modified biochar electrode in electrical adsorption.

[0017] KOH activation significantly improved the physical and chemical properties of rice husk biochar. The specific surface area of ​​activated biochar increased from 34.34m 2 / g increased to 220.02m 2 / g, pore volume from 0.024cm 3 / g expanded to 0.156cm 3 / g, forming a hierarchical pore structure with micropores and mesopores. This structure provides abundant adsorption sites and efficient mass transfer channels for methyl orange molecules.

[0018] Cyclic voltammetry tests revealed that the modified biochar electrode exhibited significant double-layer capacitance, with a charge transfer resistance of only 12Ω, indicating rapid electron transport. Electrochemical impedance spectroscopy further confirmed the electrode's low internal resistance and high conductivity, providing a foundation for its efficient electrosorption.

[0019] From the structure of methyl orange molecule, we can see that the sulfonic acid group (-SO3H) in the molecule will be completely ionized, releasing hydrogen ions (H + ), forming a negatively charged sulfonate (-SO3 - The presence of this negative charge gives the methyl orange molecule a certain polarity, which can produce a stronger electrostatic interaction with the anode and enhance the electrosorption effect.

[0020] The present invention has the following beneficial effects:

[0021] (1) Compared with conventional adsorption, electrosorption has a higher removal rate of azo dyes under the dual action of electric field and biochar;

[0022] (2) The electrons provided by the electric field gradually oxidize methyl orange into substances with smaller molecular weight, and finally into CO2 and H2O, thus preventing methyl orange from polluting the environment again. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The removal rate of different MO initial concentrations during RHAC electrosorption for 300 min;

[0024] Figure 2 This is the removal rate diagram of different MO initial concentrations when RHAC conventional adsorption reaches equilibrium;

[0025] Figure 3 This is the removal rate of 20 mg / LMO during RHC electrosorption from 0 to 300 min;

[0026] Figure 4 are the pore size distribution diagrams of rice husk charcoal; among them, (a) is the pore size distribution diagram of RHC; (b) is the pore size distribution diagram of RHAC. DETAILED DESCRIPTION

[0027] The following will clearly and explicitly describe the technical solutions in the embodiments of the invention in conjunction with the accompanying drawings used in the implementation of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0029] Example 1

[0030] This embodiment provides a modified biochar electrode, the preparation method of which includes:

[0031] Step 1: 30g of rice husks were washed with deionized water and air-dried under ventilation. The husks were then dried in a 60°C constant-temperature forced-air drying oven to a constant weight. The dried husks were immersed in a 5% to 15% potassium hydroxide solution (168g by weight) and stirred continuously for 12 hours using a constant-temperature magnetic stirrer. After immersion, the filtrate was drained and the husks were dried in a 60°C constant-temperature forced-air drying oven to a constant weight.

[0032] Step 2: The rice husk treated with potassium hydroxide is placed in a tubular furnace, and high-purity nitrogen is introduced for displacement and exhaust for 30 minutes. The furnace temperature is raised to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, and carbonized at this temperature for 2 hours. After the furnace temperature is naturally cooled to room temperature, the pyrolysis product is taken out and washed alternately with HCl and deionized water. The use of HCl should not be excessive to avoid destroying the properties of the pyrolysis product. After washing to neutrality, it is placed in a constant temperature forced air drying oven at 60°C and dried to constant weight. It is then ground through a 60-mesh sieve to obtain rice husk charcoal (RHAC).

[0033] Step 3: Several 2cm×2cm pieces of nickel foam were sequentially soaked in anhydrous ethanol and 3mol / L HCl, while ultrasonically treating for 10 minutes. Finally, they were rinsed with deionized water and dried in a 60°C constant-temperature forced-air drying oven. Polyvinylidene fluoride (PVDF) was used as a binder, and carbon black was used as a conductive agent. The three components were placed in an agate mortar at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) organic solvent was added dropwise to grind the three components uniformly to obtain an electrode slurry. The electrode slurry was evenly coated on one side of the pretreated nickel foam using a spatula, ensuring a smooth coating with no obvious holes or accumulation. The biochar powder loading was controlled to approximately 50mg. The coated nickel foam electrode was dried to a constant weight to obtain a modified biochar electrode.

[0034] Electroadsorption test

[0035] The modified biochar electrode was used as the anode, the graphite sheet was used as the cathode, the distance between the electrodes was 1.5 cm, the electrolyte Na2SO4 concentration was 0.1 mol / L, the voltage was 1.5 V, and the total solution was 100 mL.

[0036] The test results are as follows Figure 1As shown in the figure, the initial concentrations of MO are 20 mg / L, 40 mg / L, 50 mg / L, and 100 mg / L, respectively. At 300 min, the removal rate of MO at 20 mg / L reaches 85.62%. The removal rate of MO increases with time and can reach 100% in a sufficiently long time.

[0037] Comparative Example 1

[0038] This comparative example provides a modified biochar, the preparation method of which includes:

[0039] Step 1: 30g of rice husks were washed with deionized water and air-dried under ventilation. The husks were then dried in a 60°C constant-temperature forced-air drying oven to a constant weight. The dried husks were immersed in a 5% to 15% potassium hydroxide solution (168g by weight) and stirred continuously for 12 hours using a constant-temperature magnetic stirrer. After immersion, the filtrate was drained and the husks were dried in a 60°C constant-temperature forced-air drying oven to a constant weight.

[0040] Step 2: Place the rice husk treated with potassium hydroxide into a tubular furnace, introduce high-purity nitrogen for 30 minutes of displacement and exhaust, raise the furnace temperature to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, and carbonize at this temperature for 2 hours; after the furnace temperature naturally cools to room temperature, take out the pyrolysis product and wash it alternately with HCl and deionized water. Do not use excessive HCl to avoid destroying the properties of the pyrolysis product. After washing to neutrality, place it in a 60°C constant temperature forced air drying oven to dry to constant weight, grind it through a 60-mesh sieve, and obtain rice husk charcoal (RHAC).

[0041] Conventional adsorption test

[0042] 100 mL of MO solution with different concentrations was placed in a conical flask, and 50 mg of RHAC was used as the adsorbent. After RHAC was added to the conical flask, the conical flask was immediately transferred to a constant temperature water bath shaker for shaking. Samples were taken at intervals until the adsorption reached equilibrium. The results are shown in Figure 2. Figure 2 As shown in the figure, the initial MO concentrations were 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 70 mg / L, and 100 mg / L, respectively. As the MO concentration increased, the removal rate gradually decreased. At the concentrations set in the experiment, when adsorption reached equilibrium, the maximum MO removal rate was 10.15% at 20 mg / L. Compared to conventional adsorption, electrosorption achieved the highest MO removal rate at 20 mg / L, reaching 85.62%, significantly higher than conventional adsorption, demonstrating the superiority of electrosorption over conventional adsorption.

[0043] Comparative Example 2

[0044] This comparative example provides an unmodified biochar electrode, the preparation method of which includes:

[0045] Step 1: Wash 30 g of rice husk with deionized water and dry it naturally in a ventilated environment. Then, dry it in a constant temperature forced air drying oven at 60°C to constant weight.

[0046] Step 2: The clean rice husk was placed in a tubular furnace for pyrolysis, and high-purity nitrogen was introduced for displacement and evacuation for 30 minutes. The furnace temperature was raised to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, and carbonized at this temperature for 2 hours. After the furnace temperature naturally cooled to room temperature, the husk was taken out and ground through a 60-mesh sieve to obtain unmodified rice husk charcoal (RHC).

[0047] Step 3: Several 2cm×2cm pieces of nickel foam were sequentially soaked in anhydrous ethanol and 3mol / L HCl, while ultrasonically treating for 10 minutes. Finally, they were rinsed with deionized water and dried in a 60°C constant-temperature forced-air drying oven. Polyvinylidene fluoride (PVDF) was used as a binder, and carbon black was used as a conductive agent. The three components were mixed in an agate mortar at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) organic solvent was added dropwise to grind the three components uniformly to obtain an electrode slurry. The electrode slurry was evenly coated on one side of the pretreated nickel foam using a spatula, ensuring a smooth coating with no obvious holes or accumulation. The biochar powder loading was controlled to approximately 50mg. The coated nickel foam electrode was dried to a constant weight to obtain an unmodified biochar electrode.

[0048] Electroadsorption test

[0049] The unmodified biochar electrode was used as the anode, the graphite sheet was used as the cathode, the distance between the electrodes was 1.5 cm, the electrolyte Na2SO4 concentration was 0.1 mol / L, the voltage was 1.5 V, and the total solution was 100 mL.

[0050] The test results are as follows Figure 3 As shown in Figure 3, at 300 min, the unmodified biochar electrode achieved a removal efficiency of 40.75% at an initial MO concentration of 20 mg / L, significantly lower than the 85.62% achieved by the RHAC electrode. The superior electrosorption performance of the RHAC electrode further demonstrates that the high surface area and high porosity of the KOH-activated RHAC facilitate electrosorption.

[0051] Specific surface area and pore volume analysis

[0052] The specific surface area and pore size of RHC and RHAC were analyzed and the results are shown in Table 1 and Figure 4 As shown in Table 1 and Figure 4As can be seen, RHAC has a larger specific surface area and pore volume than RHC. The pore size distribution of RHAC is primarily concentrated in micropores and mesopores, with the mesopores playing a key role in determining the adsorption rate. A comparison of specific surface area and pore size indicates that RHAC has superior adsorption performance to RHC. The high porosity, large specific surface area, and optimal pore structure of the electrode material not only provide abundant active sites but also shorten the mass transfer and diffusion pathways for solutes, resulting in faster adsorption rates and greater adsorption capacity during electrosorption.

[0053] Table 1 Specific surface area and pore volume of samples

[0054]

[0055] The present invention is described in accordance with the above embodiments, which demonstrates that the electrosorption effect of the RHAC electrode of the present invention is superior to conventional adsorption of RHAC and electrosorption of RHC electrodes, thus overcoming the limitation that a single adsorption technology cannot completely degrade and remove pollutants.

[0056] The present invention has been described based on the above embodiments. It should be understood that the above embodiments do not limit the present invention in any form. Any technical solutions obtained by equivalent replacement or equivalent transformation fall within the scope of the present invention.

Claims

1. A modified biochar electrode for electrosorption and a preparation method thereof, characterized in that: The following steps are involved: (1) impregnating rice husks with potassium hydroxide solution, draining the filtrate and drying; (2) The impregnated rice husks were placed in a tube furnace for high-temperature pyrolysis. After the pyrolysis was completed, the pyrolysis products were washed, dried, and sieved to obtain rice husk charcoal (RHAC); (3) Rice husk charcoal is mixed with a binder, a conductive agent and an organic solvent in a certain proportion to obtain an electrode slurry, which is then evenly coated on a single surface of the pretreated nickel foam.

2. A modified biochar electrode for electrosorption and a preparation method thereof according to claim 1, characterized in that: In the step (1), the mass fraction of the potassium hydroxide solution is 5% to 15%, the mass ratio of rice husk to potassium hydroxide is 1:5-6, and while soaking, a constant temperature magnetic stirrer is used to continuously stir for 12 hours.

3. The modified biochar electrode for electrosorption and the preparation method thereof according to claim 1, characterized in that: The drying condition in step (1) is constant temperature forced air drying at 60°C.

4. The modified biochar electrode for electrosorption and the preparation method thereof according to claim 1, characterized in that: In the step (2), the furnace temperature is raised to 600° C. at a heating rate of 5° C. / min, and carbonization is carried out at this temperature for 2 hours, with nitrogen as the protective gas.

5. A modified biochar electrode for electrosorption and a preparation method thereof according to claim 4, characterized in that: The pyrolysis product of step (2) is washed alternately with hydrochloric acid and deionized water until neutral, dried in a constant temperature forced air drying oven at 60° C., and ground through a 60-mesh sieve to obtain rice husk charcoal (RHAC).

6. The modified biochar electrode for electrosorption and the preparation method thereof according to claim 1, characterized in that: In step (3), polyvinylidene fluoride (PVDF) is used as a binder and carbon black is used as a conductive agent. According to the mass ratio of RHAC, carbon black and PVDF of 8:1:1, the three are placed in an agate mortar, and an appropriate amount of N-methylpyrrolidone (NMP) organic solvent is added dropwise to grind the three evenly to obtain an electrode slurry.

7. A modified biochar electrode for electrosorption and a preparation method thereof according to claim 6, characterized in that: Use a scraper to evenly coat the electrode slurry on one side of the pretreated nickel foam, controlling the mass loading of the biochar powder to about 50 mg; dry the coated nickel foam electrode to a constant weight.

8. A modified biochar electrode for electrosorption, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 7.

9. The modified biochar electrode for electrosorption according to claim 8, characterized in that: The modified biochar electrode is applied to the electrosorption of azo dyes.

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