Method for treating heavy metal wastewater by using zinc modified biochar electrode material

Abstract

The application discloses a method for treating heavy metal wastewater by using zinc modified biochar electrode material, which comprises the following steps: preparing zinc modified biochar electrode material by using zinc modified biochar, wherein the zinc modified biochar comprises porous biochar material, and zinc oxide is embedded in the porous biochar material; assembling a capacitive deionization device by using the zinc modified biochar electrode material as a working electrode; and performing electric adsorption treatment on the heavy metal wastewater by using the capacitive deionization device to complete the treatment of the heavy metal wastewater. In the application, when the capacitive deionization device with the zinc modified biochar electrode material as the working electrode is used to treat the heavy metal wastewater, the heavy metal in the water body can be quickly and completely adsorbed, so that the heavy metal in the wastewater can be effectively removed, and the method has the advantages of simple process, convenient operation, low cost, high treatment efficiency, good removal effect, green environmental protection and the like, and has high application value and wide application prospect in the treatment of heavy metal contaminated water body.

Description

Technical Field

[0001] This invention belongs to the field of capacitive deionization and electroadsorption, and relates to a method for treating heavy metal wastewater, specifically a method for treating heavy metal wastewater using zinc-modified biochar electrode material. Background Technology

[0002] During the mining and smelting of mineral resources, heavy metals, either in their elemental form or as metal ions, are transferred to the surrounding environment through surface water bodies, easily causing large-scale water and soil pollution. The hazards of heavy metals, especially those classified as Class I hazardous substances such as total lead and total cadmium, are particularly severe. Lead, as a typical heavy metal pollutant, cannot be eliminated through degradation but circulates in the environment in various ways. Once ingested, lead can cause varying degrees of damage to the digestive and nervous systems. For children, lead poisoning can lead to developmental delays, intellectual disabilities, and other illnesses. Cadmium irritates the respiratory tract, and a single ingestion of high concentrations can cause severe damage to internal organs, even death. Therefore, how to treat heavy metal pollution is currently one of the important research directions in the environmental field.

[0003] Currently, the main methods for treating heavy metals include adsorption, chemical precipitation, ion exchange, and electrolysis. Adsorption (e.g., CN115430698A) has a long reaction time, a complex pretreatment process, and relatively fixed implementation scenarios, making it difficult to promote in the market. Chemical precipitation (e.g., CN113142464A) is relatively mature and simple, but it requires the addition of large amounts of chemical reagents during the reaction, which easily generates solid waste and does not meet current green environmental protection requirements. Ion exchange (e.g., CN105271468A) is a relatively green and economical treatment method, but it is more suitable for single scenarios, and the ion exchange resin in the system needs to be replaced regularly. Furthermore, ion exchange has marginal utility, becoming increasingly difficult at lower concentrations. Electrolysis (e.g., CN206266366U) is a treatment method similar to capacitor deionization technology, which can recover heavy metals and has a convenient and simple device architecture, but it consumes a large amount of energy, making it difficult to maintain economic benefits. Therefore, it is essential to obtain a method for treating heavy metals that is energy-efficient, produces no secondary pollution, has good removal effect, and has a fast processing rate.

[0004] Capacitive deionization (CDI) technology, developed in the late 20th century, is a water treatment technology with advantages such as low energy consumption, no secondary pollution, recyclability, and fast treatment rate, making it promising for widespread applications. Theoretically, CDI can treat any electron-carrying pollutant; however, most research on CDI devices has been limited to seawater desalination, primarily focusing on the removal of alkali and valuable metals, with limited research on the removal of heavy metal ions. In recent years, biochar has attracted increasing attention due to its abundant raw material sources and unique economic and environmental advantages. However, existing biochars are mostly microporous, making it difficult for solutions to penetrate them, significantly reducing the effective specific surface area and resulting in low adsorption capacity, thus limiting their application in CDI. Furthermore, existing biochars also suffer from low effective capacitance, poor conductivity, and severe hydrophobicity. Therefore, when used as an active material in the preparation of CDI electrode materials, they easily generate significant mass transfer resistance within the electrode, not only reducing the electrode's conductivity but also limiting its adsorption capacity. Furthermore, although existing biomass-derived porous carbon materials with framework structures, which can be directly used as CDI electrode materials, can overcome the shortcomings of powdered or granular carbon materials in terms of high resistance to current and mass transfer, the CDI electrode materials prepared from them only possess electroadsorption capabilities and lack redox capabilities. Therefore, they cannot achieve the conversion of heavy metal ions. As a result, the adsorption capacity of these CDI electrode materials for heavy metal ions remains low, and the adsorption rate is also low, making it difficult to quickly and thoroughly remove heavy metal ions. This greatly limits the widespread application of biochar electrode materials in capacitive deionization technology. Therefore, obtaining a biochar electrode material with both electroadsorption and redox capabilities is of great significance for expanding the application of capacitive deionization technology in the treatment of heavy metal wastewater. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for treating heavy metal wastewater using zinc-modified biochar electrode materials that is simple in process, convenient in operation, low in cost, high in treatment efficiency, good in removal effect, and environmentally friendly.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0007] A method for treating heavy metal wastewater using zinc-modified biochar electrode material includes the following steps:

[0008] S1. Zinc-modified biochar is prepared into zinc-modified biochar electrode material; the zinc-modified biochar includes porous biochar material; zinc oxide is embedded in the porous biochar material;

[0009] S2. Using zinc-modified biochar electrode material as the working electrode, a capacitor deionization device is assembled.

[0010] S3. Use a capacitor deionization device to electro-adsorb heavy metal wastewater to complete the treatment of heavy metal wastewater.

[0011] In a further improvement to the above method, the zinc-modified biochar is prepared by calcining and acid washing using biomass as raw material and zinc chloride as modifier; the acid washing solution used in the acid washing process is a hydrochloric acid solution with a concentration of 6mol / L to 8mol / L; and the temperature of the hydrochloric acid solution is 60℃ to 80℃.

[0012] A further improvement to the above method, the preparation method of the zinc-modified biochar includes the following steps:

[0013] (a) Mix biomass material, zinc chloride, and water, stir, and dry to obtain an activated precursor;

[0014] (b) The activated precursor was calcined to obtain biochar material;

[0015] (c) The biochar material was acid-washed, cleaned, and dried using hydrochloric acid solution to obtain zinc-modified biochar.

[0016] In a further improvement to the above method, in step (a), the mass ratio of the biomass material to zinc chloride is 1:1 to 3; the amount of water added is 3 times the total mass of the biomass material and zinc chloride; before use, the biomass material is further subjected to the following steps: drying the biomass material at a temperature of 60℃ to 80℃ for 12h to 18h, pulverizing it, and passing it through a sieve with a mesh size greater than 200 to obtain biomass powder; the biomass material is at least one of walnut shells, straw, sawdust, fruit peel, and lignin; the stirring time is ≥2h; the drying is carried out at a temperature of 100℃ to 120℃; and the drying time is ≥12h.

[0017] In a further improvement to the above method, in step (b), the calcination is carried out under an inert atmosphere; the inert atmosphere is nitrogen, helium, or argon; the heating rate during the calcination process is 5℃ / min to 10℃ / min; the calcination temperature is 550℃ to 650℃; and the calcination time is 20min to 40min.

[0018] In a further improvement to the above method, in step (c), the pickling time is 2 to 3 hours; the cleaning is performed sequentially using anhydrous ethanol and water; the drying temperature is 60°C to 80°C, and the drying time is 12 to 18 hours.

[0019] A further improvement to the above method, in step S1, the preparation method of the zinc-modified biochar electrode material includes the following steps:

[0020] (1) Mix zinc-modified biochar, binder and solvent, stir to make electrode slurry;

[0021] (2) The electrode slurry is mixed with carbon felt, ultrasonicated, and dried to obtain a carbon felt electrode;

[0022] (3) The carbon felt electrode is attached to the conductive substrate to obtain the zinc-modified biochar electrode material.

[0023] In a further improvement to the above method, in step (1), the mass ratio of the zinc-modified biochar to the binder is 4 to 9:1; the binder is at least one of polyvinyl alcohol and polyvinylidene fluoride; the mass-volume ratio of the zinc-modified biochar to the solvent is 1 g: 10 mL to 100 mL; the solvent is N-methylpyrrolidone; and the stirring time is ≥4 h.

[0024] The above method is further improved in the following way: before the carbon felt is used, it is further treated as follows: the carbon felt is cut into sheet material with a length and width of 5cm×5cm, ultrasonically cleaned in ethanol for more than 2 hours, ultrasonically cleaned in water for more than 2 hours, and then dried.

[0025] In a further improvement to the above method, in step (3), a conductive carbon adhesive is used to attach the carbon felt electrode to a conductive substrate; the conductive substrate is one of titanium plate, copper foil, aluminum foil, graphite paper, and graphite plate; before use, the conductive substrate is further treated as follows: the conductive substrate is placed in an oxalic acid solution and heated to 85°C to 100°C, maintained for 1.5 hours to 3 hours, and then washed with water and ethanol in sequence, and dried; the mass fraction of the oxalic acid solution is 5% to 20%.

[0026] A further improvement to the above method is that, in step S3, the flow rate of the heavy metal wastewater in the capacitive deionizer is controlled to be 15 mL / min during the electro-adsorption treatment process. -1 ~20mL min -1 .

[0027] In a further improvement to the above method, in step S3, the residence time of the heavy metal wastewater in the capacitor deionization device is 1.5 hours to 2.5 hours.

[0028] In a further improvement to the above method, in step S3, the voltage of the capacitor deionization device is controlled to be 1V to 1.2V during the electro-adsorption treatment process.

[0029] In a further improvement to the above method, in step S3, the heavy metal in the heavy metal wastewater is at least one of lead and cadmium; and the initial concentration of the heavy metal in the heavy metal wastewater is ≤100 mg / L. -1 The initial pH value of the heavy metal wastewater is <7.5.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] (1) In view of the shortcomings of existing electrode materials used in capacitive deionization technology, such as small adsorption capacity, few active sites, only having desalination function, and difficulty in quickly and thoroughly adsorbing heavy metal ions, this invention creatively proposes a method for treating heavy metal wastewater using zinc-modified biochar electrode material. The zinc-modified biochar electrode material is made of zinc-modified biochar as the working electrode, assembled into a capacitive deionization device, and the capacitive deionization device is used to treat heavy metal wastewater by electro-adsorption. The zinc-modified biochar includes porous biochar material, in which zinc oxide is embedded. In this invention, the zinc-modified biochar possesses advantages such as a large specific surface area, abundant mesopores / structure, and numerous adsorption sites. This not only enhances the adsorption capacity of the zinc-modified biochar electrode material but also improves its adsorption rate, resulting in excellent electroadsorption performance. Simultaneously, the zinc oxide embedded in the porous biochar within the zinc-modified biochar electrode material exhibits excellent semiconductor properties, electrocatalytic activity, and redox capabilities, enabling the zinc-modified biochar electrode material to alter the valence or form of heavy metals. This further significantly enhances its ability to remove heavy metals. Therefore, when using a capacitive deionization device with zinc-modified biochar electrode material as the working electrode to treat heavy metal wastewater, it can rapidly and thoroughly adsorb heavy metals from the water, achieving effective removal of heavy metals from wastewater. This method offers advantages such as simple process, convenient operation, low cost, high treatment efficiency, good removal effect, and environmental friendliness, demonstrating high application value and broad application prospects in the treatment of heavy metal-polluted water bodies.

[0032] (2) In this invention, the zinc-modified biochar is prepared by calcination and acid washing using biomass as raw material and zinc chloride as modifier. The acid washing solution used in the acid washing process is a hydrochloric acid solution with a concentration of 6 mol / L to 8 mol / L and a temperature of 75°C. Specifically, the biomass material, zinc chloride, and water are first mixed, stirred, and dried to obtain an activated precursor. The activated precursor is then calcined to obtain biochar material. Finally, the biochar material is acid washed, cleaned, and dried using hydrochloric acid solution to obtain zinc-modified biochar. In this invention, using zinc chloride as a modifier has the following advantages: (a) During the mixing process with biomass, zinc chloride will swell the biomass components, promoting the depolymerization reaction. (b) During the pyrolysis process, i.e. the carbonization reaction of biomass to biochar, zinc chloride always exists in liquid form and is called molten salt. Therefore, it can produce a strong dehydration effect, which can be used to lower the pyrolysis reaction temperature of different components in biomass and change the pyrolysis reaction path of biomass, effectively inhibiting tar formation. This can promote the formation of open pore structures and form porous biochar with different pore size distributions. It can also effectively regulate the characteristics of modified biochar, giving it a high specific surface area and a large number of mesoporous structures. This is beneficial to improving the adsorption performance of zinc-modified biochar for heavy metal ions. At the same time, during the carbonization reaction of biomass, levoglucone is formed through the dehydration effect of zinc chloride. Then, after high-temperature dehydration, decarbonylation, aromatization and intramolecular condensation, biochar with many heavy metal adsorption sites is finally formed. Based on this, the biochar is cleaned with a hydrochloric acid solution at a concentration of 6 mol / L to 8 mol / L and a temperature of 75°C. This not only effectively removes impurities such as tar and molten salts generated during calcination, thus facilitating the unblocking of mesopores in the biochar, but also utilizes the oxidizing properties of the heated hydrochloric acid solution to further enhance the hydrophilicity of the biochar, thereby improving its adsorption performance. More importantly, by optimizing the concentration of hydrochloric acid and the washing time, it is ensured that the zinc oxide embedded inside the biochar is not dissolved, thus ensuring that the zinc-modified biochar possesses redox capabilities. This is a crucial guarantee for the efficient adsorption of heavy metals by zinc-modified biochar. The resulting zinc-modified biochar has a large specific surface area, abundant mesopores / structures, multiple adsorption sites, and possesses both electroadsorption and redox functions. Furthermore, the method for preparing zinc-modified biochar in this invention has advantages such as simple process, convenient operation, wide availability of raw materials, simple preparation conditions, low production cost, high economic benefits, short production cycle, and environmental friendliness, making it suitable for large-scale preparation and industrial application.

[0033] (3) In the present invention, the method for preparing zinc-modified biochar optimizes the biochar properties by setting the mass ratio of biomass material to zinc chloride to 1:1 to 3, the calcination temperature to 550℃ to 650℃, and the calcination time to 20 min to 40 min. This optimizes the biochar properties, giving it a higher specific surface area and a greater number of mesoporous structures. This results in zinc-modified biochar with better adsorption performance for heavy metal ions. This is because if the amount of zinc chloride is too low, the regulatory effect of zinc chloride on biomass will be poor, and it will be difficult to activate the biochar, resulting in a lower degree of activation of the biochar. This will result in a smaller specific surface area and fewer mesoporous structures in the biochar. If the amount of zinc chloride is too high, the molten salt formed by the excessive zinc chloride will easily block the pores inside the biochar, which will also reduce the specific surface area of ​​the biochar and reduce the number of mesoporous structures. It will also easily lead to waste of reagents and a lower yield. If the calcination temperature is too low, the biomass raw material may not carbonize properly, the activator will not be able to perform its pore-forming and modification effects, and raw materials will be wasted. If the calcination temperature is too high, the carbon skeleton of the biochar itself may collapse, and the carbon elements contained in the biomass may graphitize, altering the properties of the biochar. At the same time, both excessively high and low temperatures will lead to a decrease in the quality of the finished biochar product. If the calcination time is too short, the biomass may not be fully calcined, resulting in a low degree of carbonization and insufficient reaction between zinc chloride and biomass. If the calcination time is too long, on the one hand, the high-temperature reaction will not continue, and on the other hand, the calcination process requires a large amount of inert gas and energy, which will also result in unnecessary waste and a significant increase in production costs. At the same time, prolonged calcination can also cause the biochar skeleton to collapse, thereby reducing the specific surface area, porosity, and stability of the biochar.

[0034] (4) In this invention, conductive carbon adhesive is used to attach carbon felt electrodes to a conductive substrate, which is not only simple to operate, but also can obtain electrode materials with more stable structure.

[0035] (5) In this invention, the flow rate of heavy metal wastewater in the capacitive deionization device during the electro-adsorption treatment process is optimized to 15 mL / min. -1 ~20mL min -1The residence time of heavy metal wastewater in the capacitor deionization device is 1.5 to 2.5 hours. During the electroadsorption treatment, the voltage of the capacitor deionization device is controlled at 1V to 1.2V, which is more conducive to the rapid and thorough adsorption and removal of heavy metals in the wastewater. This is because if the flow rate of heavy metal wastewater is too low, it will reduce the time and efficiency of pollutant removal, while if the flow rate is too high, the force generated by the water flow will wash away the metal ions, preventing them from adhering to the electrode material. If the residence time is too short, the metal pollutants cannot be completely removed, while if the residence time is too long, it will waste electricity. If the voltage is too low, it will not be able to form a sufficient electric field force to migrate the metal ions, while if the voltage is too high, it will cause polarization of the electrode material and water electrolysis reaction, thus failing to remove the metal ions. Attached Figure Description

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0037] Figure 1 This is a SEM image of the zinc-modified biochar prepared in Example 1 of this invention.

[0038] Figure 2 This is an EDS O elemental distribution diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0039] Figure 3 This is an EDS Zn elemental distribution diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0040] Figure 4 This is an XPS image of the zinc-modified biochar prepared in Example 1 of this invention.

[0041] Figure 5 This is the FITR diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0042] Figure 6 This is the nitrogen adsorption-desorption isotherm diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0043] Figure 7 This is a pore size distribution diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0044] Figure 8 The image shows the CV diagram of the zinc-modified biochar electrode material prepared in Example 1 of this invention.

[0045] Figure 9 This is a diagram showing the treatment effect of zinc-modified biochar electrode material in Example 1 of the present invention for treating lead wastewater with a concentration of 100 mg / L.

[0046] Figure 10 This is a diagram showing the treatment effect of using zinc-modified biochar electrode material to treat cadmium wastewater with a concentration of 100 mg / L in Example 1 of the present invention.

[0047] Figure 11 This is an XPS image of the zinc-modified biochar electrode material used in Example 1 of the present invention after treating heavy metal lead wastewater. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. All materials and instruments used in the following embodiments are commercially available.

[0049] Example 1:

[0050] A method for treating heavy metal wastewater using zinc-modified biochar electrode material includes the following steps:

[0051] (1) Preparation of zinc-modified biochar

[0052] (1.1) Dry the walnut shells at 80℃ for 12 hours, grind them into powder using a grinder, and pass them through a sieve with a mesh size greater than 200 to obtain walnut shell powder.

[0053] (1.2) According to the mass ratio of biomass material to zinc chloride of 1:1, and the amount of water added is 3 times the total mass of biomass material and zinc chloride, weigh walnut shell powder (e.g., weigh 1g of walnut powder) and mix it with zinc chloride, add deionized water (e.g., add 3mL of deionized water), stir for 2h (stirring for 2h to 10h is acceptable, but it should not be too short or too long. If the time is too short, the zinc chloride modifier will not be able to react and mix fully with the biochar, thus failing to form a precursor with uniform texture. If the time is too long, the mixture will easily solidify and form a caking, thus failing to carry out the next step of pyrolysis modification), so that the walnut shell powder and zinc chloride are fully mixed and heat is released. After the stirring is completed, transfer the mixture to a 105℃ oven and place it for 12h to obtain the activated precursor.

[0054] (1.3) The activated precursor was transferred to a tube furnace and heated to 600°C at a heating rate of 10°C / min under a nitrogen atmosphere and held for 30 min to obtain biochar material.

[0055] (1.4) The biochar material was placed in a water bath and the concentration of hydrochloric acid solution was controlled at 8 mol / L and the temperature at 75℃. The biochar material was cleaned for 2 hours to remove impurities such as tar and molten salt from the biochar, while retaining the zinc oxide (ZnO) embedded in the biochar. The biochar material was washed repeatedly with pure water and ethanol three times in sequence and dried in an oven at 60℃ to obtain zinc-modified biochar.

[0056] The zinc-modified biochar prepared in this step includes porous biochar material, and zinc oxide is embedded in the porous biochar material.

[0057] Figure 1 This is a SEM image of the zinc-modified biochar prepared in Example 1 of this invention. Figure 1 It can be seen that zinc-modified biochar exhibits a relatively regular and smooth surface crystal structure after modification, and generates more mesoporous channels in the carbon framework that are conducive to electron transfer and metal adhesion. The characteristics reflected by SEM surface features indicate that the role of zinc chloride in biochar modification is not merely that of a pore-forming agent as previously mentioned, but also involves a certain degree of electrocatalytic effect.

[0058] Figure 2 This is an EDS O elemental distribution diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0059] Figure 3 This is an EDS Zn elemental distribution diagram of the zinc-modified biochar prepared in Example 1 of the present invention.

[0060] Depend on Figure 2 and Figure 3 It can be seen that the zinc-modified biochar prepared by this invention contains Zn elements, and the distribution of O elements highly overlaps with that of Zn elements, indicating that after acid washing, some Zn is still embedded in the biochar in the form of oxides.

[0061] Figure 4 This is an XPS image of the zinc-modified biochar prepared in Example 1 of this invention. Figure 4 It can be seen that the peak binding energy of Zn element in the zinc-modified biochar prepared by the present invention is consistent with the binding energy of ZnO, indicating that Zn exists in the biochar in the state of ZnO.

[0062] Figure 5 This is the FITR spectrum of the zinc-modified biochar prepared in Example 1 of this invention. Figure 5 It is evident that zinc-modified biochar exhibits significant modification in its surface functional groups. The ether peaks, occurring in the 1000-1200 nm wavelength range, represent one of the effective functional groups for adsorbing metals. Simultaneously, the redox properties of zinc-modified biochar originate from the π-π bonds in the quinone and aromatic ring structures at wavelength 1600 nm. Carbonyl groups in the same wavelength range can undergo nucleophilic reduction reactions and can also be used to remove heavy metals. Furthermore, the carboxylic acid groups at wavelength 3400 nm increase the hydrophilicity of the biochar, further enhancing its ability to remove heavy metals. Additionally, the carboxyl peak at the same position allows hydrogen (free radicals) to be released and become negatively charged, thereby strengthening the capacitive deionization effect.

[0063] Figure 6This is the nitrogen adsorption-desorption isotherm diagram of the zinc-modified biochar prepared in Example 1 of this invention. Figure 6 It can be seen that this isotherm belongs to the BET adsorption isotherm, meaning that pressure and adsorption capacity are basically linearly distributed. Further analysis reveals that the adsorption capacity increases sharply in the pressure range of 0.4-0.8, indicating that the biochar in this patent is mainly mesoporous, which is more conducive to ion transport and solution flow pathways. In addition, the highest adsorption capacity of this sample exceeds 1200 cm⁻¹. 3 The result of g indicates that the zinc-modified biochar prepared in this invention has excellent adsorption properties.

[0064] Figure 7 This is a pore size distribution diagram of the zinc-modified biochar prepared in Example 1 of this invention. Figure 7 It can be seen that the maximum pore volume of the zinc-modified biochar prepared by this invention exceeds 1.75 cm³. 3 The sample exhibits a high porosity (g) and a wide pore size distribution range of 0-150 nm, indicating a higher pore density. Furthermore, due to... Figure 7 It can be seen that the zinc-modified biochar prepared by the present invention has a relatively uniform pore size distribution and a relatively symmetrical curve, mainly consisting of mesopores (2-50nm).

[0065] (2) Zinc-modified biochar is used to make zinc-modified biochar electrode materials.

[0066] (2.1) Weigh 2.7g of zinc-modified biochar and 0.9g of polyvinyl alcohol, mix them evenly, add 30mL of N-methylpyrrolidone (NMP), stir for 4h to form a paste slurry, and obtain the electrode slurry.

[0067] (2.2) The electrode slurry was mixed with carbon felt (purchased from Tianjin Carbon Plant, high-purity graphite fiber felt, 20cm x 30cm in length and width), and ultrasonicated for 6 hours to ensure thorough mixing of the slurry and carbon felt. The mixture was then dried in an oven at 80℃ for 12 hours to obtain the carbon felt electrode. Before use, the carbon felt underwent the following treatment: it was cut into sheets 5cm x 5cm in length and width, ultrasonically cleaned in ethanol for 2 hours, then ultrasonically cleaned in water for 2 hours, and finally dried in an oven at 80℃.

[0068] (2.3) The carbon felt electrode was attached to the titanium plate (excluding the titanium plate) using conductive carbon adhesive (SEM double-sided carbon conductive tape purchased from Shunsheng Electronics Technology Co., Ltd., size 5mm*20m; other similar conductive double-sided carbon adhesives used for SEM and EDS experiments are also applicable) to obtain the zinc-modified biochar electrode material. Before use, the titanium plate underwent the following treatment: a 5cm×5cm titanium plate was placed in a 10% oxalic acid solution (an oxalic acid solution with a mass fraction of 5%–20% can be used, but excessively high concentrations may generate vapors that damage the respiratory tract) and heated to 100°C, maintaining a boiling state for 2 hours to etch the titanium plate. It was then cleaned sequentially with water and ethanol to remove residual impurities and dried.

[0069] Figure 8 This is a CV diagram of the zinc-modified biochar electrode material prepared in Example 1 of this invention. Figure 8 It can be seen that the CV curve of the zinc-modified biochar electrode material prepared in this invention presents a triangular shape, the closed image is not symmetrical, and the redox peak is prominent. This indicates that the electrochemical reaction is irreversible, and a change in the state of matter must have occurred during the reaction.

[0070] (3) A capacitor deionization device is assembled using zinc-modified biochar electrode material as the working electrode.

[0071] (4) Using a capacitor deionization device, the heavy metal lead wastewater (the wastewater has a pH of 7 and a concentration of 100 mg / L) was treated separately. -1 ) and cadmium-containing wastewater (the wastewater has a pH of 7 and a concentration of 100 mg / L) -1 Electroadsorption treatment is performed, specifically as follows:

[0072] (4.1) The heavy metal wastewater to be treated is fed into the capacitor deionization device. Before energizing, the flow rate is 15 mL / min. -1 First, circulate the solution for 30 minutes to eliminate the influence of physical adsorption on the deionization performance of the capacitor.

[0073] (4.2) Connect the power supply and provide a constant voltage. Under the condition of a voltage of 1.2V, use the capacitor deionization device to perform electro-adsorption treatment on the heavy metal wastewater. The metal ions in the solution flowing through the capacitor deionization device are transferred to the electrode with the opposite charge under the action of the electric field force. The treated solution flows back to the water tank. This cycle is repeated. After 2 hours of power supply, disconnect the power supply to complete the treatment of heavy metal wastewater.

[0074] In this embodiment, the treatment effect of zinc-modified biochar electrode material on heavy metal lead wastewater and heavy metal cadmium wastewater with a concentration of 50 mg / L was also investigated, with other conditions being the same.

[0075] Control group: Electrode materials were prepared using unmodified biochar instead of zinc-modified biochar to treat lead and cadmium wastewater with a concentration of 100 mg / L, under the same conditions. The preparation method of unmodified biochar was essentially the same as that of zinc-modified biochar, except that zinc chloride was not added in the preparation of unmodified biochar.

[0076] During the electroadsorption treatment process, samples were taken under different treatment time conditions to detect the concentration of heavy metals in the wastewater, and the removal rate of heavy metals by different electrode materials was calculated. The results are as follows: Figure 9 , Figure 10 As shown.

[0077] Figure 9 This image shows the treatment effect of using zinc-modified biochar electrode material to treat lead wastewater with a concentration of 100 mg / L in Example 1 of this invention. Figure 9 It can be seen that zinc-modified biochar has a very good removal effect on lead ions in capacitive deionization. After reaching adsorption equilibrium through physical adsorption for 30 minutes, the capacitive deionization device is started to operate. It can be seen that after the power is turned on, the lead ion removal rate increases exponentially, and finally reaches adsorption equilibrium in about 90 minutes. The results of ICP metal ion concentration test on the sample solution extracted at different time periods show that the removal efficiency of lead ions using zinc-modified biochar exceeds 93%, and the effluent concentration meets the national wastewater discharge standard (<1mg / L) (GB8978-1996).

[0078] Figure 10 This image shows the treatment effect of using zinc-modified biochar electrode material to treat cadmium wastewater with a concentration of 100 mg / L in Example 1 of this invention. Figure 10 It can be seen that zinc-modified biochar has a very good removal effect on heavy metal cadmium ions in capacitive deionization. After reaching adsorption equilibrium through physical adsorption for 30 minutes, the capacitive deionization device is powered on. It can be seen that after power-on operation, the removal rate of lead ions increases exponentially, and finally reaches adsorption equilibrium at about 60 minutes. The results of ICP metal ion concentration test by extracting sample solutions at different time periods show that the removal efficiency of cadmium ions using zinc-modified biochar exceeds 70%, achieving excellent results.

[0079] After the electroadsorption treatment was completed, the adsorption amount of heavy metals by different electrode materials was measured, and the results are shown in Table 1.

[0080] Table 1 Adsorption capacity of different electrode materials for heavy metal ions

[0081]

[0082] Adsorption capacity is one of the most important indicators of a material's adsorption performance. The adsorption capacity can be determined by comparing the ratio of pollutant removal to biochar usage. Experimental tests and calculations, as shown in Table 1, reveal that at low concentrations (50 mg / L), due to the marginal effect, the lower the target pollutant concentration, the worse the effect. Therefore, using unmodified biochar as an electrode material cannot achieve the desired effect. However, zinc-modified biochar overcomes similar shortcomings through the introduction of functional groups and its redox properties. Similarly, at high concentrations, zinc-modified biochar significantly outperforms unmodified biochar in adsorption capacity, indicating that zinc chloride is a highly effective modifier.

[0083] Figure 11 This is an XPS image of the zinc-modified biochar electrode material used in Example 1 of this invention after treating lead-containing wastewater. Figure 11 It can be seen that Pb was generated after treatment with zinc-modified biochar electrode material. 4+ The substance, namely Pb in the wastewater 2+ The valence state changed, and it transformed into Pb. 4+ , combined Figure 8 The results show that the zinc-modified biochar electrode material used in this invention can induce redox reactions.

[0084] In summary, the method for treating heavy metal wastewater using zinc-modified biochar electrode material can quickly and thoroughly adsorb heavy metals in water, thereby achieving effective removal of heavy metals from wastewater. It has advantages such as simple process, convenient operation, low cost, high treatment efficiency, good removal effect, and green environmental protection. It has high application value and broad application prospects in the treatment of heavy metal polluted water bodies.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for treating heavy metal wastewater using zinc-modified biochar electrode material, characterized in that, Includes the following steps: S1. Zinc-modified biochar is prepared into zinc-modified biochar electrode material; the zinc-modified biochar includes porous biochar material; zinc oxide is embedded in the porous biochar material; the zinc-modified biochar is obtained by calcination and acid washing using biomass as raw material and zinc chloride as modifier; the acid washing solution used in the acid washing process is a hydrochloric acid solution with a concentration of 6mol / L to 8mol / L; the temperature of the hydrochloric acid solution is 60℃ to 80℃; S2. Using zinc-modified biochar electrode material as the working electrode, a capacitor deionization device is assembled. S3. Use a capacitor deionization device to electro-adsorb heavy metal wastewater to complete the treatment of heavy metal wastewater.

2. The method according to claim 1, characterized in that, The method for preparing the zinc-modified biochar includes the following steps: (a) Mix biomass material, zinc chloride, and water, stir, and dry to obtain an activated precursor; (b) The activated precursor was calcined to obtain biochar material; (c) The biochar material was acid-washed, cleaned and dried with hydrochloric acid solution to obtain zinc-modified biochar.

3. The method according to claim 2, characterized in that, In step (a), the mass ratio of the biomass material to zinc chloride is 1:1 to 3; the amount of water added is 3 times the total mass of the biomass material and zinc chloride; before use, the biomass material further includes the following steps: drying the biomass material at a temperature of 60℃ to 80℃ for 12h to 18h, pulverizing it, and passing it through a sieve with a mesh size greater than 200 to obtain biomass powder; the biomass material is at least one of walnut shells, straw, sawdust, fruit peel, and lignin; the stirring time is ≥2h; the drying is carried out at a temperature of 100℃ to 120℃; the drying time is ≥12h. In (b), the calcination is carried out under an inert atmosphere; the inert atmosphere is nitrogen, helium or argon; the heating rate during the calcination process is 5℃ / min to 10℃ / min; the calcination temperature is 550℃ to 650℃; and the calcination time is 20min to 40min. In step (c), the pickling time is 2h to 3h; the cleaning is performed by sequentially using anhydrous ethanol and water; the drying temperature is 60℃ to 80℃, and the drying time is 12h to 18h.

4. The method according to any one of claims 1 to 3, characterized in that, In step S1, the preparation method of the zinc-modified biochar electrode material includes the following steps: (1) Mix zinc-modified biochar, binder and solvent, stir to make electrode slurry; (2) The electrode slurry is mixed with carbon felt, ultrasonicated, and dried to obtain a carbon felt electrode; (3) The carbon felt electrode is attached to the conductive substrate to obtain zinc-modified biochar electrode material.

5. The method according to claim 4, characterized in that, In (1), the mass ratio of zinc-modified biochar to binder is 4-9:1; the binder is at least one of polyvinyl alcohol and polyvinylidene fluoride; the mass-volume ratio of zinc-modified biochar to solvent is 1g:10mL-100mL; the solvent is N-methylpyrrolidone; and the stirring time is ≥4h. In the (2) above, the carbon felt is further subjected to the following treatment before use: the carbon felt is cut into sheet material with a length and width of 5cm×5cm, ultrasonically cleaned in ethanol for more than 2 hours, ultrasonically cleaned in water for more than 2 hours, and then dried. In step (3), a conductive carbon adhesive is used to attach the carbon felt electrode to the conductive substrate; the conductive substrate is one of titanium plate, copper foil, aluminum foil, graphite paper, and graphite plate; before use, the conductive substrate is further treated as follows: the conductive substrate is placed in an oxalic acid solution and heated to 85°C~100°C, kept for 1.5 hours to 3 hours, and then washed with water and ethanol in sequence, and dried; the mass fraction of the oxalic acid solution is 5%~20%.

6. The method according to any one of claims 1 to 3, characterized in that, In step S3, the flow rate of the heavy metal wastewater in the capacitive deionization device is controlled at 15 mL / min during the electro-adsorption treatment process. -1 ~20 mL min -1 .

7. The method according to claim 6, characterized in that, In step S3, the residence time of the heavy metal wastewater in the capacitor deionization device is 1.5 hours to 2.5 hours.

8. The method according to claim 6, characterized in that, In step S3, the voltage of the capacitor deionization device is controlled to be 1V to 1.2V during the electro-adsorption process.

9. The method according to claim 6, characterized in that, In step S3, the heavy metal in the heavy metal wastewater is at least one of lead and cadmium; the initial concentration of the heavy metal in the heavy metal wastewater is ≤100 mg / L. -1 The initial pH value of the heavy metal wastewater is <7.5.

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