Magnetic biochar for wastewater treatment and preparation method and application thereof

Abstract

This invention relates to the field of wastewater treatment technology, and discloses a magnetic biochar for wastewater treatment, its preparation method, and its application. The magnetic biochar comprises porous sludge carbon formed by the pyrolysis and carbonization of dry Fenton sludge particles; a porous bamboo powder carbon layer formed by the pyrolysis and carbonization of bamboo powder is bonded to the surface of the porous sludge carbon, the porous bamboo powder carbon layer being doped with Fe particles and / or Fe3O4 particles; and magnetite particles are bonded to the outer surface of the porous bamboo powder carbon layer. The magnetic biochar of this invention has a structure from the inside out consisting of porous sludge carbon – a porous bamboo powder carbon layer doped with Fe particles and / or Fe3O4 particles – magnetite particles. Utilizing the properties of these three materials and their synergistic effect under a specific structure, the removal efficiency of Cr(VI) in wastewater can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a magnetic biochar for wastewater treatment, its preparation method, and its application. Background Technology

[0002] Chromium, due to its hardness, wear resistance, and high-temperature resistance, is widely used in electroplating, leather tanning, catalysis, and other fields. However, the resulting chromium-containing wastewater causes serious pollution when discharged into the environment. In particular, hexavalent chromium (Cr(VI)) has high solubility and mobility, easily seeping into groundwater and entering the food chain. It is also easily absorbed by the human body, producing carcinogenic and mutagenic effects, posing a serious threat to human health and ecosystems.

[0003] The removal of Cr(VI) from wastewater mainly includes physical methods (adsorption separation, membrane separation), chemical methods (precipitation reaction, ion exchange reaction, electrochemical reaction, photocatalytic reaction), and biological methods (physical adsorption, biochemical reaction). Among these, biochar adsorption is low-cost and simple to operate, but it also has limitations such as poor pH sensitivity, low adsorption efficiency, and difficulty in recovery. Iron-based magnetic biochar is a functional environmental material produced by combining iron-containing substances (iron source) with biomass precursors during or after pyrolysis. It combines the adsorption properties of biochar with the reducing, catalytic, and magnetic properties of iron-based compounds (especially ferrous / triferric oxides, such as magnetite and maghemite).

[0004] Patent CN115888712A discloses a method for preparing and applying Fenton sludge-based magnetic biochar catalytic material. The method involves dehydrating, drying, and grinding Fenton sludge, followed by pyrolysis under anaerobic conditions (pyrolysis temperature 600-800℃, heating rate 10℃ / min) to obtain the Fenton sludge-based magnetic biochar catalytic material. Since the Fenton reaction depends on the conversion between Fe(II) and Fe(III), Fenton sludge contains a large amount of iron, which can be directly formed into magnetic biochar after pyrolysis and carbonization (the organic matter in Fenton sludge can form biochar, and the iron source can form Fe3O4 and Fe...). 0 Iron-based materials such as FeO and Fe-C are used, but since most of the iron-based materials are located inside the Fenton sludge-based magnetic biochar, and less iron-based materials are exposed on the surface, the complexation and reduction efficiency of Cr(VI) is low when used for the treatment of Cr(VI)-containing wastewater. Summary of the Invention

[0005] To address the technical problem of low complexation and reduction efficiency of Cr(VI) in existing sludge-based magnetic biochar, this invention provides a magnetic biochar for wastewater treatment, its preparation method, and its application. The magnetic biochar of this invention has a structure consisting of, from the inside out, porous sludge carbon, a porous bamboo powder carbon layer doped with Fe particles and / or Fe3O4 particles, and magnetite particles. Through the synergistic effect of these three materials under this specific structure, the removal efficiency of Cr(VI) in wastewater can be effectively improved.

[0006] The specific technical solution of this invention is as follows: In a first aspect, the present invention provides a magnetic biochar for wastewater treatment, comprising porous sludge carbon formed by the pyrolysis and carbonization of dry Fenton sludge particles; the porous sludge carbon has a porous bamboo powder carbon layer formed by the pyrolysis and carbonization of bamboo powder on its surface, the porous bamboo powder carbon layer being doped with Fe particles and / or Fe3O4 particles; and magnetite particles are bonded to the outer surface of the porous bamboo powder carbon layer.

[0007] In the magnetic biochar of this invention, porous sludge carbon formed by the pyrolysis and carbonization of dry Fenton sludge particles is used as the core layer, and porous bamboo powder carbon formed by the pyrolysis and carbonization of bamboo powder is used as the shell layer. This core-shell structure design can achieve the following advantages compared with simple blending: (1) Porous sludge carbon constitutes the core layer structure of the entire magnetic biochar material. It has high mechanical strength and mineral skeleton. Porous bamboo powder carbon is combined with the porous sludge carbon as a shell layer, which can enhance the integrity of the overall structure and make the magnetic biochar material less prone to collapse during long-term operation or repeated use.

[0008] (2) Under the combined effect of physical structural defects (interface lattice mismatch) and chemical atmosphere during calcination, a large number of oxygen-containing functional groups (such as -OH, -COOH) can be formed at the junction of porous sludge carbon core layer and porous bamboo powder carbon shell layer. These oxygen-containing functional groups are good coordination groups and can efficiently capture Cr(VI) in water through electrostatic interaction or complexation. In addition, these oxygen-containing functional groups can also transfer electrons, promote the transfer of electrons from biochar matrix or microorganisms to Cr(VI) adsorbed in biochar, accelerate its reduction to low-toxicity Cr(III), thereby improving the removal efficiency of magnetic biochar for Cr(VI).

[0009] (3) Dry Fenton sludge particles can form a relatively loose and rough structure under a suitable pyrolysis and carbonization process, and the resulting porous sludge carbon is rich in mineral ash. The alkaline mineral components in the carbon can also neutralize the acidity of the influent. These characteristics are conducive to the proliferation and biofilm formation of microorganisms in the porous sludge carbon core layer during wastewater treatment. Bamboo powder, after being spread on the surface of the porous sludge carbon and pyrolyzed and carbonized, can form a relatively dense surface area, which can partially shield the direct toxicity of toxic substances (such as high concentrations of Cr(VI)) to the microorganisms in the core layer. Under the combined effect of the above, microorganisms can be better utilized to improve the removal efficiency of Cr(VI) and organic matter in wastewater.

[0010] (4) By doping Fe particles and / or Fe3O4 particles into the porous bamboo carbon layer and combining magnetite particles on the outer surface of the porous bamboo carbon layer, the surface of the magnetic biochar can have more reduction active sites. The relatively small pore structure in the porous bamboo carbon layer is conducive to the adsorption of Cr(VI), and then the porous bamboo carbon can act as an electron transfer medium to promote the redox reaction between Fe(O) and Fe(II) doped in the porous bamboo carbon layer and Cr(VI) adsorbed in the porous bamboo carbon layer, accelerating the reduction of Cr(VI) to the less toxic Cr(III). In this way, the small pore structure of the porous bamboo carbon layer, combined with the Fe particles and / or Fe3O4 particles doped in it, can improve the removal efficiency of Cr(VI) in wastewater.

[0011] The magnetic biochar of this invention can remove Cr(VI) from wastewater through the following mechanism: the porous structure of the magnetic biochar can physically adsorb Cr(VI), the magnetite particles on the surface can complex Cr(VI), and at the same time, abundant oxygen-containing functional groups (such as -OH, -COOH) can be formed at the interface between the porous sludge carbon core layer and the porous bamboo powder carbon shell layer, as well as on the surface of the porous bamboo powder carbon shell layer, which can enhance the complexation and electrostatic adsorption capacity for Cr(VI). Through the above methods, the magnetic biochar can adsorb Cr(VI) and reduce the Cr(VI) content in the wastewater. In addition, Fe(O) and Fe(II) in the magnetic biochar can reduce the highly toxic Cr(VI) to the less toxic Cr(III), and at the same time, Fe(O) and Fe(II) are converted into Fe(III), and then Cr(III) and Fe(III) can form a coprecipitate and be removed.

[0012] As an optional implementation, the mass ratio of bamboo powder, dry Fenton sludge particles and magnetite particles is 0.2~0.4:1:0.1~0.2.

[0013] As an optional implementation, the mass ratio of the bamboo powder to the Fe element doped in the porous bamboo carbon layer is 1:0.1~0.3.

[0014] Secondly, the present invention provides a method for preparing the magnetic biochar, comprising: S1: Mix bamboo powder with iron-containing... 3+ The solution was mixed, ball-milled, and then dried. S2: In an inert atmosphere, dry Fenton sludge particles are heated to 290-310℃ at 8-12℃ / min and kept at 30-40min, then heated to 490-510℃ at 3-6℃ / min and kept at 490-510℃ for 1-3h. S3: Spread the S1 product onto the surface of the S2 product, raise the temperature to 680-700℃ in an H2 atmosphere at 5-10℃ / min and keep it for 0.5-1h to obtain a preliminary composite material; keep it at 680-700℃, switch to a CO2 atmosphere, spray the nano magnetite powder onto the surface of the preliminary composite material, and keep it at that temperature for 25-35min.

[0015] The above preparation method helps to form a structure with a loose and rough porous sludge carbon core layer and a porous bamboo powder carbon shell layer with a small pore size. Specifically: (1) When containing Fe 3+ Before the bamboo powder (product S1) is spread onto the surface of the dry Fenton sludge particles, the dry Fenton sludge particles are pre-calcined (step S2). This helps to avoid the rapid release of a large amount of gas during the pyrolysis and carbonization of the dry Fenton sludge particles, which would cause the porous bamboo powder carbon shell to have an excessively large pore size.

[0016] (2) In step S2, during the pre-calcination of dry Fenton sludge particles, the first stage involves raising the temperature at 8-12℃ / min to 290-310℃ and holding for 30-40 min to allow the organic matter in the dry Fenton sludge particles to undergo preliminary decomposition and remove volatiles. The second stage involves raising the temperature at 3-6℃ / min to 490-510℃ and holding for 1-3 h to allow preliminary carbonization and the formation of a porous framework. Dividing the pre-calcination into these two stages is beneficial for forming a stable sludge carbon framework. Furthermore, both stages of calcination are carried out in an inert atmosphere without the introduction of CO2, which helps to avoid excessive etching of the carbon material by CO2 before a stable framework is formed, thus preventing structural collapse. Therefore, utilizing the specific calcination procedure and atmosphere in step S2 helps to form a more porous and coarse structure in the porous sludge carbon core layer.

[0017] (3) In step S3, by raising the temperature to 680-700℃ at a rate of 5-10℃ / min and holding it for 0.5-1h in an atmosphere containing H2, Fe can be... 3+ Reduced to Fe3O4 and Fe 0 Simultaneously, bamboo powder undergoes pyrolysis and carbonization on the core layer surface to form a carbon shell, during which the core layer completes full carbonization and pore development. Afterwards, switching to a CO2 atmosphere allows for controlled etching of the carbon layer, resulting in the formation of more microporous structures within the shell.

[0018] Furthermore, in the preparation process of this invention, the nano-magnetite powder is sprayed onto the surface of the preliminary composite material before the final roasting step (roasting in a CO2 atmosphere), which can reduce its agglomeration at high temperatures, thereby giving the magnetic biochar surface more reducing active sites, which helps to improve the removal efficiency of Cr(VI) in wastewater.

[0019] As an optional implementation, in step S3, the H2-containing atmosphere is composed of H2 and an inert gas, wherein the volume percentage of H2 is 5-10%; the CO2-containing atmosphere is composed of CO2 and an inert gas, wherein the volume percentage of CO2 is 8-12%.

[0020] As an optional implementation, in step S1, the ball mill rotates at a speed of 300-500 rpm for a time of 1.5-2.5 h.

[0021] As an optional implementation, in step S1, the particle size of the bamboo powder is 5~25μm; in step S2, the particle size of the dry Fenton sludge particles is 400~600μm; and in step S3, the particle size of the nano magnetite powder is 40~60nm.

[0022] As an optional implementation, in step S2, the preparation steps of the dry Fenton sludge particles include: air-drying, baking and crushing the Fenton sludge in sequence, and then sieving it.

[0023] Thirdly, the present invention provides the application of the magnetic biochar described above in the treatment of wastewater containing +6 valent Cr.

[0024] As an optional implementation, the magnetic biochar is used to remove +6 valent Cr from wastewater.

[0025] As an optional implementation, the application steps include: adding activated sludge and magnetic biochar to both the anoxic and aerobic zones; passing wastewater containing +6 valent Cr sequentially through the anoxic and aerobic zones; intermittently stirring in the anoxic zone to control the dissolved oxygen concentration of the wastewater at 0.2~0.5 mg / L and the hydraulic retention time at 3~5 h; and intermittently aerating in the aerobic zone to control the dissolved oxygen concentration of the wastewater at 2.0~4.0 mg / L and the hydraulic retention time at 3~5 h.

[0026] Compared with the prior art, the present invention has the following advantages: (1) The magnetic biochar of the present invention has a structure consisting of porous sludge carbon, porous bamboo powder carbon layer doped with Fe particles and / or Fe3O4 particles, and magnetite particles from the inside out. By utilizing the characteristics of these three materials and their synergistic effect under a specific structure, the removal efficiency of Cr(VI) in wastewater can be effectively improved.

[0027] (2) The magnetic biochar preparation method of the present invention helps to form a structure with a loose and rough core layer and a small pore size in the shell layer, thereby promoting the proliferation and biofilm formation of microorganisms in the porous sludge carbon core layer. At the same time, it partially shields the direct toxicity of toxic substances to microorganisms in the core layer, thereby using microorganisms to improve the removal efficiency of Cr(VI) and organic matter in wastewater. Attached Figure Description

[0028] Figure 1 SEM images of different magnetic biochars. Among them: Figure 1 (A) is the magnetic biochar prepared in Example 1; Figure 1 (B) is the magnetic biochar prepared in Comparative Example 2; Figure 1 (C) is the magnetic biochar prepared in Comparative Example 3.

[0029] Figure 2 SEM images of porous bamboo powder charcoal and porous sludge charcoal are shown. Among them: Figure 2 (A) is porous bamboo powder charcoal; Figure 2 (B) is porous sludge carbon.

[0030] Figure 3 XRD patterns of different biochars.

[0031] Figure 4 This is the wastewater treatment test apparatus used in the test examples of the present invention. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0033] First, the present invention relates to a magnetic biochar for wastewater treatment, comprising porous sludge carbon formed by pyrolysis and carbonization of dry Fenton sludge particles; the porous sludge carbon has a porous bamboo powder carbon layer formed by pyrolysis and carbonization of bamboo powder on its surface, the porous bamboo powder carbon layer being doped with Fe particles and / or Fe3O4 particles; and magnetite particles are bonded to the outer surface of the porous bamboo powder carbon layer.

[0034] In some specific embodiments, the mass ratio of bamboo powder, dry Fenton sludge particles and magnetite particles is 0.2~0.4:1:0.1~0.2.

[0035] In some specific embodiments, the mass ratio of the bamboo powder to the Fe element doped in the porous bamboo carbon layer is 1:0.1~0.3.

[0036] Second, the present invention relates to a method for preparing the magnetic biochar, comprising: S1: Mix bamboo powder with iron-containing... 3+ The solution was mixed, ball-milled, and then dried. S2: In an inert atmosphere, dry Fenton sludge particles are heated to 290-310℃ at 8-12℃ / min and kept at 30-40min, then heated to 490-510℃ at 3-6℃ / min and kept at 490-510℃ for 1-3h. S3: Spread the S1 product onto the surface of the S2 product, raise the temperature to 680-700℃ in an H2 atmosphere at 5-10℃ / min and keep it for 0.5-1h to obtain a preliminary composite material; keep it at 680-700℃, switch to a CO2 atmosphere, spray the nano magnetite powder onto the surface of the preliminary composite material, and keep it at that temperature for 25-35min.

[0037] In some specific embodiments, in step S1, the ball mill rotates at a speed of 300-500 rpm for a time of 1.5-2.5 h.

[0038] In some specific embodiments, in step S3, the H2-containing atmosphere is composed of H2 and an inert gas, wherein the volume percentage of H2 is 5-10%.

[0039] In some specific embodiments, in step S3, the CO2-containing atmosphere is composed of CO2 and an inert gas, wherein the volume percentage of CO2 is 8-12%.

[0040] In some specific embodiments, in step S1, the particle size of the bamboo powder is 5~25μm; in step S2, the particle size of the dry Fenton sludge particles is 400~600μm; and in step S3, the particle size range of the nano magnetite powder is 40~60nm.

[0041] In some specific embodiments, step S2, the preparation steps of the dry Fenton sludge particles include: air-drying, baking and crushing the Fenton sludge in sequence, and then sieving it.

[0042] Third, the present invention relates to the application of the magnetic biochar described above in the treatment of wastewater containing +6 valent Cr.

[0043] In some specific embodiments, the magnetic biochar is used to remove +6 valent Cr from wastewater.

[0044] In some specific embodiments, the application steps include: adding activated sludge and magnetic biochar to both the anoxic and aerobic zones; passing wastewater containing +6 valent Cr sequentially through the anoxic and aerobic zones; intermittently stirring in the anoxic zone to control the dissolved oxygen concentration of the wastewater at 0.2~0.5 mg / L and the hydraulic retention time at 3~5 h; and intermittently aerating in the aerobic zone to control the dissolved oxygen concentration of the wastewater at 2.0~4.0 mg / L and the hydraulic retention time at 3~5 h.

[0045] The present invention will now be described with reference to specific embodiments and comparative examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Example 1

[0046] The magnetic biochar of this embodiment was prepared through the following steps: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0047] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0048] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 300°C at a rate of 10°C / min and hold for 30 min, then heat it to 500°C at a rate of 5°C / min and hold for 60 min. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0049] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The furnace was then returned to the tube furnace and heated to 700°C at a rate of 7°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 16 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed onto the surface of the hot preliminary composite material through the inlet, and held for 30 min. The material was then naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar, with the surface morphology as shown in the figure. Figure 1 As shown in (A), the XRD pattern is as follows: Figure 3 As shown in “WJBC+Fe3O4”. Example 2

[0050] This embodiment differs from Example 1 in that the calcination conditions are modified, while the remaining raw materials and steps are the same. The specific steps for preparing magnetic biochar in this embodiment are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0051] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0052] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 290°C at a rate of 8°C / min and hold for 40 min, then heat it to 490°C at a rate of 3°C / min and hold for 3 h. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0053] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The product was then returned to the tube furnace and heated to 700°C at a rate of 10°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 16 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed from the inlet onto the surface of the hot preliminary composite material and held for 25 min. The material was then naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar. Example 3

[0054] This embodiment differs from Example 1 in that the calcination conditions are modified, while the remaining raw materials and steps are the same. The specific steps for preparing magnetic biochar in this embodiment are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0055] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0056] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 310°C at a rate of 12°C / min and hold for 30 min, then heat it to 510°C at a rate of 6°C / min and hold for 60 min. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0057] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The product was then returned to the tube furnace and heated to 690°C at a rate of 5°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 1 h to form a preliminary composite material. The temperature was then maintained at 690°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 16 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed from the inlet onto the surface of the hot preliminary composite material, and held for 35 min. The material was then naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar. Example 4

[0058] This embodiment differs from Example 1 in that the proportions of dry Fenton sludge granules, bamboo powder, and nano-magnetite powder are varied, while the remaining raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this embodiment are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0059] S2: Bamboo powder pretreatment Take 20 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0060] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 300°C at a rate of 10°C / min and hold for 30 min, then heat it to 500°C at a rate of 5°C / min and hold for 60 min. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0061] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The product was then returned to the tube furnace and heated to 700°C at a rate of 7°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 20 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed from the inlet onto the surface of the hot preliminary composite material and held for 30 min. The material was then naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar. Example 5

[0062] This embodiment differs from Example 1 in that the proportions of dry Fenton sludge granules, bamboo powder, and nano-magnetite powder are varied, while the remaining raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this embodiment are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0063] S2: Bamboo powder pretreatment Take 40 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0064] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 300°C at a rate of 10°C / min and hold for 30 min, then heat it to 500°C at a rate of 5°C / min and hold for 60 min. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0065] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The product was then returned to the tube furnace and heated to 700°C at a rate of 7°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 10 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed from the inlet onto the surface of the hot preliminary composite material, and held for 30 min. The material was then naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar.

[0066] Comparative Example 1 This comparative example differs from Example 1 in that the composite method between dry Fenton sludge particles and bamboo powder-Fe(III) complex is changed to simple blending, while the remaining raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this comparative example are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0067] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0068] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 300°C at a rate of 10°C / min and hold for 30 min, then heat it to 500°C at a rate of 5°C / min and hold for 60 min. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0069] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, it was mixed and stirred with the bamboo powder-Fe(III) composite obtained in step S2 for 2 h. Then, it was placed back into the tube furnace and heated to 700 °C at a rate of 7 °C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700 °C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 16 g of nano-magnetic mineral powder with a particle size of 40–60 nm was uniformly sprayed from the inlet onto the surface of the hot preliminary composite material, and held for 30 min. Finally, it was naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar.

[0070] Comparative Example 2 This comparative example is based on Example 1, but without the use of Fenton sludge; all other raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this comparative example are as follows: S1: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0071] S2: Composite roasting The bamboo powder-Fe(III) composite obtained in step S1 was placed in a tube furnace and heated to 700°C at a rate of 7°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form porous bamboo powder char. Then, while maintaining the temperature at 700°C, the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%), and 16 g of magnetite nanoparticles with a particle size of 40-60 nm were uniformly sprayed onto the surface of the incandescent porous bamboo powder char through the inlet, and held for 30 min. Afterward, it was naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar, the surface morphology of which is as follows. Figure 1 As shown in (B), the XRD pattern is as follows: Figure 3 As shown in “JMBC+Fe3O4”.

[0072] Comparative Example 3 This comparative example differs from Example 1 in that it does not use bamboo powder; all other raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this comparative example are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0073] S2: Composite roasting 100 g of the dry Fenton sludge particles obtained in step S1 were placed in a tube furnace. Under a pure nitrogen atmosphere, the temperature was first increased to 300°C at a rate of 10°C / min and held for 30 min, then increased to 500°C at a rate of 5°C / min and held for 60 min. After natural cooling to room temperature, the temperature was switched to a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%) and increased to 700°C at a rate of 7°C / min, held for 30 min, to form porous sludge carbon. Then, the temperature was maintained at 700°C, and the temperature was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 16 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed onto the surface of the hot porous sludge carbon through the inlet and held for 30 min. After natural cooling to room temperature under pure nitrogen protection, the carbon was removed from the tube furnace to obtain magnetic biochar, the surface morphology of which is as follows. Figure 1 As shown in (C), the XRD pattern is as follows: Figure 3 As shown in “WNBC+Fe3O4”.

[0074] Comparative Example 4 This comparative example differs from Example 1 in that it does not pre-calcine the dry Fenton sludge particles. Instead, a bamboo powder-Fe(III) composite is coated onto the surface of the particles before calcination. All other raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this comparative example are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0075] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0076] S3: Compound roasting Take 100 g of the dry Fenton sludge particles obtained in step S1, and uniformly spread the bamboo powder-Fe(III) composite obtained in step S2 onto its surface. Place it in a tube furnace and heat it to 300°C at a rate of 10°C / min under a pure nitrogen atmosphere. Hold for 30 min, then heat it to 500°C at a rate of 5°C / min and hold for 60 min. Allow it to cool naturally to room temperature. Then switch to a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%) and heat it to 700°C at a rate of 7°C / min. Hold for 30 min to form a preliminary composite material. Then maintain the temperature at 700°C and switch to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). Spray 16 g of nano-magnetite powder with a particle size of 40~60 nm uniformly onto the surface of the hot preliminary composite material through the injection port and hold for 30 min. Then, under the protection of pure nitrogen, it is naturally cooled to room temperature and removed from the tube furnace to obtain magnetic biochar.

[0077] Comparative Example 5 This comparative example differs from Example 1 in that the stepwise Fenton sludge pre-calcination process is performed in one step, while the remaining raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this comparative example are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0078] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0079] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 500°C at a rate of 10°C / min, hold for 60 min, and then cool naturally to room temperature to obtain the pre-calcined product.

[0080] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The product was then returned to the tube furnace and heated to 700°C at a rate of 7°C / min under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%). 16 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed from the inlet onto the surface of the hot preliminary composite material and held for 30 min. The material was then naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar.

[0081] Comparative Example 6 This comparative example differs from Example 1 in that the timing of spraying the nano-magnetite powder was advanced; all other raw materials and steps are the same as in Example 1. The specific steps for preparing magnetic biochar in this comparative example are as follows: S1: Fenton sludge pretreatment The waste Fenton sludge was air-dried, oven-dried, and pulverized in sequence, and then sieved to obtain dry Fenton sludge particles with a particle size of 425~600 μm.

[0082] S2: Bamboo powder pretreatment Take 30 g of bamboo powder with a particle size of 5~25 μm, add it to 100 mL of 0.2 g / mL FeCl3·6H2O aqueous solution, place it in a planetary ball mill, and ball mill at 400 rpm for 2 h. After drying, bamboo powder-Fe(III) complex is obtained.

[0083] S3: Fenton sludge pre-roasting Take 100 g of the dry Fenton sludge particles obtained in step S1 and put them into a tube furnace. Under a pure nitrogen atmosphere, first heat the furnace to 300°C at a rate of 10°C / min and hold for 30 min, then heat it to 500°C at a rate of 5°C / min and hold for 60 min. Then cool it naturally to room temperature to obtain the pre-calcined product.

[0084] S4: Compound roasting After removing the pre-calcined product from the tube furnace in step S3, the bamboo powder-Fe(III) composite obtained in step S2 was uniformly spread onto its surface. The material was then returned to the tube furnace, and 16 g of nano-magnetic mineral powder with a particle size of 40-60 nm was uniformly sprayed onto the material surface through the inlet. Under a mixed atmosphere of nitrogen and hydrogen (hydrogen volume percentage of 5%), the temperature was increased to 700°C at a rate of 7°C / min and held for 30 min to form a preliminary composite material. The temperature was then maintained at 700°C, and the atmosphere was switched to a mixed atmosphere of nitrogen and CO2 (CO2 volume percentage of 10%) for 30 min. Finally, the material was naturally cooled to room temperature under pure nitrogen protection and removed from the tube furnace to obtain magnetic biochar.

[0085] Test Example: Electroplating Wastewater Treatment Experiment Magnetic biochar prepared according to the methods of each embodiment and comparative example was used to conduct wastewater treatment experiments using the same batch of electroplating wastewater (water quality conditions are shown in Table 1) from a centralized treatment station of an electroplating plant in Zhejiang Province. The structure of the experimental apparatus is as follows. Figure 4 As shown, a sequencing batch reactor (SBR) was used, consisting of interconnected anoxic and aerobic tanks, each with an effective volume of 8 L. Each tank contained 20 g of activated sludge (taken from the anoxic and aerobic tanks of an electroplating wastewater treatment plant, and acclimated to actual wastewater for 2 weeks after retrieval) and 10 g of magnetic biochar. The wastewater treatment process was as follows: wastewater was first introduced into the anoxic tank and intermittently stirred at 60 rpm, with the dissolved oxygen concentration controlled at 0.2–0.5 mg / L. After 3 hours, the wastewater from the anoxic tank was introduced into the aerobic tank, and the dissolved oxygen concentration was controlled at 2.0–4.0 mg / L through intermittent aeration. The wastewater was discharged after 3 hours. Samples were taken to detect the Cr and COD content in the water, and the Cr removal rate and COD removal rate were calculated. The results are shown in Table 2.

[0086] Table 1. Water quality of electroplating wastewater Table 2. Removal efficiency of Cr and COD from electroplating wastewater Based on the test results in Table 2, it can be seen that: (1) Compared with using bamboo powder alone (Comparative Example 2) and Fenton sludge alone (Comparative Example 3) as raw materials for biochar formation, this invention uses a combination of these two materials and combines the porous bamboo powder carbon layer with the porous sludge carbon surface (Example 1), which effectively improves the removal efficiency of magnetic biochar for Cr and COD in wastewater, and the effect is better than the non-core-shell structure formed by simple blending (Comparative Example 1). The reason is that: more oxygen-containing functional groups can be formed at the interface between the porous sludge carbon core layer and the porous bamboo powder carbon shell layer, thereby improving the adsorption effect of magnetic biochar on Cr(VI); in addition, the loose, rough porous sludge carbon core layer rich in mineral ash can provide a good carrier for the reproduction and biofilm formation of microorganisms, while the relatively dense porous bamboo powder carbon shell layer can partially shield the direct toxicity of toxic substances to the microorganisms in the core layer, thereby better utilizing microorganisms to improve the removal efficiency of Cr(VI) and organic matter in wastewater. Figure 2 (A) is porous bamboo powder carbon formed solely by roasting bamboo powder, and its XRD pattern is as follows: Figure 3 As shown in "JMBC", Figure 2 (B) is porous sludge carbon formed solely from the roasting of waste Fenton sludge, and its XRD pattern is shown below. Figure 3 As shown in “WNBC”, the morphology of porous sludge carbon is more porous than that of porous bamboo powder carbon. Furthermore, the relatively small pore structure in the porous bamboo powder carbon layer is conducive to the adsorption of Cr(VI), and the porous bamboo powder carbon can then act as an electron transport medium, promoting the absorption of Fe(VI) by the porous bamboo powder carbon layer. 0 Fe(II) reacts with Cr(VI) adsorbed in the porous bamboo carbon layer to carry out an oxidation-reduction reaction, thereby accelerating the removal of Cr from the wastewater.

[0087] (2) Compared with Comparative Example 4, Example 1 pre-calcined the dry Fenton sludge particles before uniformly spreading the bamboo powder-Fe(III) composite onto the surface of the dry Fenton sludge particles, which effectively improved the removal efficiency of magnetic biochar for Cr and COD in wastewater. The reason for this is that pre-calcining the dry Fenton sludge particles helps to avoid the rapid release of a large amount of gas during the pyrolysis and carbonization of the dry Fenton sludge particles, which would cause the porous bamboo powder carbon shell to have an excessively large pore size. The smaller pore structure of the porous bamboo powder carbon shell is beneficial for shielding the direct toxicity of toxic substances to the microorganisms in the core layer, and it is also beneficial for improving the adsorption effect of Cr(VI) in wastewater.

[0088] (3) In the process of pre-calcining dry Fenton sludge particles, compared with the one-step method (Comparative Example 5), the pre-calcination process is designed into two stages according to the procedure in this invention (Example 1), which can improve the removal efficiency of magnetic biochar for Cr and COD in wastewater. The reason is that: through the first stage of pre-calcination, the organic matter in the dry Fenton sludge particles can be initially decomposed and volatile matter can be removed. Then, in the second stage of pre-calcination, a porous skeleton can be initially carbonized. In this way, due to the formation of a stable sludge carbon skeleton, the collapse of the pores caused by the rapid decomposition of organic matter and the rapid release of gas is avoided, so that the porous sludge carbon core layer can have a more loose and rough structure, which is conducive to the reproduction and biofilm formation of microorganisms.

[0089] (4) After completing the pre-calcination of the dry Fenton sludge particles, Example 1 adopted the sequence of spreading bamboo powder-Fe(III) composite → calcination in H2 atmosphere → spraying nano-magnetite powder → calcination in CO2 atmosphere. Comparative Example 6 advanced the spraying of nano-magnetite powder to before calcination in H2 atmosphere. From the wastewater treatment test results, the Cr removal efficiency of Example 1 was significantly higher than that of Comparative Example 6. The reason is that spraying nano-magnetite powder onto the surface of the preliminary composite material before the final calcination step (calcination in CO2 atmosphere) can reduce its agglomeration at high temperature, thereby giving the magnetic biochar surface more reducing active sites, which helps to improve the removal efficiency of Cr(VI) in wastewater.

Claims

1. A magnetic biochar for wastewater treatment, characterized in that, It includes porous sludge carbon formed by the pyrolysis and carbonization of dry Fenton sludge particles; the surface of the porous sludge carbon is bonded with a porous bamboo powder carbon layer formed by the pyrolysis and carbonization of bamboo powder, the porous bamboo powder carbon layer being doped with Fe particles and / or Fe3O4 particles; and magnetite particles are bonded to the outer surface of the porous bamboo powder carbon layer.

2. The magnetic biochar according to claim 1, characterized in that, The mass ratio of bamboo powder, dry Fenton sludge particles, and magnetite particles is 0.2~0.4:1:0.1~0.

2.

3. The magnetic biochar according to claim 1 or 2, characterized in that, The mass ratio of bamboo powder to Fe element doped in the porous bamboo carbon layer is 1:0.1~0.

3.

4. A method for preparing magnetic biochar according to any one of claims 1 to 3, characterized in that, include: S1: Mix bamboo powder with iron-containing... 3+ The solution was mixed, ball-milled, and then dried. S2: In an inert atmosphere, dry Fenton sludge particles are heated to 290-310℃ at 8-12℃ / min and kept at 30-40min, then heated to 490-510℃ at 3-6℃ / min and kept at 490-510℃ for 1-3h. S3: Spread the S1 product onto the surface of the S2 product, raise the temperature to 680-700℃ in an H2 atmosphere at 5-10℃ / min and keep it for 0.5-1h to obtain a preliminary composite material; keep it at 680-700℃, switch to a CO2 atmosphere, spray the nano magnetite powder onto the surface of the preliminary composite material, and keep it at that temperature for 25-35min.

5. The preparation method according to claim 4, characterized in that, In step S3, the H2-containing atmosphere is composed of H2 and an inert gas, wherein the volume percentage of H2 is 5-10%; the CO2-containing atmosphere is composed of CO2 and an inert gas, wherein the volume percentage of CO2 is 8-12%.

6. The preparation method according to claim 4, characterized in that, In step S1, the ball mill rotates at a speed of 300-500 rpm for 1.5-2.5 hours.

7. The preparation method according to claim 4, characterized in that, In step S1, the particle size of the bamboo powder is 5~25μm; in step S2, the particle size of the dry Fenton sludge particles is 400~600μm; in step S3, the particle size of the nano magnetite powder is 40~60nm.

8. The application of magnetic biochar according to any one of claims 1 to 3 in the treatment of wastewater containing +6 valent Cr.

9. The application according to claim 8, characterized in that, The magnetic biochar is used to remove +6 valent Cr from wastewater.

10. The application according to claim 8 or 9, characterized in that, The application steps include: adding activated sludge and magnetic biochar to both the anoxic and aerobic zones; passing wastewater containing +6 valent Cr sequentially through the anoxic and aerobic zones; intermittently stirring in the anoxic zone to control the dissolved oxygen concentration of the wastewater at 0.2~0.5 mg / L and the hydraulic retention time at 3~5 h; and intermittently aerating in the aerobic zone to control the dissolved oxygen concentration of the wastewater at 2.0~4.0 mg / L and the hydraulic retention time at 3~5 h.

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