Method for preparing biochar-steel slag composite adsorbent by synergizing steel slag and biomass

Abstract

This invention relates to the technical field of solid waste resource utilization and environmental pollution control, and particularly to a method for co-preparing a biochar-steel slag composite adsorbent using steel slag and biomass. First, the raw steel slag is pretreated by coarse grinding, then mechanically activated and modified by high-energy ball milling to obtain finely ground steel slag powder. Next, the finely ground steel slag powder is mixed with a sodium bicarbonate solution and subjected to ultrasonic treatment in a water bath. Then, biochar bran is added in a specific ratio, and further ultrasonic treatment is performed for chemical composite modification. Finally, after solid-liquid separation, washing, and drying, a biochar-steel slag composite structure adsorbent is obtained. This invention, through a combined mechanical and chemical technique, significantly increases the specific surface area of ​​the material, exposes the internal active components, constructs a composite structure, and enriches the surface active functional groups, exhibiting excellent adsorption performance and rapid adsorption kinetics for heavy metal ions in wastewater.

Description

Technical Field

[0001] This invention relates to the technical field of solid waste resource utilization and environmental pollution control, and in particular to a method for the synergistic preparation of biochar-steel slag composite adsorbent from steel slag and biomass. Background Technology

[0002] Steel slag accounts for approximately 15% to 20% of crude steel production and is a major solid waste generated during steel smelting, with a low comprehensive utilization rate. Steel slag is rich in active components such as dicalcium silicate (C2S), tricalcium silicate (C3S), and iron oxides, showing promising applications in environmental remediation, particularly in the treatment of heavy metal wastewater. However, when raw steel slag is used directly as an adsorbent, it generally suffers from drawbacks such as small specific surface area, dense microstructure, and few active sites, making it difficult to fully expose its internal active components, resulting in low adsorption capacity and slow adsorption rate for pollutants. On the other hand, the development of the edible fungi industry has generated a large amount of biomass waste such as mushroom residue. Improper disposal of this residue can also cause environmental pollution. Biochar prepared from mushroom residue through pyrolysis possesses abundant functional groups and a well-developed porous structure, is lightweight, and is an excellent adsorbent carrier.

[0003] Current technologies for modifying steel slag often employ single methods such as acid / alkali activation, high-temperature reconstruction, or mechanical crushing, which suffer from high costs, risks of secondary pollution, or complex operations. While there are reports of preparing adsorbent materials by combining steel slag with biomass, these are mostly simple physical mixtures, failing to form effective chemical bonds and structural composites. The stability and surface functional group diversity of the adsorbents are limited, hindering significant improvements in adsorption performance and limiting the large-scale application of steel slag in environmental pollution control. Therefore, developing a green, efficient, and synergistic treatment method that simultaneously addresses the stability issues of steel slag and enhances its adsorption performance, while achieving high-value utilization of both solid wastes, is of significant practical importance. Summary of the Invention

[0004] The purpose of this invention is to provide a method for the synergistic preparation of biochar-steel slag composite adsorbent from steel slag and biomass, aiming to solve the problems mentioned in the background art, where the modification of steel slag often adopts single acid or alkali activation or high-temperature reconstruction and mechanical crushing, which have the problems of high cost, risk of secondary pollution or complicated operation.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for synergistic preparation of biochar-steel slag composite adsorbent from steel slag and biomass, comprising the following steps:

[0006] (1) Steel slag pretreatment: The raw steel slag is crushed, screened, and then coarsely ground to obtain coarsely ground steel slag;

[0007] (2) Mechanical activation modification: The coarse steel slag obtained in step (1) is placed in a high-energy ball mill for mechanical activation ball milling to obtain fine steel slag powder;

[0008] (3) Chemical composite modification: The finely ground steel slag powder obtained in step (2) is mixed with sodium bicarbonate solution and subjected to a first water bath ultrasonic treatment. Then, bacterial bran biochar is added and a second water bath ultrasonic treatment is carried out to obtain a modified mixture.

[0009] (4) Post-processing: The modified mixture obtained in step (3) is subjected to solid-liquid separation. The resulting solid product is washed and dried to obtain the biochar-steel slag micro-nano composite adsorbent.

[0010] Furthermore, in step (1), the parameters of the coarse grinding process are: ball milling time of 4800 min, ball milling frequency of 50 Hz, grinding ball diameter of 10 mm, weight ratio of grinding ball to steel slag of 10:1, and the particle size of steel slag after coarse grinding passes through a 150-250 mesh sieve.

[0011] Preferably, the raw steel slag is converter slag, and the coarse grinding pretreatment can be carried out using a drum mill or a planetary ball mill;

[0012] In step (2), the parameters of the mechanically activated ball mill are: ball milling time of 10-20h, ball milling speed of 480-720r / min, mass ratio of grinding balls to coarsely ground steel slag of 3-6:1, and grinding ball gradation of balls with diameters of 8-12mm, 6-9mm and 3-5mm in a quantity ratio of (0-1):(1-2):(6-10).

[0013] Preferably, the ball milling time is 20 hours, the ball milling intensity is 480 r / min, the ball-to-material ratio is 6:1, and the grinding ball gradation is balls with diameters of 10 mm, 8 mm, and 3.5 mm in a quantity ratio of 0:2:8 or 1:1:8.

[0014] Furthermore, in step (2), a dispersed phase is added during the mechanical activation ball milling process. The dispersed phase is Al2O3 or SiO2, and its addition amount is 0-10% of the mass of the coarsely ground steel slag.

[0015] Preferably, the amount of dispersed phase added is 5-10% of the mass of the coarsely ground steel slag.

[0016] Furthermore, in step (2), the obtained finely ground steel slag powder has a particle size that passes through a 400-600 mesh sieve, a specific surface area of ​​0.8-4.1 m² / g, an average particle size of 6.5-15.0 μm, and a size distribution of β-C2S phase, RO phase and CaAl2Si2O8 phase of 2.16~3.54 μm, 0.45~0.89 μm and 0.84~2.26 μm, respectively.

[0017] Preferably, the finely ground steel slag powder has a particle size that passes through a 500-mesh sieve.

[0018] Furthermore, in step (3), the concentration of the sodium bicarbonate solution is 1-3 mol / L, and the solid-liquid ratio of the finely ground steel slag powder to the sodium bicarbonate solution is (2-6):100 g / mL.

[0019] Preferably, the solid-liquid ratio of finely ground steel slag powder to sodium bicarbonate solution is 4:100 g / mL;

[0020] The parameters for the first water bath ultrasonic treatment are: water bath temperature of 20-30℃ and treatment time of 15-25 min;

[0021] The parameters for the second water bath ultrasonic treatment are: water bath temperature of 50-70℃ and treatment time of 0.8-1.2h;

[0022] Both ultrasonic treatments were performed at a frequency of 40 kHz and a power of 200 W.

[0023] Furthermore, in step (3), the mass ratio of the amount of the added mushroom biochar to the mass of the finely ground steel slag powder is (3-5):1, preferably 4:1. The particle size of the mushroom biochar is 100-200 mesh, preferably 110 mesh, and its specific surface area is 266.63 m² / g, and its pore volume is 0.136 cm³ / g.

[0024] The preparation method of the biochar is as follows: after the biomass raw material is pretreated by air drying, baking, crushing and sieving, the powder is placed in a crucible, compacted and sealed, and then pyrolyzed in a resistance furnace at 350-650℃ under a protective atmosphere or limited oxygen conditions for 2-4 hours with a heating rate of 5-15℃ / min. After natural cooling, it is ground through a 100-200 mesh sieve to obtain the product.

[0025] Preferably, the pyrolysis temperature of the bacterial residue biochar is 650℃, the pyrolysis time is 2 hours, and the heating rate is [missing information]. ℃ / min;

[0026] The biomass raw material is one or more of mushroom residue, straw, rice husk, and sawdust, with mushroom residue being the preferred choice.

[0027] Furthermore, in step (4), the drying temperature is 80-120℃, the time is 20-40min, the solid-liquid separation is performed by vacuum filtration, the pumping speed is 1.8m³ / h, and the filter membrane pore size is 0.45μm.

[0028] A biochar-steel slag micro-nano composite adsorbent, wherein the adsorbent has a micro-nano structure of in-situ composite steel slag particles on the surface of biochar, with a specific surface area of ​​8.5-11 m² / g and the surface is rich in active functional groups such as hydroxyl and carbonate groups.

[0029] Application of a biochar-steel slag micro-nano composite adsorbent in the treatment of wastewater containing heavy metal ions, including Pb 2+ Cd 2+ Hg 2+ At least one of them.

[0030] Furthermore, the treatment conditions are as follows: wastewater pH is 3.5-5.5, wastewater heavy metal concentration can be selected from 0 to 500 mg / L, adsorbent dosage is 0.5-5 g / L, reaction temperature is 15-35℃, reaction time is 0.5-24 h, oscillation speed is 120-160 r / min, and the reaction equipment can be a constant temperature oscillator or a constant temperature incubator, etc.

[0031] Preferably, the heavy metal ion is Pb. 2+ The pH was 4.5, the concentration was 300 mg / L, the adsorbent dosage was 1 g / L, the reaction temperature was 25℃, the reaction time was 40 min, the shaking speed was 150 r / min, and the adsorption efficiency reached more than 90% of the maximum adsorption capacity.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] 1. High reactivity: The process fully activates the active mineral components such as calcium, iron, and silicon in steel slag, while retaining the well-developed pore structure and abundant surface functional groups of biochar. The two materials complement each other and work synergistically to significantly improve the overall reactivity and adsorption site density of the materials.

[0034] 2. Stable structure: By combining mechanical and chemical methods, not only is the stability problem of steel slag solved and its heavy-duty characteristics improved, but a stable composite structure is also constructed, and the material has good recyclability.

[0035] 2. Excellent adsorption performance: The prepared adsorbent exhibits extremely high adsorption capacity and rapid adsorption equilibrium time for heavy metals such as Pb²⁺, with a saturated adsorption capacity >250mg / g, which is significantly better than that of unmodified steel slag and single materials.

[0036] 3. Green and low-cost process: Using water as a solvent and ultrasound assistance, it adopts a mild path that combines salt modification and mechanical activation. It does not require high temperature and high pressure and complex equipment. It abandons high-energy-consuming and high-polluting processes such as strong acid and strong alkali etching and high-temperature roasting and melting. The raw material cost is low, there is no risk of secondary pollution, it meets the requirements of clean production, and it is easy to operate and promote industrialization.

[0037] 4. Multi-mechanism adsorption: It combines ion exchange, chemical precipitation, and surface complexation into a triple chemical adsorption mechanism, as well as physical adsorption. It is mainly achieved through the complexation of lead ions by CS-H gel, oxygen-containing functional groups, and metal sites on the RO phase surface; the precipitation of lead ions by carbonate groups; and the exchange of lead ions by calcium and magnesium ions.

[0038] 5. Significant environmental benefits: It has achieved the synergistic treatment and high-value utilization of two typical solid wastes, steel slag and biomass waste, reducing the pressure of stockpiling on the environment and realizing "treating waste with waste". Attached Figure Description

[0039] Figure 1 Schematic diagram of the preparation process mechanism of the biochar-steel slag composite adsorbent in this invention;

[0040] Figure 2 Microscopic morphology diagram of the biochar-steel slag composite adsorbent in this invention;

[0041] Figure 3 Particle size diagram of the biochar-steel slag composite adsorbent in this invention;

[0042] Figure 4 Mineral phase composition diagram of the biochar-steel slag composite adsorbent prepared in Example 1 of this invention;

[0043] Figure 5 A diagram of the surface functional groups of the biochar-steel slag composite adsorbent prepared in Example 1 of this invention;

[0044] Figure 6 Specific surface area and pore size distribution of the biochar-steel slag composite adsorbent prepared in Example 1 of this invention;

[0045] Figure 7 The different adsorbents (MS1-MS12) prepared in Examples 1-12 of this invention for Pb 2+ A comparison chart of adsorption capacity and efficiency. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below. For implementation steps where specific conditions are not specified, conventional conditions or conditions recommended by the manufacturer shall apply. Except for steel slag and biochar, all other reagents or instruments used, unless otherwise specified, are commercially available products. The raw steel slag used in this embodiment is a typical alkaline industrial waste slag discharged from a large-scale modern steel plant in Southwest China, collected from the steel slag treatment workshop. The biochar used in this embodiment is mushroom biochar obtained by pyrolysis of edible fungi residue cultivated in the laboratory using Fenjiu liquor lees as raw material. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] Example 1

[0048] 1. Preparation of inoculum residue biochar

[0049] Using edible fungus lees as a culture medium, the lees were air-dried and then dried in an oven at 60℃. The lees were then pulverized and passed through a 0.5mm sieve. 10g of the pretreated lees powder was placed in a 50mL porcelain crucible, compacted, covered with 20μm tin foil, sealed, and placed in a resistance furnace. The temperature was increased to 300℃ at 10℃ / min and pyrolyzed at a constant temperature for 2h. After the pyrolysis, the temperature was reduced to room temperature at 10℃ / min. The product was then removed, ground, and passed through a 110-mesh sieve to obtain lees biochar. The biochar was found to have a specific surface area of ​​266.627m² / g and was rich in functional groups such as hydroxyl (-OH) and carboxyl (-COOH).

[0050] 2. Characteristic Analysis of Steel Slag Pretreatment and Coarse Grinding Steel Slag

[0051] Raw steel slag with an original particle size of 10-20 cm and a dense structure was taken. After preliminary crushing, it was put into a planetary ball mill (XQM-4L) for coarse grinding pretreatment. The ball milling parameters were: time 4800 min, frequency 50 Hz, grinding ball diameter 10 mm, and ball material to steel slag weight ratio of 10:1. After ball milling, it was passed through a 200-mesh sieve to obtain coarsely ground steel slag.

[0052] After pretreatment, the coarsely ground steel slag is refined from centimeter-sized lumps into 1-100 μm grayish-brown powder, with a bimodal particle size distribution (0.5-1 μm and 10-20 μm). Chemically, the internal minerals are exposed, and free calcium oxide hydrates / carbonizes to generate highly active Ca(OH)2 and CaCO3. In terms of mineral phase structure, the outer brittle phase is broken, forming a crystalline system with β-C2S, C2F, and RO phases as its core, fully exposing the active calcium components. Porosity and adsorption properties are significantly improved, and the original ore Pb... 2+The adsorption capacity was only 60.34 mg / g, but after coarse grinding, it jumped to 161.88 mg / g, an increase of 168.3% compared with the original ore.

[0053] 3. Characteristic Analysis of Mechanically Activated Modified and Finely Ground Steel Slag

[0054] Take 10g of coarsely ground steel slag and place it in a high-energy ball mill. The ball milling parameters are: time 20h, speed 480r / min, ball-to-material mass ratio 6:1, and grinding ball gradation of Φ10mm:Φ8mm:Φ3.5mm=0:2:8 (quantity ratio). After ball milling, pass through a 500-mesh sieve to obtain finely ground steel slag powder.

[0055] After high-energy ball milling, the average particle size of the three main phases (RO phase, anorthite phase, and β-C2S phase) of finely ground steel slag (S1) decreased by 22.9%, 31.4%, and 19.7%, respectively, while the specific surface area significantly increased to 15.6 m² / g. This phase refinement provided a structural basis for the full exposure of active sites; Pb 2+ The adsorption capacity reached 197.13 mg / g, which is 226.7% higher than that of the original ore, verifying the effectiveness of mechanical activation.

[0056] 4. Chemical composite modification and construction of composite structures

[0057] To overcome the performance bottleneck of single mechanical modification, chemical composite modification was carried out on finely ground steel slag (S1). 4g of S1 sample was weighed and placed in a beaker containing 100mL of 2mol / L NaHCO3 solution. The beaker was placed in a 25℃ constant temperature water bath, and ultrasonic treatment was performed for 20min using an ultrasonic cleaner (40KHz, 200W). Subsequently, 16g of inoculum biochar (HBC650) (biochar:steel slag = 4:1) was added, the temperature was raised to 60℃, and ultrasonic treatment was continued for 1h.

[0058] During chemical activation, NaHCO3 consumes unstable f-CaO to improve long-term stability and promotes the hydration of β-C2S to form CSH gel. NaHCO3 hydrolyzes to produce HCO3⁻ and CO3. 2- Ions, and Ca dissolved in steel slag 2+ Mg 2+ The reaction of free calcium oxide (f-CaO) and calcium hydroxide (CH) generates stable CaCO3 and MgCO3 nanocrystals, which act as nucleating agents. The abundant oxygen-containing functional groups (-OH, -COOH) on the surface of the biochar provide a large number of active nucleation sites, allowing the new biophase to grow in situ on its surface and in the pores, forming a tightly cross-linked "biochar-steel slag" composite structure, achieving synergy between physical and chemical processes. At the same time, the carboxyl and hydroxyl functional groups introduced by the biochar can react with Pb. 2+ It forms a stable complex, and its lightweight properties also improve the material's dispersibility and reduce particle agglomeration.

[0059] 5. Post-processing

[0060] After the reaction, solid-liquid separation was performed using a vacuum filtration device (vacuum speed 1.8 m³ / h, 0.45 μm filter membrane), and the filter cake was repeatedly washed with deionized water to remove residual NaHCO3 and soluble impurities. The filter cake was then dried in an oven at 102 °C for 30 min to obtain the steel slag-based adsorbent, named MS1.

[0061] 6. Characterization and performance of the final adsorbent (MS1)

[0062] Structure, morphology, and adsorption performance: SEM showed that MS1 formed a typical loaded composite structure, with a large number of micron-sized steel slag particles loaded on the surface and pores of the biochar framework. BET analysis showed that the specific surface area of ​​finely ground steel slag was only 1.666 m² / g, the pore volume was 0.007639 cm³ / g, and the pore size was 1.633 nm. After chemical composite modification, the specific surface area of ​​the steel slag-based adsorbent (MS1) was significantly increased to 9.541 m² / g (about 473% higher than that of finely ground steel slag), the pore volume increased to 0.02856 cm³ / g, and the average pore size broadened to 14.290 nm, with a significant improvement in the 2-10 nm mesoporous range. Under the conditions of pH=4.5, initial Pb²⁺ concentration of 200 mg / L, dosage of 0.1 g / 50 mL, and shaking reaction at 25℃ for 2 h, MS1 achieved a Pb²⁺ removal rate of 99.5% and an equilibrium adsorption capacity of 265.70 mg / g, which is approximately 340.3% higher than that of the original steel slag (60.34 mg / g). This performance improvement stems from the synergistic effect of physical adsorption and chemical activity: the well-developed micro-nano pores provide a large adsorption interface, while the functional groups such as -OH and -COOH on the surface of new biological phases like CSH gel and CaCO₃, as well as biochar, provide multiple specific chemical adsorption sites for ion exchange, surface complexation, and chemical precipitation, collectively achieving efficient and rapid capture of Pb²⁺.

[0063] Example 2

[0064] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 10 hours, ball milling intensity of 720 r / min, ball-to-material ratio of 3:1, grinding ball gradation of 0:2:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S2, and the adsorbent MS2 was prepared. Testing showed that MS2 effectively treated Pb... 2+ The adsorption capacity is 230.9661 mg / g.

[0065] Example 3

[0066] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 10 hours, ball milling intensity of 480 r / min, ball-to-material ratio of 6:1, grinding ball gradation of 0:2:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S3, and the adsorbent MS3 was prepared. Testing showed that MS3 effectively treated Pb... 2+ The adsorption capacity is 213.1295 mg / g.

[0067] Example 4

[0068] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 10 hours, ball milling intensity of 720 r / min, ball-to-material ratio of 6:1, grinding ball gradation of 0:2:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S4, and the adsorbent MS4 was prepared. Testing showed that MS4 effectively treated Pb... 2+ The adsorption capacity is 230.4664 mg / g.

[0069] Example 5

[0070] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 20 hours, ball milling intensity of 480 r / min, ball-to-material ratio of 3:1, grinding ball gradation of 0:2:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S5, and the adsorbent MS5 was prepared. Testing showed that MS5 effectively treated Pb... 2+ The adsorption capacity is 211.8076 mg / g.

[0071] Example 6

[0072] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 20 hours, ball milling intensity of 720 r / min, ball-to-material ratio of 3:1, grinding ball gradation of 0:2:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S6, and the adsorbent MS6 was prepared. Testing showed that MS6 effectively treated Pb... 2+ The adsorption capacity is 231.9979 mg / g.

[0073] Example 7

[0074] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 10 hours, ball milling intensity of 480 r / min, ball-to-material ratio of 3:1, grinding ball gradation of 0:2:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S7, and the adsorbent MS7 was prepared. Testing showed that MS7 effectively treated Pb... 2+ The adsorption capacity was 231.54 mg / g, the highest among all examples.

[0075] Example 8

[0076] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time 20 h, ball milling intensity 720 r / min, ball-to-material ratio 6:1, grinding ball gradation 0:2:8, no dispersed phase, and the resulting finely ground steel slag powder was named S8, thus preparing adsorbent MS8. Testing showed that MS8 effectively treated Pb... 2+ The adsorption capacity is 264.9830 mg / g.

[0077] Example 9

[0078] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time 20 h, ball milling intensity 480 r / min, ball-to-material ratio 6:1, grinding ball gradation (Φ10mm:Φ8mm:Φ3.5mm) 1:1:8, with no dispersed phase. The resulting finely ground steel slag powder was named S9, and adsorbent MS9 was prepared. Testing showed that MS9 effectively treated Pb... 2+ The adsorption capacity is 218.8739 mg / g.

[0079] Example 10

[0080] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 20 hours, ball milling intensity of 720 r / min, ball-to-material ratio of 6:1, grinding ball gradation of 1:1:8, and absence of dispersed phase. The resulting finely ground steel slag powder was named S10, and the adsorbent MS10 was prepared. Testing showed that MS10 effectively treated Pb... 2+ The adsorption capacity is 249.4935 mg / g.

[0081] Example 11

[0082] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 20 hours, ball milling intensity of 480 r / min, ball-to-material ratio of 6:1, grinding ball gradation of 0:2:8, and the addition of 5% Al2O3 as the dispersed phase. The resulting finely ground steel slag powder was named S11, and the adsorbent MS11 was prepared. Testing showed that MS11 effectively treated Pb... 2+ The adsorption capacity is 253.5678 mg / g.

[0083] Example 12

[0084] The process was essentially the same as in Example 1, except for the mechanical activation modification step: ball milling time of 20 hours, ball milling intensity of 480 r / min, ball-to-material ratio of 6:1, grinding ball gradation of 0:2:8, and the addition of 10% Al2O3 as the dispersed phase. The resulting finely ground steel slag powder was named S12, and the adsorbent MS12 was prepared. Testing showed that MS12 effectively treated Pb... 2+ The adsorption capacity is 252.0137 mg / g.

[0085] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass, characterized in that, Includes the following steps: (1) Steel slag pretreatment: The raw steel slag is crushed, screened, and then coarsely ground to obtain coarsely ground steel slag; (2) Mechanical activation modification: The coarse steel slag obtained in step (1) is placed in a high-energy ball mill for mechanical activation ball milling to obtain fine steel slag powder; (3) Chemical composite modification: The finely ground steel slag powder obtained in step (2) is mixed with sodium bicarbonate solution and subjected to a first water bath ultrasonic treatment. Then, bacterial bran biochar is added and a second water bath ultrasonic treatment is carried out to obtain a modified mixture. (4) Post-processing: The modified mixture obtained in step (3) is subjected to solid-liquid separation. The resulting solid product is washed and dried to obtain the biochar-steel slag micro-nano composite adsorbent.

2. The method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass according to claim 1, characterized in that, In step (1), the parameters for the coarse grinding process are: ball milling time of 4800 min, ball milling frequency of 50 Hz, grinding ball diameter of 10 mm, weight ratio of grinding ball to steel slag of 10:1, and the particle size of the steel slag after coarse grinding passes through a 150-250 mesh sieve. In step (2), the parameters of the mechanically activated ball mill are: ball milling time of 10-20h, ball milling speed of 480-720r / min, mass ratio of grinding balls to coarsely ground steel slag of 3-6:1, and grinding ball gradation of balls with diameters of 8-12mm, 6-9mm and 3-5mm in a quantity ratio of (0-1):(1-2):(6-10).

3. The method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass according to claim 1, characterized in that, In step (2), a dispersed phase is added during the mechanical activation ball milling process. The dispersed phase is Al2O3 or SiO2, and the amount added is 0-10% of the mass of the coarsely ground steel slag.

4. The method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass according to claim 1, characterized in that, In step (2), the obtained finely ground steel slag powder has a particle size that passes through a 400-600 mesh sieve, a specific surface area of ​​0.8-4.1 m² / g, and an average particle size of 6.5-15.0 μm.

5. The method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass according to claim 1, characterized in that, In step (3), the concentration of the sodium bicarbonate solution is 1-3 mol / L, and the solid-liquid ratio of the finely ground steel slag powder to the sodium bicarbonate solution is (2-6):100 g / mL. The parameters for the first water bath ultrasonic treatment are: water bath temperature of 20-30℃ and treatment time of 15-25 min; The parameters for the second water bath ultrasonic treatment are: water bath temperature of 50-70℃ and treatment time of 0.8-1.2h; Both ultrasonic treatments were performed at a frequency of 40 kHz and a power of 200 W.

6. The method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass according to claim 1, characterized in that, In step (3), the mass ratio of the amount of the bacterial bran biochar added to the mass ratio of the finely ground steel slag powder is (3-5):1; The preparation method of the biochar is as follows: the biomass raw material is pyrolyzed at 350-650℃ under a protective atmosphere or oxygen-limited conditions for 2-4 hours, the heating rate is 5-15℃ / min, and after natural cooling, it is ground through a 100-200 mesh sieve to obtain the biochar. The biomass raw materials are one or more of the following: mushroom residue, straw, rice husks, and sawdust.

7. The method for co-preparing biochar-steel slag composite adsorbent using steel slag and biomass according to claim 1, characterized in that, In step (4), the drying temperature is 80-120℃ and the time is 20-40 min. The solid-liquid separation is carried out by vacuum filtration with a pumping speed of 1.8 m³ / h and a filter membrane pore size of 0.45 μm.

8. A biochar-steel slag micro / nano composite adsorbent prepared by the method according to any one of claims 1-7, characterized in that, The adsorbent has a micro-nano structure of in-situ composite steel slag particles on the surface of biochar, with a specific surface area of ​​8.5-11 m² / g.

9. The application of the biochar-steel slag micro-nano composite structure adsorbent according to claim 8 in the treatment of wastewater containing heavy metal ions, characterized in that, The heavy metal ions include Pb. 2+ Cd 2+ Hg 2+ At least one of them.

10. The application of the biochar-steel slag micro-nano composite adsorbent according to claim 9 in the treatment of wastewater containing heavy metal ions, characterized in that, The treatment conditions are as follows: wastewater pH value is 3.5-5.5, adsorbent dosage is 0.5-5 g / L, reaction temperature is 15-35℃, and reaction time is 0.5-24 h.

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