Straw-based magnesium-iron modified biochar and industrial wastewater treatment method thereof

Abstract

This invention provides a straw-based magnesium-iron modified biochar and its industrial wastewater treatment method, belonging to the field of water treatment materials technology. The method includes: S1, mixing and impregnating straw powder with magnesium or iron salt solutions and conducting a hydrothermal reaction to promote metal ion penetration into the fiber interior and pre-complexation with oxygen-containing functional groups, obtaining a deeply modified precursor; S2, pyrolyzing the precursor under a protective atmosphere with limited oxygen, achieving in-situ simultaneous biomass carbonization and metal crystallization, thus firmly anchoring the metal active sites in the generated carbon skeleton. This invention effectively avoids metal agglomeration, endowing the material with a well-developed microporous-mesoporous structure and extremely low metal dissolution rate; it is not only resistant to acid leaching and desorption and has a long cycle life, but also specifically overcomes the "negative removal" defect of primary carbon in phosphorus removal, achieving highly efficient synergistic purification of total phosphorus, COD, and ammonia nitrogen in industrial wastewater.

Description

Technical Field

[0001] This invention belongs to the field of water treatment materials technology, and specifically relates to a straw-based magnesium-iron modified biochar and its industrial wastewater treatment method. Background Technology

[0002] With the acceleration of industrialization, especially in arid and semi-arid regions (such as Xinjiang in my country), the combined wastewater discharged from industrial parks is typically characterized by complex composition, high chemical oxygen demand (COD), high ammonia nitrogen, high total phosphorus (TP), and high salinity. This type of wastewater has poor biodegradability, making it difficult to achieve discharge standards using traditional biological treatment processes. Therefore, finding efficient and economical deep purification technologies has become an urgent need in the field of water pollution control. Adsorption methods, due to their advantages of simple operation, strong adaptability, and no secondary byproducts, are widely used in the final-end enhanced treatment of industrial wastewater. Biochar is often used for industrial wastewater adsorption, but raw straw-based biochar has inherent defects: its surface is electronegative and extremely lacks basic active sites, making it not only difficult to adsorb anionic phosphorus in wastewater, but also releasing its own phosphates, causing serious "negative removal"; at the same time, its limited specific surface area also results in extremely low adsorption capacity for high concentrations of COD and ammonia nitrogen. First, existing processes lack pre-bonding of metals and biomass at the molecular level, resulting in metal crystalline phases that only physically adhere to the carbon surface. When treating acidic, high-salt, and complex wastewater, the active metal sites are easily detached and dissolved, leading not only to a rapid decline in adsorption performance but also to secondary pollution of the water body.

[0003] Second, due to the lack of strong anchoring of metal components, a large amount of them will be lost in the essential industrial "acid pickling, desorption and regeneration" process, resulting in the material being completely scrapped after only 1-2 cycles, leading to high operating costs. Third, conventional high-concentration impregnation modification can easily cause severe aggregation of metal salts on the surface of biochar, directly blocking the original microporous structure of the material, resulting in a decrease in specific surface area instead of an increase, which greatly weakens the mass transfer and interception efficiency of macromolecular organic matter (COD).

[0004] Therefore, a straw-based magnesium-iron modified biochar and its industrial wastewater treatment method are proposed. Summary of the Invention

[0005] In view of this, the present invention provides a straw-based magnesium-iron modified biochar and its industrial wastewater treatment method to solve or alleviate the technical problems existing in the prior art, and at least provides a beneficial option.

[0006] The technical solution of this invention is achieved as follows: a method for preparing straw-based magnesium-iron modified biochar, the specific steps of which are as follows: S1. Preparation of metal deep-modification precursors Corn stalks or cotton stalks are selected, and after washing and drying, their particle size is controlled to be 60-100 mesh using mechanical crushing technology. This particle size range can ensure that the cellulose, hemicellulose and lignin structure inside the stalks are fully exposed, providing abundant microscopic channels for the penetration of metal ions.

[0007] Mix straw powder with modified metal salt solution (magnesium chloride hexahydrate solution or ferric chloride hexahydrate solution) at a solid-liquid ratio of 1 gram: 10 milliliters.

[0008] First, the dense encapsulation structure of the straw cell wall is broken by 30 minutes of ultrasonic treatment, utilizing the cavitation effect of ultrasound. Subsequently, the mixture was kept at a constant temperature of 25°C and 200 rpm for 12 hours, followed by 12 hours of static impregnation, so that magnesium ions (concentration of 1.5 mol / L) or iron ions (concentration of 0.3 mol / L) could diffuse fully from the surface of the biomass into the deep pores under the influence of concentration gradient and mechanical drive.

[0009] Hydrothermal pre-complexation reaction: The impregnated mixture is placed in a reactor and reacted at 180℃ for 2 hours. Under subcritical hydrothermal conditions, the macromolecular components in the straw undergo partial hydrolysis, releasing a large number of active oxygen-containing functional groups such as hydroxyl and carboxyl groups. These functional groups undergo strong chemical chelation and complexation reactions with the infiltrated metal ions, thereby achieving the initial fixation of metal components on the biomass matrix at the molecular level, forming a precursor for deep metal modification.

[0010] S2, In-situ Synchronized Pyrolysis and Site Anchoring The precursor obtained in step S1 was pyrolyzed at a rate of 10 °C / min to 600 °C under a nitrogen protective atmosphere for 2 hours.

[0011] During oxygen-limited pyrolysis, the biomass matrix undergoes carbonization, dehydrogenation, and aromatization, reconstructing into a highly conductive carbon framework. Simultaneously, the metal components pre-complexed in the framework undergo in-situ crystalline phase transformation, generating nanoscale active oxide crystals.

[0012] Because the metal ions are deeply bound to the matrix through hydrothermal action before pyrolysis, the generated metal oxides are firmly "anchored" to the inner wall of the generated carbon skeleton or embedded in the microporous structure, forming a very strong chemical bond. This not only prevents the aggregation of metal active sites at high temperatures, but also ensures that the material has an extremely low metal leaching rate in subsequent wastewater treatment applications.

[0013] Furthermore, the modified biochar prepared by the above method in this invention: Through the combined effects of hydrothermal and pyrolysis, the material forms a rich microporous-mesoporous composite structure, with the specific surface area of ​​magnesium-modified biochar reaching 341-391 m². 2 / g, the specific surface area of ​​iron-modified biochar reaches 360-403m². 2 / g.

[0014] Magnesium-modified biochar has highly active crystalline magnesium oxide and magnesium hydroxide loaded on its surface; iron-modified biochar has crystalline iron(III) oxide and iron(II) oxide loaded on its surface, which have magnetic and redox activities.

[0015] This invention further provides the application of the above-mentioned materials in the advanced treatment of industrial wastewater: Add modified biochar at a dosage of 3 g / L to wastewater containing COD, ammonia nitrogen, or total phosphorus, and stir for 160-240 minutes.

[0016] When treating high-phosphorus wastewater, magnesium-modified biochar is selected. By adjusting the pH of the wastewater to 7-11, the electrostatic attraction, surface complexation, and precipitation of magnesium salts are utilized to achieve efficient removal of total phosphorus.

[0017] When treating high COD wastewater, iron-modified biochar is selected. Under pH 5-9 conditions, the porous adsorption capacity and electron transfer characteristics of iron oxides are utilized to achieve rapid interception and transformation of organic pollutants.

[0018] Using a 1.5 mol / L hydrochloric acid solution as the desorption solvent, the adsorption-saturated biochar was desorbed at a solid-liquid ratio of 1 g: 20 mL (500 rpm, 4 hours). The regenerated material after acid washing can be reused, reducing industrial operating costs.

[0019] The embodiments of the present invention have the following advantages due to the adoption of the above technical solutions: I. This invention employs a dual coupling process of "hydrothermal pre-complexation" and "in-situ pyrolysis" to promote the deep chelation of oxygen-containing groups generated by straw hydrolysis with metal ions and simultaneous in-situ carbonization. This achieves the effect of chemically anchoring the active sites of metals in the carbon skeleton, thereby realizing an extremely low metal leaching rate and fundamentally eliminating secondary heavy metal pollution in water bodies.

[0020] Second, this invention achieves strong chemical bonding between the in-situ generated metal oxide nanocrystals and the carbon matrix through molecular-level precursor pre-complexation, thereby enhancing the acid leaching and desorption resistance of the material and realizing the efficient recycling of modified biochar, which significantly reduces the long-term operating cost of industrial wastewater.

[0021] Third, this invention achieves the effect of avoiding surface aggregation and expanding the microporous-mesoporous composite structure by directly performing oxygen-limited pyrolysis on the precursor pre-bound with metal ions and using metal salt as an in-situ activator. This enables targeted overcoming of the "negative removal" defect of the original straw charcoal phosphorus removal and improves the material's high-capacity synergistic purification capacity for macromolecular organic matter (COD), ammonia nitrogen and total phosphorus in complex wastewater.

[0022] The above overview is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features of the invention will become readily apparent from the accompanying drawings and the following detailed description. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart of the preparation method steps of the present invention. Detailed Implementation

[0025] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0026] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0027] like Figure 1 As shown, this embodiment of the invention provides a method for preparing straw-based magnesium-iron modified biochar, comprising the following steps: S1. Straw powder is mixed and impregnated with a modified metal salt solution, and a hydrothermal reaction is carried out. The hydrothermal environment promotes the penetration of metal ions into the interior of the straw fibers and pre-complexes them with oxygen-containing functional groups to obtain a precursor for deep metal modification. The modified metal salt solution is a magnesium salt solution or an iron salt solution. The straw is corn straw or cotton straw, with a particle size of 60-100 mesh; the modified metal salt solution is magnesium chloride hexahydrate solution or ferric chloride hexahydrate solution; The mixed impregnation includes: first, ultrasonic treatment for 30 minutes, then constant temperature oscillation at 25°C for 12 hours, and finally static impregnation for 12 hours; The solid-liquid ratio of the straw powder to the modified metal salt solution is 1g:10mL; when using a magnesium salt solution, its concentration is 1.5mol / L; when using an iron salt solution, its concentration is 0.3mol / L. The hydrothermal reaction conditions are: reaction at 180℃ for 2 hours; S2. The precursor is subjected to oxygen-limited pyrolysis under a protective atmosphere. By synchronizing the biomass carbonization and metal component crystallization processes in situ, the metal active sites are anchored in the generated carbon skeleton structure to obtain the straw-based magnesium-iron modified biochar. The conditions for oxygen-limited pyrolysis are as follows: under a nitrogen atmosphere, the temperature is increased to 600°C at a heating rate of 10°C / min, and then held at that temperature for 2 hours.

[0028] Example 1: Preparation of magnesium-modified cotton straw biochar S1, Metal Deep-Modified Precursor: Take dry cotton stalks, crush them in a pulverizer, and then pass them through a 60-100 mesh sieve to obtain cotton stalk powder.

[0029] Weigh 10g of straw powder and add it to 100mL of magnesium chloride hexahydrate solution with a concentration of 1.5mol / L.

[0030] Place the mixture in an ultrasonic cleaner and ultrasonically treat for 30 minutes.

[0031] It was then moved to a desktop constant temperature oscillator and oscillated continuously for 12 hours at 25°C and 200 rpm.

[0032] After oscillation, allow the fibers to stand at room temperature for 12 hours to allow magnesium ions to fully diffuse into the fiber interior.

[0033] The mixed solid-liquid mixture was transferred to a 150 mL hydrothermal reactor lined with polytetrafluoroethylene and reacted at 180°C for 2 h.

[0034] After cooling, the solid was filtered and dried in an oven at 80°C to constant weight.

[0035] S2, In-situ Synchronous Pyrolysis: The dried precursor was placed in a tube furnace and heated under a nitrogen protective atmosphere (flow rate 100 mL / min).

[0036] The heating rate was set to 10°C / min, and the temperature was raised to 600°C and then kept at that temperature for 2 hours for pyrolysis.

[0037] After the furnace temperature drops to room temperature, remove the product and rinse it repeatedly with deionized water until the washing solution is neutral and no chloride ions are detected.

[0038] Finally, the mixture was dried at 105°C to obtain magnesium-modified cotton straw biochar. Example 2: Preparation of iron-modified corn straw biochar S1. Precursor preparation: Corn stalk powder (60 mesh) was used, and the impregnation solution was changed to a 0.3 mol / L ferric chloride hexahydrate solution. The remaining conditions for ultrasonication, oscillation (12 h), standing (12 h), and hydrothermal reaction (180°C, 2 h) were the same as in Example 1.

[0039] S2. Pyrolysis anchoring: Under nitrogen protection, the temperature was increased to 600°C at 10°C / min and pyrolyzed for 2 hours. After cooling, washing, and drying, iron-modified corn straw biochar was obtained.

[0040] Example 3: Preparation of magnesium-modified corn straw biochar Corn stalks were used as the substrate and the preparation was carried out according to the process described in Example 1 (1.5 mol / L magnesium chloride hexahydrate, hydrothermal treatment at 180°C, pyrolysis at 600°C).

[0041] Example 4: Preparation of iron-modified cotton straw biochar Cotton stalks were used as the substrate and prepared according to the process described in Example 2 (0.3 mol / L ferric chloride hexahydrate, hydrothermal treatment at 180°C, pyrolysis at 600°C).

[0042] Comparative Example 1: Raw cotton stalk biochar The sieved cotton stalk powder was placed directly into a tube furnace and heated to 600°C at a rate of 10°C / min under nitrogen protection, and held at that temperature for 2 hours. No hydrothermal or metal modification treatment was performed.

[0043] Comparative Example 2: Magnesium-modified biochar prepared by conventional impregnation method First, prepare raw cotton stalk biochar according to the method of Comparative Example 1.

[0044] The prepared raw biochar was added to a 1.5 mol / L magnesium chloride hexahydrate solution, shaken for 12 hours, and then dried directly.

[0045] Calcination was carried out twice at 600°C for 1 hour.

[0046] Test Example 1: The materials obtained in the above embodiments and comparative examples were characterized and analyzed, and the results are shown in Table 1 below: Table 1: Test Results of Physicochemical Properties of Biochar Materials

[0047] The modified biochar prepared in this embodiment of the invention has a higher specific surface area than the original biochar (more than 4 times higher) and that prepared by traditional modification methods. This indicates that the "hydrothermal-pyrolysis synchronization" process can effectively widen the pores and make the metal active sites uniformly distributed.

[0048] Test Example 2: Performance Evaluation of Industrial Wastewater Treatment The experiment used comprehensive wastewater from an industrial park, with the following water quality indicators: COD 450 mg / L, total phosphorus (TP) 15 mg / L, and ammonia nitrogen 45 mg / L.

[0049] Experimental conditions: biochar dosage 3 g / L, reaction time 160 min, temperature 25°C.

[0050] Table 2: Comparison of the purification effects of different materials on industrial wastewater

[0051] In this test, the original straw charcoal (Comparative Example 1) showed a secondary release of phosphorus (negative removal rate), while the magnesium-modified charcoal (Example 1) achieved a removal rate of over 84%, demonstrating the strong binding of magnesium active sites to phosphorus.

[0052] COD removal advantages: Iron-modified carbon (Example 4) has the best COD removal effect, up to 95.37%, which far exceeds traditional methods.

[0053] The performance of Example 1 is significantly better than that of Comparative Example 2, which confirms the key role of the "hydrothermal pre-complexation" of the present invention in improving adsorption performance.

[0054] Test Example 3: Cyclic Stability and Metal Loss Test Desorption and regeneration experiments were conducted on Examples 1 and 2.

[0055] Desorption process: Use 1.5 mol / L hydrochloric acid solution, solid-liquid ratio 1 g: 20 mL, stir at 500 rpm for 4 h.

[0056] Reuse: After 5 "adsorption-regeneration" cycles, the retention rate of pollutant removal and the amount of metal ions dissolved into the water are tested.

[0057] Table 3: Recycling performance and metal stability test (after 5th cycle)

[0058] As can be seen from Table 3, the biochar prepared by the present invention (Examples 1 and 4) has a much higher performance retention rate after 5 cycles than the traditional impregnation method (Comparative Example 2). The metal leaching of the materials in the examples is extremely low, only 0.15-0.22 mg / L, while the metal leaching of the materials prepared by the traditional method is as high as 4.85 mg / L.

[0059] Test Example 4: Adsorption Kinetics and Mechanism Analysis (1) Adsorption kinetics test At 25°C and pH=7, 3 g / L of Example 2 was added to wastewater containing 450 mg / L COD, and samples were taken and measured at different time points (10, 30, 60, 120, 160, and 240 minutes).

[0060] Test Results: The goodness of fit between the experimental data and the quasi-second-order dynamic model (R0) 2 It is as high as 0.99.

[0061] This indicates that the COD adsorption process of the biochar is mainly controlled by chemisorption, including electron sharing or electron transfer, which verifies the strong chemical affinity between the iron active sites and organic pollutants.

[0062] (2) Phosphorus removal mechanism test Example 1 after total phosphorus adsorption was analyzed using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR).

[0063] Test results: After adsorption, obvious precipitates of magnesium phosphate and magnesium hydrogen phosphate appeared on the surface of the material, and the intensity of the hydroxyl peak on the surface was significantly weakened.

[0064] This demonstrates that the main logic behind the phosphorus removal of this material is achieved through the surface complexation of magnesium oxide active sites and the synergistic effect of chemical precipitation with phosphate ions.

[0065] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various variations or substitutions within the technical scope disclosed in the present invention, and these should all be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing straw-based magnesium-iron modified biochar, characterized in that, Includes the following steps: S1. Straw powder is mixed and impregnated with a modified metal salt solution, and a hydrothermal reaction is carried out. The hydrothermal environment promotes the penetration of metal ions into the interior of the straw fibers and pre-complexes them with oxygen-containing functional groups to obtain a precursor for deep metal modification. The modified metal salt solution is a magnesium salt solution or an iron salt solution. S2. The precursor is subjected to oxygen-limited pyrolysis under a protective atmosphere. By synchronizing the biomass carbonization and metal component crystallization processes in situ, the metal active sites are anchored in the generated carbon skeleton structure to obtain the straw-based magnesium-iron modified biochar.

2. The method for preparing straw-based magnesium-iron modified biochar according to claim 1, characterized in that, In step S1: the straw is corn straw or cotton straw, with a particle size of 60-100 mesh; the modified metal salt solution is magnesium chloride hexahydrate solution or ferric chloride hexahydrate solution.

3. The method for preparing straw-based magnesium-iron modified biochar according to claim 1, characterized in that, In step S1: the mixed impregnation includes: first, ultrasonic treatment for 30 minutes, then constant temperature oscillation at 25°C for 12 hours, and finally static impregnation for 12 hours.

4. The method for preparing straw-based magnesium-iron modified biochar according to claim 1, characterized in that, In step S1: the solid-liquid ratio of the straw powder to the modified metal salt solution is 1g:10mL; when using magnesium salt solution, its concentration is 1.5mol / L; when using iron salt solution, its concentration is 0.3mol / L.

5. The method for preparing straw-based magnesium-iron modified biochar according to claim 1, characterized in that, In step S1, the hydrothermal reaction is carried out at 180°C for 2 hours.

6. The method for preparing straw-based magnesium-iron modified biochar according to claim 1, characterized in that, In step S2: the conditions for oxygen-limited pyrolysis are: under a nitrogen atmosphere, the temperature is increased to 600°C at a heating rate of 10°C / min, and the pyrolysis is maintained at this temperature for 2 hours.

7. A straw-based magnesium-iron modified biochar obtained by the preparation method of straw-based magnesium-iron modified biochar according to any one of claims 1 to 6, characterized in that: The specific surface area of ​​the magnesium-modified biochar is 341-391 m². 2 / g, with crystalline magnesium oxide and magnesium hydroxide loaded on the surface; the specific surface area of ​​the iron-modified biochar is 360-403 m² / g. 2 / g, with crystalline iron(III) oxide and iron(II) oxide loaded on the surface.

8. A method for applying straw-based magnesium-iron modified biochar as described in claim 7 in industrial wastewater treatment, characterized in that: The straw-based magnesium-iron modified biochar was added to industrial wastewater containing chemical oxygen demand (COD), ammonia nitrogen, or total phosphorus for adsorption reaction. The amount of biochar added was 3 g / L, and the adsorption reaction time was 160-240 minutes.

9. The application method according to claim 8, characterized in that: When removing total phosphorus from wastewater, magnesium-modified biochar is used, and the pH of the wastewater is adjusted to 7-11. When removing chemical oxygen demand (COD) from wastewater, iron-modified biochar is used, and the pH of the wastewater is adjusted to 5-9.

10. The application method according to claim 8, characterized in that, It also includes a desorption and regeneration step after adsorption saturation: using a 1.5 mol / L hydrochloric acid solution as the desorption liquid, the modified biochar after adsorption saturation is added to the desorption liquid at a solid-liquid ratio of 1 gram: 20 milliliters, stirred at 500 rpm for 4 hours, filtered, washed and dried.

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