Composite magnesium iron modified biochar and industrial wastewater purification method thereof

Abstract

This invention provides a composite magnesium-iron modified biochar and its method for purifying industrial wastewater, belonging to the technical field of functional materials for water treatment. The composite biochar of this invention uses porous biochar as a substrate, and through bimetallic impregnation, in-situ co-precipitation nucleation with pH control, and programmed temperature calcination, a composite crystalline phase of magnesium-iron layered bimetallic oxide and magnesium-iron spinel is anchored in-situ on its surface and internal pores. This material exhibits excellent magnetic field response characteristics and abundant bimetallic active sites. When added to composite wastewater, the bimetallic synergistic mechanism enables one-step simultaneous adsorption and degradation of COD, ammonia nitrogen, and total phosphorus; after the reaction, only an external magnetic field is needed to quickly achieve solid-liquid separation. This invention completely solves the problem of powder material recovery, and is resistant to desorption and regeneration, exhibiting excellent long-term recycling performance.

Description

Technical Field

[0001] This invention relates to the field of functional materials for water treatment, and in particular to a composite magnesium-iron modified biochar and its method for purifying industrial wastewater. Background Technology

[0002] Industrial complex wastewater (such as combined wastewater from chemical industrial parks and wastewater from large-scale livestock farming) typically contains high concentrations of chemical oxygen demand (COD), ammonia nitrogen, and total phosphorus, and also has high salinity. In existing wastewater treatment processes, the removal of different pollutants often corresponds to different optimal reaction conditions. For example, phosphorus removal often relies on precipitation under alkaline conditions, while the degradation of complex organic matter usually occurs under acidic or neutral conditions. This necessitates that existing water treatment projects typically employ multi-stage series processes and frequent pH adjustments, resulting in long process flows, high reagent consumption, and high operating costs. The use of modified biochar for adsorption and purification has become a research hotspot in recent years. However, existing modified biochar technologies have the following problems when treating complex wastewater; I. Existing single-metal modified (such as pure magnesium or pure iron modified) biochar has limited functions. Magnesium-modified materials have specific adsorption of phosphate but are not active enough in degrading macromolecular organic matter. Although iron-modified materials are beneficial for electron transfer to remove COD, they have extremely poor affinity for anionic phosphorus. It is difficult for both to achieve simultaneous and efficient removal of multiple pollutants in the same reaction system. Second, existing bimetallic composite modification methods mostly employ simple physical mixing or conventional co-impregnation pyrolysis. Magnesium and iron form independent phases on the carbon framework, lacking microscopic lattice-level bonding. This prevents the generation of synergistic effects of electron transfer and complementary active sites, resulting in low overall purification efficiency.

[0003] Therefore, a composite magnesium-iron modified biochar and its industrial wastewater purification method are proposed. Summary of the Invention

[0004] In view of this, the present invention provides a composite magnesium-iron modified biochar and its industrial wastewater purification method to solve or alleviate the technical problems existing in the prior art, and at least provides a beneficial option.

[0005] The technical solution of the present invention is achieved as follows: a composite magnesium-iron modified biochar, with porous biochar as the base, wherein magnesium-iron layered bimetallic oxide and magnesium-iron spinel composite crystal phase are in situ anchored on its surface and in its internal pores; the composite magnesium-iron modified biochar has magnetic field response characteristics, and its surface is enriched with a large number of basic hydroxyl groups and coordinated unsaturated metal active sites. The substrate is obtained by pyrolysis of corn stalks or cotton stalks, and its particle size is 60-100 mesh; in the composite crystalline phase, the molar ratio of magnesium to iron is 2:1 to 4:1.

[0006] The preparation method of the composite magnesium-iron modified biochar includes the following steps: S1. Bimetallic ion loading: Biochar is dispersed in a mixed solution containing magnesium and iron salts (total metal concentration 0.5-2.0 mol / L) to obtain a bimetallic saturated impregnation system. S2. In-situ co-precipitation nucleation: Under continuous stirring, an alkaline precipitant is slowly added to the system, and the pH value of the system is strictly controlled to be kept constant at 9.5-10.5, so that magnesium ions and iron ions co-precipitate on the surface of the carbon substrate and self-assemble to form a layered bimetallic hydroxide composite precursor. S3. Crystal phase evolution and solidification: The composite precursor is aged at 60-80℃, washed, and then calcined at 400-550℃ for 2-4 hours under nitrogen protection with a heating rate of 5℃ / min to cause a topological transformation of the layered structure and generate a magnetic composite crystal phase (LDO / spinel).

[0007] An industrial wastewater purification method based on the above-mentioned composite magnesium-iron modified biochar includes the following steps: P1. Adsorption performance: Add 2-5 g / L of composite magnesium-iron modified biochar to composite industrial wastewater (containing high concentrations of COD, ammonia nitrogen and total phosphorus) with the initial pH controlled between 6 and 9, and react for 120-240 minutes; utilize the synergistic effect of bimetallic sites to achieve one-step simultaneous purification of multiple pollutants in wastewater.

[0008] P2. External Magnetic Separation and Recovery: After adsorption, a magnetic field is applied outside the reaction vessel. The magnetic response characteristics of the material cause the suspended carbon to rapidly aggregate, achieving solid-liquid separation. The separated carbon can be recycled after desorption and regeneration.

[0009] The embodiments of the present invention have the following advantages due to the adoption of the above technical solutions: I. This invention utilizes a specific pH-controlled co-precipitation coupled with topological transformation to enable magnesium and iron to self-assemble into a composite lattice at the atomic level. In this unique structure, the enriched magnesium active sites target and capture phosphate ions through strong electrostatic and surface coordination interactions; simultaneously, the introduction of iron generates abundant oxygen vacancies within the lattice and opens electron transfer channels, greatly enhancing the degradation capacity for complex macromolecular organic matter (COD). Thus, without the need for deep pH pre-adjustment, dual-effect purification of total phosphorus and organic matter can be achieved simultaneously, significantly simplifying the existing treatment process for complex wastewater.

[0010] Second, while constructing an active crystalline phase, this invention utilizes the phase transformation of iron to endow the carbon substrate with excellent magnetic susceptibility. After the adsorption reaction is completed, only a conventional magnetic field needs to be applied outside the reaction system, and the suspended powdered modified biochar can quickly aggregate, settle, and separate from the water in a very short time, fundamentally solving the engineering problems of traditional powdered adsorbents being difficult to retrieve and easily causing the water to turn black.

[0011] 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

[0012] 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.

[0013] Figure 1 This is a flowchart illustrating the preparation of composite magnesium-iron modified biochar and its method for purifying industrial wastewater according to the present invention. Detailed Implementation

[0014] 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.

[0015] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0016] like Figure 1 As shown, this embodiment of the invention provides a composite magnesium-iron modified biochar, which uses porous biochar as a substrate, with magnesium-iron layered bimetallic oxide and magnesium-iron spinel composite crystal phases anchored in situ on its surface and in its internal pores. The composite magnesium-iron modified biochar has magnetic field response characteristics and its surface is enriched with a large number of basic hydroxyl groups and coordinated unsaturated metal active sites. The substrate is obtained by pyrolysis of corn stalks or cotton stalks, and its particle size is 60-100 mesh; In the composite crystalline phase, the molar ratio of magnesium to iron is 2:1 to 4:1.

[0017] Example 1: Preparation of composite magnesium-iron modified biochar with a magnesium to iron molar ratio of 2:1 S1. Washed and dried cotton stalks were pyrolyzed in a tube furnace at 500°C under limited oxygen for 2 hours. After cooling, the stalks were mechanically pulverized and passed through an 80-mesh sieve to obtain a raw biochar substrate. 10 grams of this raw biochar were weighed and added to 200 ml of a mixed salt solution containing 0.4 mol / L magnesium chloride, 0.2 mol / L ferric chloride, a magnesium to iron molar ratio of 2:1, and a total bimetallic concentration of 0.6 mol / L. The suspension was ultrasonically dispersed in an ultrasonic cleaner for 30 minutes to form a bimetallic saturated impregnation system.

[0018] S2. Transfer the above system to a thermostatic magnetic stirrer and stir vigorously at 300 rpm. Simultaneously, use a peristaltic pump to slowly add an alkaline precipitant, a mixed aqueous solution of 2.0 mol / L sodium hydroxide and 0.5 mol / L sodium carbonate, at a rate of 2 mL / min. During the addition, monitor the pH value in real time using a high-precision pH meter and strictly control the system to maintain a constant pH of 10.0. After the addition is complete, continue stirring for 2 hours to allow magnesium and iron ions to co-precipitate in situ in the pores and surface of the carbon substrate, self-assembling to form a layered bimetallic hydroxide composite precursor.

[0019] S3. The above-mentioned composite precursor suspension was placed in a constant temperature water bath at 65°C and allowed to age for 18 hours. After aging, it was filtered and repeatedly washed with deionized water until the filtrate was neutral. The obtained solid was vacuum dried at 80°C for 12 hours. Subsequently, it was placed in a tube furnace and heated to 450°C at a programmed heating rate of 5°C / min under a protective atmosphere of high-purity nitrogen (flow rate 100 mL / min), and calcined at this temperature for 3 hours. During the calcination process, the layered structure underwent a topological transformation. After naturally cooling to room temperature, composite magnesium-iron modified biochar with magnetic field responsiveness was obtained.

[0020] Example 2: Preparation of composite magnesium-iron modified biochar with a magnesium to iron molar ratio of 3:1 The preparation steps are basically the same as in Example 1, the main difference being the adjustment of the concentration ratio of the mixed salt solution and the calcination temperature: In step S1, the concentration of magnesium chloride is adjusted to 0.6 mol / L, the concentration of ferric chloride is maintained at 0.2 mol / L, the molar ratio of magnesium to iron is 3:1, and the total metal concentration is 0.8 mol / L.

[0021] In step S3, the constant temperature calcination temperature is adjusted to 500℃. The remaining steps and parameters are the same as in Example 1.

[0022] Comparative Example 1: Preparation of physically mixed bimetallic biochar Preparation of control materials using purely physical mixtures: The cotton stalk-based biochar was impregnated in a single metal impregnation method, with each solution being a 0.6 mol / L magnesium chloride solution and a 0.6 mol / L ferric chloride solution.

[0023] After drying, the biochar was calcined in nitrogen at 450°C to obtain pure magnesium-modified biochar and pure iron-modified biochar.

[0024] The prepared pure magnesium-modified biochar and pure iron-modified biochar were physically ground and mixed at a mass ratio of 2:1 to form Comparative Example 1.

[0025] Comparative Example 2: Preparation of Uncalcined Layered Bimetallic Hydroxide Composite Carbon Perform steps S1 and S2 as described in Example 1. In step S3, the aged and washed precursor is directly placed in a 60°C vacuum drying oven for constant temperature drying to obtain the product. This product does not undergo programmed calcination at 400-550°C, meaning that no topological transformation occurs to form oxygen vacancies and a spinel composite phase.

[0026] Test Example 1: Characterization of the Physicochemical Properties and Microstructure of Materials X-ray diffraction (XRD), specific surface area and magnetic field response characteristics of the materials in Example 1 and the comparative example were tested and analyzed.

[0027] (1) The results of microscopic crystal phase analysis confirmed the composite phase structure of the present invention: The diffraction pattern of Comparative Example 1 (physical mixture) shows only isolated magnesium oxide diffraction peaks and weak iron oxide diffraction peaks, indicating that the two phases exist completely independently.

[0028] The spectrum of Example 1 (composite carbon with a magnesium to iron molar ratio of 2:1) not only retains some characteristic peaks of layered bimetallic oxides, but also shows clear and sharp characteristic peaks of magnesium-iron spinel at a specific diffraction angle, proving that magnesium and iron are not simply physically stacked, but rather self-assembled into a true composite lattice structure through in-situ co-precipitation and calcination.

[0029] Advantages of the process of this invention: The original biochar had a specific surface area of ​​only 82.4 m² / g; the specific surface area of ​​Example 1 increased significantly to 415.8 m² / g; while the uncalcined Comparative Example 2 had only 142.3 m² / g. This demonstrates the crucial role of topological transformation calcination in step S3 in opening up pores and increasing specific surface area.

[0030] Tests revealed that the saturation magnetization of Example 1 reached 24.5 emu / g, exhibiting excellent magnetic response capability; while the uncalcined Comparative Example 2 showed no obvious magnetic response signal, proving that the magnetism depends on the magnesium-iron spinel composite crystal phase generated by the calcination phase transformation.

[0031] Test Example 2: Performance Evaluation of One-Step Purification of Industrial Complex Wastewater Experimental water sample: taken from the comprehensive wastewater discharge of a chemical industrial park. The water quality indicators are: initial hydrogen ion concentration index (pH value) of 7.2, initial chemical oxygen demand concentration of 600 mg / L, total phosphorus concentration of 25 mg / L, and ammonia nitrogen concentration of 50 mg / L. Experimental Procedure: At room temperature (25℃), the materials from the examples and comparative examples were added to 500 ml of the above wastewater at a dosage of 3 g / L, respectively. No pH pre-adjustment was required, and the mixture was stirred at a constant speed for 180 minutes. After the reaction, the removal efficiency and solid-liquid separation of each group were tested. The test results are shown in Table 1.

[0032] Table 1: Comparison of the effects of different materials on one-step purification of composite wastewater

[0033] in conclusion: Comparative Example 1 (pure physical mixing) showed low removal rates for all pollutants; while Example 1 achieved a total phosphorus removal rate of 92.4% and a chemical oxygen demand removal rate of 88.6%. This demonstrates that the "electrostatic coordination of magnesium active sites" and "electron transfer of iron active sites" in the composite crystalline phase produced a combined synergistic enhancement effect, realizing a one-step comprehensive purification of multiple pollutants.

[0034] Although Comparative Example 2 (uncalcined) has a certain phosphorus removal capacity, it has a very poor ability to degrade organic pollutants (chemical oxygen demand) (only 55.8%) due to the lack of oxygen vacancies and magnesium iron spinel crystal phase, and it does not have magnetic separation characteristics at all.

[0035] After the adsorption process is complete, all suspended powdered modified biochar is completely adsorbed and aggregated to the vessel wall within 15 seconds by applying a conventional magnetic field outside the reaction vessel, and the purified water instantly becomes clear again. In contrast, the non-magnetic comparative example must rely on time-consuming and energy-intensive high-speed centrifugation or filtration membrane systems to achieve solid-liquid separation.

[0036] Test Example 3: Evaluation of Desorption-Regeneration and Long-Term Cycling Stability To verify the industrial reusability of the material of the present invention, multiple cycles of desorption and regeneration tests were conducted on the material of Example 1 that was saturated with adsorption. Regeneration method: After rapidly separating the material from Example 1 using an external magnetic field, it was immersed in a low-concentration hydrochloric acid solution (0.1 mol / L) and shaken for 2 hours to release the adsorbed pollutants. Subsequently, a second separation was performed using an external magnetic field, followed by washing with deionized water until neutral. After drying, the regenerated carbon was re-immersed in composite wastewater of the same concentration for a new round of adsorption tests.

[0037] The purification efficiency was tested after five consecutive cycles.

[0038] Table 2: Overall purification efficiency retention after 5 consecutive cycles in Example 1

[0039] in conclusion: Test data fully demonstrates that after five consecutive rigorous cycles of "desorption treatment-magnetic separation and recovery," Example 1 still maintains a comprehensive removal rate of over 85% of the initial efficiency for both chemical oxygen demand (COD) and total phosphorus in the composite wastewater. This indicates that after the topological transformation in step S3, the layered bimetallic oxide and magnesium-iron spinel composite lattice structure anchored in situ on the biochar substrate is extremely robust, greatly resisting the leaching and stripping of metal active sites under desorption conditions. This fully verifies the long-term stability and outstanding economic reuse value of this invention in practical industrial applications.

[0040] Although Comparative Example 2 (uncalcined) has a certain phosphorus removal capacity, its COD degradation capacity is extremely poor (only 55.8%) due to the lack of oxygen vacancies and spinel crystal phase, and it is not magnetic.

[0041] After the adsorption of the material in the example is complete, simply attach a common strong magnet to the outside of the beaker, and all the powder will be completely adsorbed to the container wall within 15 seconds, and the water will instantly become clear again; while the comparative example, which lacks magnetism, requires time-consuming and energy-intensive high-speed centrifugation or filtration membranes for separation.

[0042] 4. Test Example 3: Evaluation of Desorption-Regeneration and Long-Term Cycling Stability To verify the industrial reusability of the material of the present invention, multiple cycle tests were conducted on the material of Example 1 that was saturated with adsorption. After separating the material from Example 1 using an external magnetic field, it was immersed in 0.1 mol / L low-concentration hydrochloric acid and shaken for 2 hours for desorption. Subsequently, it was separated again using a magnetic field, rinsed with deionized water until neutral, dried, and then used as regenerated carbon to be re-immersed in wastewater of the same concentration for a new round of adsorption testing. This cycle was repeated 5 times to determine the retention rate of its TP and COD removal efficiency.

[0043] Table 3: Purification efficiency retention after 5 consecutive cycles in Example 1

[0044] Data demonstrates that after five cycles of the "acid leaching-desorption-magnetic separation" process, Example 1 still maintains a comprehensive removal rate of over 85% of the initial efficiency for multiple pollutants in the wastewater. This indicates that the composite lattice anchored in situ after the topological phase transition in step S3 is extremely robust, greatly resisting the leaching and stripping of active metal sites under strong acid conditions, thus verifying the protective features of claim 10.

[0045] 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 composite magnesium-iron modified biochar, characterized in that: The composite magnesium-iron modified biochar uses porous biochar as a base, and magnesium-iron layered bimetallic oxide and magnesium-iron spinel composite crystal phase are in situ anchored on its surface and in its internal pores. The composite magnesium-iron modified biochar has magnetic field responsiveness and its surface is enriched with a large number of basic hydroxyl groups and coordinated unsaturated metal active sites.

2. The composite magnesium-iron modified biochar according to claim 1, characterized in that: The substrate is obtained by pyrolysis of corn stalks or cotton stalks, and its particle size is 60-100 mesh; In the composite crystalline phase, the molar ratio of magnesium to iron is 2:1 to 4:

1.

3. The composite magnesium-iron modified biochar according to claim 1, characterized in that, The composite magnesium-iron modified biochar is prepared by a method including the following steps: S1. Bimetallic ion loading: Biochar is dispersed in a mixed solution containing magnesium and iron salts to obtain a bimetallic saturated impregnation system. S2. In-situ co-precipitation nucleation: Under continuous stirring, an alkaline precipitant is added dropwise to the system to adjust the system's hydrogen ion concentration index (pH value) to 9.5-10.5, so that magnesium ions and iron ions co-precipitate on the carbon substrate surface and self-assemble to form a layered bimetallic hydroxide composite precursor. S3. Crystal phase evolution and solidification: The composite precursor is aged at 60-80°C, washed, and then calcined at a programmed temperature of 400-550°C under a protective atmosphere to cause a topological transformation of the layered bimetallic hydroxide and generate the magnetic composite crystal phase.

4. The composite magnesium-iron modified biochar according to claim 3, characterized in that: In step S1, the total concentration of magnesium and iron ions in the mixed solution of magnesium and iron salts is 0.5-2.0 mol / L; In step S2, the alkaline precipitant is a mixed aqueous solution of sodium hydroxide and sodium carbonate; In step S3, the heating rate of the calcination is 5°C / min, and the constant temperature calcination time is 2-4 hours.

5. A method for purifying industrial wastewater based on the composite magnesium-iron modified biochar according to any one of claims 1 to 4, characterized in that, Includes the following steps: P1. Simultaneous adsorption and degradation: The composite magnesium-iron modified biochar is added to a composite industrial wastewater containing high concentrations of chemical oxygen demand (COD), ammonia nitrogen, and total phosphorus. The reaction is carried out under stirring conditions. Through the synergistic effect of the bimetallic sites, a one-step simultaneous purification of multiple pollutants in the wastewater is achieved. P2. External magnetic separation and recovery: After the adsorption reaction is completed, a magnetic field is applied outside the reaction vessel. The magnetic response characteristics of the material are used to make the suspended modified biochar rapidly aggregate, thereby achieving solid-liquid separation.

6. The industrial wastewater purification method according to claim 5, characterized in that, In step P1: The initial hydrogen ion concentration index (pH value) of the composite industrial wastewater is controlled between 6 and 9, without the need for deep acid-base pre-adjustment. The dosage of the composite magnesium-iron modified biochar is 2-5 g / L, and the reaction time is 120-240 minutes.

7. The industrial wastewater purification method according to claim 5, characterized in that, In step P1: the synergistic effect of the bimetallic sites specifically includes: magnesium active sites targeting and removing phosphate ions in wastewater through electrostatic attraction, surface complexation and co-precipitation; iron active sites and oxygen vacancies intercepting and degrading macromolecular organic pollutants in wastewater through electron transfer and redox mechanisms.

8. The industrial wastewater purification method according to claim 5, characterized in that: The composite industrial wastewater is either wastewater from a comprehensive industrial park or high-concentration organic wastewater from agricultural sources, characterized by high salt, high phosphorus, and biodegradability.

9. The industrial wastewater purification method according to claim 5, characterized in that, It also includes a desorption and regeneration step after step P2: the modified biochar recovered by magnetic separation is subjected to oscillation desorption treatment using a low-concentration acidic solution or alkaline desorbent, so that the adsorbed pollutants are released from the pores and active sites; then, a second separation is performed using an external magnetic field, and after washing and drying, the regenerated modified biochar is recycled into step P1.

10. The industrial wastewater purification method according to claim 9, characterized in that: After at least five cycles of desorption, regeneration, and recycling, the composite magnesium-iron modified biochar still maintains a combined removal rate of over 85% of the initial efficiency for chemical oxygen demand (COD) and total phosphorus in the wastewater.

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