High-crystallinity modified iron oxide as well as preparation method and application thereof

Abstract

The invention provides a high-crystallinity modified iron oxide as well as a preparation method and application thereof, and relates to the technical field of organic solid waste treatment. The preparation method comprises the following steps: preparing a precursor MIL-88B by utilizing a hydrothermal method; the precursor MIL-88B is subjected to calcination treatment, and an iron oxide with the MIL-88B as the precursor is obtained; the temperature condition of the calcination treatment is 550-850 DEG C. The high-crystallinity modified iron oxide prepared by the preparation method improves methane production efficiency and stability, avoids competition of iron reducing bacteria, is stable in long-term operation and low in cost, and has both environmental and economic benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of organic solid waste treatment, and in particular to a high-crystallinity modified iron oxide, a preparation method thereof, and an application thereof. Background Art

[0002] In today's global context of increasing attention to sustainable development and environmental protection, the efficient treatment and resource utilization of organic solid waste has become an important topic in the field of environmental science. With the acceleration of urbanization and the expansion of industrial production, the amount of organic solid waste generated continues to increase. How to properly treat these wastes while achieving resource recovery and energy utilization is a problem that needs to be solved urgently. Anaerobic digestion technology, as a green and low-carbon method for treating organic solid waste, has attracted widespread attention due to its significant advantages in reducing waste volume, reducing greenhouse gas emissions, and producing renewable energy. Through anaerobic digestion, the organic matter in organic solid waste can be decomposed by microorganisms in an oxygen-free environment and eventually converted into valuable energy products such as biomethane, providing a new approach to solving the problems of energy shortage and environmental pollution.

[0003] However, existing anaerobic digestion technology still faces many challenges in practical application. Although anaerobic digestion can convert organic waste into biomethane and realize the resource utilization of waste, the stability and efficiency of this technology are limited by many factors during actual operation. First, the composition of organic solid waste is complex, and it contains a large amount of difficult-to-degrade organic matter. These substances may cause the reaction rate to be slow during the anaerobic digestion process, affecting the operating efficiency of the entire system. Secondly, if the intermediate products produced during the anaerobic digestion process, such as volatile fatty acids, cannot be decomposed by microorganisms in time, they may accumulate in the reactor, inhibit the activity of microorganisms, and thus affect the stable operation of the system. In addition, the presence of some toxic and harmful substances may also have an inhibitory effect on microorganisms, further reducing the efficiency of anaerobic digestion. These factors work together to make anaerobic digestion technology face many difficulties in large-scale application, limiting its further promotion in the field of organic solid waste treatment.

[0004] In existing anaerobic digestion technologies, the use of iron oxides is considered a potential solution for enhancing the conversion of organic waste to methane. However, conventional iron oxides present significant stability issues in practical applications. During the anaerobic digestion process, conventional iron oxides are prone to undergoing valence state changes, with Fe³⁺ partially reduced to Fe²⁺, resulting in changes in their chemical properties and crystal structure. These valence and structural changes not only reduce the catalytic activity of the iron oxide but also impair its electron transfer capacity. Furthermore, conventional iron oxides are prone to enriching iron-reducing bacteria under anaerobic conditions. Iron-reducing bacteria use Fe³⁺ as an electron acceptor, reducing Fe³⁺ to Fe²⁺, thereby promoting the decomposition of organic matter. However, this iron reduction reaction causes iron-reducing bacteria to compete with methanogens for key intermediates (such as acetic acid and hydrogen), thereby reducing methane yield. Excessive iron-reducing bacteria can also inhibit the activity of methanogens, further reducing the efficiency of the anaerobic digestion system. The stability issues of conventional iron oxides become more pronounced during long-term anaerobic digestion. Due to changes in valence and structure, conventional iron oxide gradually loses its catalytic activity over long-term operation, leading to a gradual decline in methane production efficiency. Furthermore, conventional iron oxide is easily lost during the reaction or becomes encapsulated in biological sludge, making it difficult to recycle and reuse, increasing operating costs.

[0005] In summary, existing anaerobic digestion technology has many defects in the treatment of organic solid waste. On the one hand, the accumulation of complex organic components and intermediates leads to a slow reaction rate, affecting the stability and efficiency of the system; on the other hand, conventional iron oxide has poor stability during the anaerobic digestion process, and is prone to valence and structural changes, which triggers the enrichment of iron-reducing bacteria, leading to competition with methanogens and reducing the yield of methane. These problems not only limit the large-scale application of anaerobic digestion technology in the treatment of organic solid waste, but also restrict its potential in resource recovery and energy utilization. Therefore, there is an urgent need to develop a new technology or new method that can effectively solve the above problems, so as to improve the efficiency and stability of anaerobic digestion and promote the efficient treatment and resource utilization of organic solid waste.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of the present invention is to provide a high-crystallinity modified iron oxide and its preparation method and application. The high-crystallinity modified iron oxide prepared by the preparation method improves the methane production efficiency, has stability to avoid competition from iron-reducing bacteria, is stable in long-term operation and has low cost, and has both environmental and economic benefits.

[0008] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted: In a first aspect, the present invention provides a method for preparing a modified iron oxide with high crystallinity, comprising: The precursor MIL-88B was prepared by hydrothermal method; The precursor MIL-88B is calcined to obtain iron oxide with MIL-88B as a precursor.

[0009] In an optional embodiment, the temperature condition of the calcination treatment is 550°C~850°C.

[0010] In an optional embodiment, the holding time of the calcination treatment is not less than 2 hours.

[0011] In an optional embodiment, the precursor MIL-88B is prepared by a hydrothermal method, comprising: mixing N,N-dimethylformamide, FeCl3·6H2O, terephthalic acid, and NaOH to obtain a first mixture; The first mixture is subjected to a hydrothermal treatment and dried to obtain the precursor MIL-88B.

[0012] In an optional embodiment, the heating target temperature of the hydrothermal treatment is 100°C.

[0013] In an optional embodiment, during the hydrothermal treatment, the time for maintaining the constant temperature after heating is not less than 12 hours.

[0014] In a second aspect, the present invention provides a modified iron oxide with high crystallinity, which is prepared using the method for preparing the modified iron oxide with high crystallinity as described in any one of the aforementioned embodiments.

[0015] In a third aspect, the present invention provides a method for preparing a supported iron oxide, comprising: Dispersing the modified iron oxide with high crystallinity as described in the previous embodiment in water, and adding sodium alginate to form a second mixture; heating the second mixture while continuously stirring to form a mixed solution; adding the mixed solution into a calcium chloride solution to carry out a curing reaction, and obtaining a cured product after washing; After drying the solidified product, a supported iron oxide is obtained; The temperature of the heating treatment is 80°C; and / or, The stirring rate of the continuous stirring is 600 rpm; and / or, The continuous stirring time is 30 minutes; and / or, The mixed solution is added to the calcium chloride solution by adding the mixed solution dropwise to the calcium chloride solution at a rate of 2 mL / min; and / or, The calcium chloride solution is a calcium chloride solution with a mass concentration of 2%; and / or, The curing reaction time is 30 minutes; and / or, The drying temperature of the drying process is 60° C.; and / or, The drying time of the drying process is 4 hours.

[0016] In a fourth aspect, the present invention provides a supported iron oxide prepared using the method for preparing the supported iron oxide described in the aforementioned embodiment.

[0017] In a fifth aspect, the present invention provides a use of the highly crystalline modified iron oxide as described in the aforementioned embodiment or the supported iron oxide as described in the aforementioned embodiment in the anaerobic digestion treatment of organic solid waste.

[0018] The preparation method of high-crystallinity modified iron oxide provided in this application, first, high-crystallinity MDF shows excellent methane production efficiency in anaerobic digestion, and the higher its crystallinity, the better the methane production effect. For example, MDF850 obtained by calcining at 850°C has the highest methane production, reaching 334.01±4.21 mL / g VSadded. Secondly, MDF has extremely high stability. The valence and structure of its iron hardly change during anaerobic digestion, which can effectively avoid the competition between the iron reduction reaction and the methane production process caused by the enrichment of iron-reducing bacteria, thereby significantly improving the methane production efficiency. In addition, MDF showed a stable methanogenesis-promoting effect in long-term experiments. Its cumulative methane production at different stages was higher than that of the conventional iron oxide group, and it could still maintain efficient methane production without supplementing materials. This method also has the advantages of simple preparation process, low cost, and strong adjustability. It can prepare MDF with different crystallinity according to actual needs and further optimize its performance. By improving the methane production efficiency of anaerobic digestion, MDF can not only better convert organic solid waste into biomethane and reduce greenhouse gas emissions, but also reduce waste treatment costs, with significant environmental and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 For the purpose of this application, we investigated the methane production graph of anaerobic digestion of different concentrations of MDF and conventional iron oxide in Experiment 1; Figure 2 For the purpose of this application, the XPS and XRD patterns of MDF and conventional iron oxide before and after anaerobic digestion in Experiment 1 were investigated; Figure 3 For the purpose of this application, the relative abundance graph of microbial genus levels in MDF and conventional iron oxide (5 g / L) in Experiment 1 was investigated; Figure 4 For this application, we examined the methane production graph of the long-term experiment with MDF (5 g / L) and commercial iron oxide (5 g / L) in Experiment 2; Figure 5 For the purpose of this application, the XPS spectra of conventional iron oxide and MDF after long-term reaction in Experiment 2 were investigated; Figure 6 For the purpose of this application, the XRD patterns of MDF with different crystallinity in Experiment 3 were investigated; Figure 7 The methane production diagram of anaerobic digestion of MDF with different crystallinity in Experiment 3 was investigated for this application. DETAILED DESCRIPTION

[0021] The embodiments of the present invention will be described in detail below with reference to the examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention. Where specific conditions are not specified in the examples, the methods were performed according to conventional conditions or the conditions recommended by the manufacturer. Where the manufacturers of the reagents or instruments are not specified, they are all commercially available conventional products.

[0022] The present invention provides a method for preparing a modified iron oxide with high crystallinity, comprising: Step S1, preparing the precursor MIL-88B by a hydrothermal method.

[0023] As mentioned above, the hydrothermal method involves a chemical reaction in a high-temperature / high-pressure aqueous solution environment. In this application, it is used to synthesize metal-organic framework (MOF) materials. The advantages of the hydrothermal method are mild reaction conditions and the ability to precisely control the material synthesis process.

[0024] MIL-88B is a metal-organic framework (MOF) composed of iron ions and organic ligands. The hydrothermal synthesis of MIL-88B yields a precursor material with a specific structure and porosity, providing a foundation for subsequent modification.

[0025] Step S2: calcining the precursor MIL-88B to obtain iron oxide with MIL-88B as the precursor.

[0026] As mentioned above, calcination is the process of heat treating the precursor material at high temperature, the purpose of which is to remove the organic ligands and precipitate the metal oxide from the MOF structure to form modified iron oxide (MDF) with specific properties.

[0027] Calcination temperature is a key factor affecting the crystallinity of the material. Higher calcination temperatures can increase the material's crystallinity, thereby improving its performance in anaerobic digestion. Experiments in this application show that increasing the calcination temperature increases the crystallinity of MDF and improves its methane production.

[0028] In this embodiment, by controlling the calcination temperature, MDF with high crystallinity can be prepared. Materials with high crystallinity have better stability and higher catalytic activity, and can significantly improve the methane production efficiency of anaerobic digestion. MDF shows extremely high stability during anaerobic digestion, and the valence and structure of iron hardly change, avoiding the competition between the iron reduction reaction caused by the enrichment of iron-reducing bacteria and the methane production process. In long-term experiments, MDF shows a stable effect of promoting methane production, and can maintain efficient methane production even without supplementing materials.

[0029] In summary, modified iron oxides with high crystallinity were prepared by hydrothermal method and high-temperature calcination. This material showed excellent performance in anaerobic digestion and has important application value.

[0030] In some embodiments, the calcination temperature is 550° C. to 850° C. For example, the calcination temperature may be 550° C., 600° C., 650° C., 700° C., 750° C., 800° C., 850° C., and the like.

[0031] In some embodiments, the holding time of the calcination treatment is not less than 2 hours.

[0032] Precisely controlling the holding time of the calcination treatment is crucial in preparing highly crystalline modified iron oxides. First, the holding time must be sufficiently long to ensure that the organic ligands in the precursor, MIL-88B, can completely decompose and volatilize at high temperatures, yielding pure, structurally intact iron oxides. Furthermore, sufficient holding time ensures that the iron oxides fully crystallize and form a stable structure, preventing insufficient crystallinity from impacting subsequent performance.

[0033] Crystallinity is a key factor in determining material performance. An appropriate holding time can significantly improve a material's crystallinity, thereby enhancing its methane production efficiency during anaerobic digestion. However, excessive holding time can lead to excessive sintering and increased particle size, reducing the material's specific surface area and activity. Therefore, experimental optimization of the holding time is necessary to achieve an optimal balance between crystallinity and activity.

[0034] Furthermore, controlling the holding time is crucial to ensuring uniform heating of the material at high temperatures. Since temperature distribution in a high-temperature furnace can be uneven, a reasonable holding time can prevent chemical property differences caused by localized overheating or underheating, thereby ensuring material uniformity. Furthermore, a suitable holding time improves the thermal and chemical stability of the material, ensuring high performance over long-term operation. It also inhibits iron reduction reactions, preventing competition from iron-reducing bacteria that reduces methane production efficiency.

[0035] In summary, the calcination holding time was set to 2 hours based on experimental optimization results. This time setting is designed to ensure that the material can be fully converted to form a highly crystalline and stable structure while avoiding excessive sintering and inhomogeneity, thereby significantly improving the performance and long-term stability of the material in anaerobic digestion.

[0036] In some embodiments, the step S1, preparing the precursor MIL-88B by a hydrothermal method, comprises: Step S1: N,N-dimethylformamide, FeCl3·6H2O, terephthalic acid and NaOH are mixed to obtain a first mixture.

[0037] In this step, N,N-dimethylformamide (DMF), FeCl3·6H2O (ferric chloride hexahydrate), terephthalic acid (BDC) and NaOH (sodium hydroxide) are mixed in a certain proportion to form a first mixture.

[0038] The selection and proportion of these raw materials are based on the chemical structure and synthesis requirements of MIL-88B: (1) N,N-dimethylformamide (DMF): acts as a solvent and provides a reaction medium.

[0039] (2) FeCl3·6H2O: provides iron source and is a key precursor for the formation of iron oxides.

[0040] (3) Terephthalic acid (BDC): As an organic ligand, it coordinates with iron ions to form the framework structure of MIL-88B.

[0041] (4) NaOH: As an alkaline regulator, it promotes the reaction and ensures the smooth coordination reaction between iron ions and organic ligands.

[0042] Step S12: hydrothermally treating the first mixture and drying the mixture to obtain the precursor MIL-88B.

[0043] In this step, the mixture is placed in a hydrothermal reactor and reacted at a specific temperature and pressure. The high temperature and pressure conditions of the hydrothermal method accelerate the reaction rate and promote the coordination of iron ions with organic ligands, forming a MIL-88B precursor with a specific structure.

[0044] After the reaction is completed, the product is taken out from the reactor and the excess solvent and water are removed by drying to obtain a dry precursor MIL-88B.

[0045] The hydrothermal method allows for precise control of the synthesis of MIL-88B, ensuring a uniform pore structure and chemical composition. This precise control is crucial for its subsequent conversion to highly crystalline iron oxide via calcination. The hydrothermal method operates under relatively mild conditions, avoiding the stringent requirements of high temperature and high pressure on equipment and raw materials, thus reducing production costs. This method exhibits excellent reproducibility and scalability, making it suitable for both laboratory research and industrial production.

[0046] MIL-88B synthesized via the hydrothermal method exhibits excellent structural integrity and chemical stability, providing a high-quality precursor for subsequent calcination. This facilitates the formation of highly crystalline and stable iron oxides during calcination. High-quality precursors are essential for the preparation of high-performance modified iron oxides. Precise control of the hydrothermal synthesis process ensures efficient methanogenesis in anaerobic digestion.

[0047] In some embodiments, the target heating temperature of the hydrothermal treatment is 100°C.

[0048] In some embodiments, during the hydrothermal treatment, the time for maintaining the constant temperature after heating is not less than 12 hours.

[0049] During the hydrothermal treatment, the material may be first heated to the target heating temperature, and then maintained at a constant temperature for at least 12 hours while maintaining the target heating temperature.

[0050] In an embodiment of the present application, a high-crystallinity modified iron oxide is provided, which is prepared using the preparation method of the high-crystallinity modified iron oxide as described in any of the aforementioned embodiments.

[0051] The present invention provides a method for preparing a supported iron oxide, comprising: In step S100 , the modified iron oxide with high crystallinity as described in the above embodiment is dispersed in water, and sodium alginate is added to form a second mixture.

[0052] In this step, the highly crystalline modified iron oxide is first dispersed in water, and then sodium alginate is added to form a second mixture.

[0053] Among them, sodium alginate acts as a thickener and stabilizer in this step, which can improve the rheological properties of the mixture and make it more suitable for subsequent molding operations.

[0054] In step S200 , the second mixture is heated and continuously stirred to form a mixed solution.

[0055] Furthermore, the temperature of the heating treatment is 80°C; Furthermore, the stirring rate of the continuous stirring is 600 rpm; Furthermore, the stirring time of the continuous stirring is 30 minutes; In this step, the second mixture is heated to 80°C and stirred continuously at 600 rpm for 30 minutes. The purpose of heating and stirring is to thoroughly mix the mixture and form a stable mixed solution. The heating temperature of 80°C ensures the dissolution of the sodium alginate and the fluidity of the mixture, but does not increase the temperature too much, which may affect the material properties.

[0056] Step S300: adding the mixed solution into a calcium chloride solution to perform a curing reaction, and obtaining a cured product after washing.

[0057] Furthermore, the calcium chloride solution is a calcium chloride solution with a mass concentration of 2%.

[0058] Furthermore, the reaction time of the curing reaction is 30 minutes.

[0059] In this step, the mixed solution can be added dropwise to a 2% mass concentration calcium chloride solution at a rate of 2 mL / min to perform a curing reaction for 30 minutes.

[0060] Sodium alginate undergoes a cross-linking reaction in a calcium chloride solution, forming a calcium alginate gel that encapsulates and fixes the iron oxide particles. This dropwise addition method effectively controls the morphology and size of the solidified product, forming a uniform microspherical structure.

[0061] Step S400, drying the solidified product to obtain supported iron oxide; Preferably, the mixed solution is added to the calcium chloride solution in the following manner: The mixed solution was added dropwise to the calcium chloride solution at a rate of 2 mL / min; Furthermore, the drying temperature of the drying process is 60°C; Furthermore, the drying time of the drying treatment is 4 hours.

[0062] After the curing reaction is complete, the solidified product is washed with water to remove residual calcium chloride and other impurities on the surface. It can then be dried at 60°C for 4 hours to obtain the supported iron oxide. The washing and drying steps can further improve the purity and stability of the product, making it more suitable for subsequent applications.

[0063] The supported iron oxide prepared by the cross-linking reaction of sodium alginate and calcium chloride has good mechanical stability and dispersibility. The supported structure can effectively prevent the agglomeration of iron oxide particles during the reaction process, increase its specific surface area and activity in applications such as anaerobic digestion, and thus further improve the efficiency of methane production. This method can achieve the regulation of the morphology, size and structure of the supported iron oxide by precisely controlling parameters such as heating temperature, stirring rate, dripping rate and curing time, ensuring the uniformity and consistency of the product. This controllability enables the method to be optimized according to different application requirements and has a wide range of applicability. The sodium alginate and calcium chloride used are common, environmentally friendly materials with a wide range of sources and low cost. In addition, the preparation method is simple to operate, the conditions are mild, and it is easy to achieve industrial production, with high cost-effectiveness and environmental friendliness.

[0064] Supported iron oxides have significant application potential in anaerobic digestion. Their supported structure enhances the stability and activity of iron oxides, promoting efficient organic solid waste degradation and methanogenesis while reducing competition from iron-reducing bacteria, thereby improving system stability and methanogenesis efficiency. Beyond anaerobic digestion, supported iron oxides can also be used in other catalytic reactions, such as gas purification and wastewater treatment. Their excellent mechanical stability and dispersibility enhance the catalyst's lifespan and catalytic efficiency, promising broad application prospects.

[0065] The present application provides a supported iron oxide in an embodiment, which is prepared using the preparation method of the supported iron oxide described in the above embodiment.

[0066] In an embodiment of the present application, there is provided a use of a highly crystalline modified iron oxide as described in the aforementioned embodiment or a supported iron oxide as described in the aforementioned embodiment in anaerobic digestion treatment of organic solid waste.

[0067] The present invention is further described below by way of specific examples. However, it should be understood that these examples are merely provided for more detailed description and are not to be construed as limiting the present invention in any form.

[0068] Example 1 In this example, a modified iron oxide with high crystallinity was prepared.

[0069] Experimental methods: (1) Prepare a first mixture of 50 mL DMF (N,N-dimethylformamide), 1.893 g FeCl3·6H2O, 1.163 g BDC (terephthalic acid), and 4 mL NaOH (2 mol / L). (2) hydrothermally heating the first mixture at 100°C for 12 hours and drying the mixture to obtain the precursor MIL-88B; (3) The precursor MIL-88B was calcined and heated at 550°C for 2 h to destroy its organic skeleton and obtain an iron oxide material (MDF) prepared with MOF as the precursor.

[0070] Example 2 In this example, a modified iron oxide with high crystallinity was prepared.

[0071] Experimental methods: The method used in this embodiment is basically the same as that in Example 1, except that the calcination heating temperature is 700°C.

[0072] Example 3 In this example, a modified iron oxide with high crystallinity was prepared.

[0073] Experimental methods: The method used in this embodiment is basically the same as that in embodiment 1, except that the calcination heating temperature is 850°C.

[0074] Example 4 In this example, supported iron oxide (supported MDF) was prepared.

[0075] Experimental methods: (1) High crystallinity modified iron oxide was prepared by the method of Example 1; (2) Dispersing 2 g of high crystallinity modified iron oxide in 50 mL of water, and adding 0.5 g of sodium alginate to form a second mixture; (3) heating the second mixture to 80° C. and stirring at 600 rpm for 30 min to form a mixed solution; (4) The mixed solution was added dropwise to 100 mL of 2% mass concentration CaCl2 solution at a rate of 2 mL / min using a rubber-tipped dropper to carry out a curing reaction for 30 minutes. The obtained solid was rinsed three times with water and dried at 60°C for 4 hours to obtain microspherical supported iron oxide.

[0076] Comparative Example 1 In this comparative example, modified iron oxide was prepared.

[0077] Experimental methods: The method used in this comparative example is basically the same as that in Example 1, except that the calcination heating temperature is 400°C.

[0078] Comparative Example 2 In this comparative example, supported iron oxide was prepared.

[0079] Experimental methods: (1) Disperse 2 g of conventional iron oxide in 50 mL of water and add 0.5 g of sodium alginate to form a second mixture; (2) heating the second mixture to 80° C. and stirring at 600 rpm for 30 min to form a mixed solution; (3) The mixed solution was added dropwise to 100 mL of 2% mass concentration CaCl2 solution at a rate of 2 mL / min using a rubber-tipped dropper to carry out a curing reaction for 30 minutes. The obtained solid was rinsed three times with water and dried at 60°C for 4 hours to obtain microspherical supported iron oxide.

[0080] Observation experiment 1: In this experiment, the effects of anaerobic digestion and methanogenesis of modified iron oxides with high crystallinity and conventional iron oxide at different concentrations were investigated.

[0081] Experimental methods: (1) Grouping: The ratio of added sludge to substrate was 2:1 (in terms of VS). Specific groups were: conventional Fe2O3 was added at concentrations of 2 g / L, 5 g / L, and 10 g / L; the high-crystallinity modified iron oxide (MDF) prepared in Example 1 was added at concentrations of 2 g / L, 5 g / L, and 10 g / L. In addition, a group of reaction bottles inoculated with only sludge was set up as a blank control.

[0082] (2) The mixed solution in the reaction bottles of different groups (the total capacity of the reaction bottle is 100mL, and the working capacity is 80mL) was fully mixed with a stirring paddle, and the pH was adjusted to 7.2~7.4. Then, a nitrogen bottle was used to purge for about one minute to create a strict anaerobic environment. Finally, it was sealed with a rubber stopper and placed in a mechanical shaker with a speed of 120rpm and a reaction temperature of 37±1℃. A set of parallel samples was set for all reaction bottles and the operation was carried out for 30 days. During the operation, the mixed solution stirred evenly in the reactor was extracted by syringe for physicochemical indicators (pH, sCOD, VFAs, NH4 + -N) determination.

[0083] (3) Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electrochemical analysis were performed on the different groups of Fe2O3 and MDF to observe the different morphological characteristics and conductive properties of the two materials. After the reaction, the used Fe2O3 and MDF were separated from the sludge and used for SEM and XPS analysis to observe the changes in the two materials before and after the anaerobic digestion reaction. Finally, microbial sequencing was performed.

[0084] (4) Analyze the effects of MDF and conventional iron oxide on the microbial community structure during anaerobic digestion.

[0085] Experimental results: (1) For the methane production effect of anaerobic digestion of different concentrations of MDF and conventional iron oxide, the experimental groups of all conductive materials were significantly higher than the blank group (P<0.05). Specifically, conventional iron oxide and MDF achieved the best performance at 5.0 g / L and 10 g / L dosages, with methane production rates of 522.96±2.00 and 547.91±2.02 mL / g VS, respectively (reference Figure 1 ).

[0086] These results confirm the promoting effect of conductive materials on methane production. However, due to the thermodynamic advantage of iron reduction at a low electron donor threshold, excessive conventional iron oxide (10 g / L) triggers competition between iron reduction and methanogenesis for acetic acid / hydrogen, thereby reducing methanogenesis efficiency, while MDF does not show excessive inhibition.

[0087] (2) Through material characterization, we observed the changes in the morphology, valence and chemical properties of conventional iron oxide and MDF before and after anaerobic digestion, and proved that MDF can remain relatively stable before and after anaerobic digestion. The XPS and XRD patterns of the two materials before and after the anaerobic digestion batch experiment of different concentrations of MDF and conventional iron oxide are shown in Figure 2. Figure 2 The XPS results are shown as Figure 2 e. Figure 2 f shows the conventional iron oxide fraction Fe after anaerobic digestion process. 3+ Converted to Fe 2+ , while the MDF materials were Fe before and after anaerobic digestion. 3+ , maintaining extremely high stability.

[0088] XRD results are as follows Figure 2 As shown in g, both materials are consistent with hematite. In addition, MDF shows a higher three-peak crystallinity than conventional iron oxide (47.20% vs. 43.66%), indicating that MDF may have a more stable morphology in the reaction system. Figure 2 j) In contrast, the MDF after the reaction has almost no change compared to the original material and still conforms to the characteristics of hematite. In contrast, the conventional iron oxide crystal form after the reaction has undergone a significant change, turning into a combination of hematite and magnetite. This result is consistent with the XPS results of the conventional iron oxide part Fe 3+ Converted to Fe 2 + consistent.

[0089] The results of XPS and XRD both show that the valence and structure of conventional iron oxide undergo significant changes after anaerobic digestion. In contrast, MDF material exhibits superior stability, and its properties remain almost unchanged before and after anaerobic digestion.

[0090] (3) Reference Figure 3 Microbial analysis showed that Trichococcus was significantly enriched only in the conventional iron oxide group, but not in the MDF group. Trichococcus is a typical iron-reducing bacterium that can use complex carbohydrates to produce lactic acid, acetic acid, and ethanol, while transferring electrons to conventional iron oxide to reduce Fe. 3+ Fe 2+ It uses carbohydrates to produce lactic acid, acetate and ethanol, is electroactive and responsible for electron transfer, and is the Fe 2+ This also indicates that conventional iron oxide will change its valence state due to the iron reduction reaction after anaerobic digestion, resulting in a decrease in methanogenesis efficiency. However, Trichococcus was not enriched in the MDF group, indicating that MDF inhibited the occurrence of iron reduction reaction, thereby avoiding the iron reduction inhibitory effect.

[0091] Observation experiment 2: In order to verify the advantages of MDF in terms of high stability and better methane production in anaerobic digestion compared with Fe2O3, the long-term stability of MDF was investigated in this experiment.

[0092] Experimental methods: In this experiment, an automatic methane potential test system (MultiTalent 203, Nova Skantek, Sweden) was used for long-term experiments.

[0093] The reactor volume was 500 mL, and the working volume was 400 mL. The hydraulic retention time (HRT) was 20 days, and the total reactor TS was controlled at 3%. Three reactor groups were set up: a blank group, an Fe2O3 group (using the supported iron oxide described in Comparative Example 2), and an MDF group (using the supported iron oxide prepared in Example 4). The experiment was conducted in three phases: (1) Sludge acclimation stage: Inoculated sludge and kitchen waste are placed in three reactors without adding any materials, so that the sludge can adapt to the acidified environment of kitchen waste. The operation is expected to last 30 days. (2) Material addition stage: After the indicators of each reactor in the first stage are basically stable, 2g of Fe2O3 and MDF are added respectively to evaluate the methane production efficiency of the two materials in the long-term experiment. The corresponding materials need to be added during the daily loading and unloading to maintain the material concentration in the reactor at 5g / L. The operation is expected to last for 50 days; (3) Material loading stage: The prepared Fe2O3 (loaded iron oxide) / MDF (loaded iron oxide) material microspheres were added to the reactor separately. The mixed solution was collected after the microspheres settled each time to ensure that the material was always present in the reactor. No material was added during sampling to maintain a constant material concentration in the reactor. The methane production promotion efficiency of the two materials in this stage was evaluated and compared with the results of the second stage to verify the high stability of MDF. The operation was expected to last 30 days.

[0094] Experimental results: Long-term experiments further verified the advantages of MDF in terms of stability and methanogenesis (ref. Figure 4 This experiment was divided into three phases. In the second phase, two powder materials were added and replenished daily to maintain a material dosage of 5g / L. In the third phase, to observe the changes in the reaction of the same batch of materials before and after long-term experiments, equal amounts of loaded conventional iron oxide and MDF materials were added without additional supplementation.

[0095] In the second stage, the blank group produced a total of 3959.19 mL of methane, the conventional iron oxide group produced a total of 4456.81 mL of methane, and the MDF group produced a total of 4761.17 mL of methane; relatively speaking, the MDF group promoted methane production by 6.8% compared with the conventional iron oxide group.

[0096] In the third phase, the blank group produced a cumulative methane of 1367.89 mL, the conventional iron oxide group produced 1522.69 mL, and the MDF group produced 1658.09 mL. Similarly, the MDF group increased methane production by 8.9% compared to the conventional iron oxide group. The difference in methane production between MDF and commercial iron oxide in the third phase suggests that MDF can better promote methane production without supplemental materials, further demonstrating that MDF has a stable methane-promoting effect comparable to conventional iron oxide in long-term experiments.

[0097] For the long-term experiments of MDF and conventional iron oxide, the XRD patterns also changed significantly, such as Figure 5 The measurement results are consistent with the aforementioned research findings, indicating that commercial iron oxide undergoes iron reduction during the long-term reaction process, with some Fe³⁺ being reduced to Fe²⁺. This iron reduction phenomenon causes the commercial iron oxide's methane production efficiency to gradually decrease. This finding further confirms the instability of commercial iron oxide during long-term operation. Furthermore, the XPS spectrum of MDF remains unchanged, indicating that its iron valence remains unchanged. This also confirms the high stability of MDF relative to commercial iron oxide, which is why MDF is so effective in promoting methane production.

[0098] Observation experiment 3: In order to explore the mechanism of the effect of the crystallinity of conductive materials on the methane production in the anaerobic digestion of organic solid waste, this experiment investigated the effects of different calcination temperatures on the crystallinity and methane production of MDF.

[0099] Experimental methods: The research content of this chapter is carried out using batch experiments. 400 (Comparative Example 1), MDF 550 (Example 1), MDF 700 (Example 2) and MDF 850 (Example 3) X-ray diffraction (XRD) analysis was performed, and the crystallinity of the material was calculated using Jade software, and then the experiment was started.

[0100] The total volume of the reaction bottle is 100 mL, the working volume is 80 mL, the ratio of sludge to substrate is 2:1 (in terms of VS), and the reaction bottles are divided into 4 groups, namely MDF 400 、MDF 550 、MDF 700 and MDF 850 and a blank group in which only inoculum sludge was added, with the addition concentration being 5 g / L.

[0101] After stirring the above reaction bottle evenly, adjust the pH to 7.5±0.1, then purge with pure nitrogen for 1 minute to remove oxygen, and then seal with a rubber stopper. Then place it in a mechanical shaker with a rotation speed of 120 rpm and a reaction temperature of 37±1°C. All reactors are set up in parallel and run for 30 days.

[0102] During operation, the mixed solution in the reactor is taken through the sampling tube for physical and chemical indicators (pH, sCOD, VFAs, NH4 + -N) determination.

[0103] After the reaction, the four groups of materials were analyzed by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) before and after the reaction. Finally, microbial sequencing was performed.

[0104] PS: The calcination temperature of 400℃ is the lowest temperature to destroy its carbon skeleton.

[0105] Experimental results: The XRD patterns of different crystallinity are as follows Figure 6 As shown, the results show that its characteristic peaks are consistent with hematite, and the relevant results are basically the same as the previous analysis results.

[0106] In addition, the crystallinity of MDF materials calcined at four different temperatures was calculated using Jade analysis software, and the MDF400 The crystallinity is 92.2%, MDF 550 The crystallinity is 95.56%, MD F700 The crystallinity is 96.77% and MDF 800 The crystallinity is 98.97%. The calculation results show that the crystallinity of iron oxide materials prepared at different temperatures under the same conditions also changes, and the crystallinity is proportional to the calcination temperature.

[0107] For the methanogenic effect ( Figure 7 ), after 18 days, the methane production of each group tended to be stable, among which MDF 400 The cumulative methane production in the group was 273.12±1.80mL / g VSadded, MDF 550 The cumulative methane production in the group was 302.17±3.68 mL / gVSadded, MDF 700 The cumulative methane production in the group was 304.79±5.28 mL / g VSadded, MDF 850 The cumulative methane production of group was the highest, reaching 334.01±4.21mL / g VSadded.

[0108] In view of this, we can draw the following basic conclusion: in anaerobic digestion, the crystallinity of the added conductive material is positively correlated with its methane production promotion efficiency, that is, the higher the crystallinity, the better the methane production effect.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a high-crystallinity modified iron oxide, characterized in that: include: The precursor MIL-88B was prepared by hydrothermal method; The precursor MIL-88B is calcined to obtain iron oxide with MIL-88B as a precursor; wherein the temperature condition of the calcination treatment is 550° C. to 850° C.

2. The method for preparing a high crystallinity modified iron oxide according to claim 1, wherein: The holding time of the calcination treatment is not less than 2 hours.

3. The method for preparing a modified iron oxide with high crystallinity according to claim 1, wherein: The precursor MIL-88B is prepared by a hydrothermal method, comprising: mixing N,N-dimethylformamide, FeCl3·6H2O, terephthalic acid, and NaOH to obtain a first mixture; The first mixture is subjected to a hydrothermal treatment and dried to obtain the precursor MIL-88B.

4. The method for preparing a modified iron oxide with high crystallinity according to claim 3, wherein: The target heating temperature of the hydrothermal treatment is 100°C.

5. The method for preparing a modified iron oxide with high crystallinity according to claim 3, wherein: During the hydrothermal treatment, the time for maintaining the constant temperature after heating is not less than 12 hours.

6. A high crystallinity modified iron oxide, characterized in that The modified iron oxide is prepared by the method for preparing the high-crystallinity modified iron oxide according to any one of claims 1 to 5.

7. A method for preparing a supported iron oxide, characterized in that: include: Dispersing the highly crystalline modified iron oxide according to claim 6 in water, and adding sodium alginate to form a second mixture; heating the second mixture while continuously stirring to form a mixed solution; adding the mixed solution into a calcium chloride solution to carry out a curing reaction, and obtaining a cured product after washing; The solidified product is dried to obtain supported iron oxide.

8. The method for preparing a supported iron oxide according to claim 7, wherein: The temperature of the heating treatment is 80°C; and / or, The stirring rate of the continuous stirring is 600 rpm; and / or, The continuous stirring time is 30 minutes; and / or, The mixed solution is added to the calcium chloride solution by dropping the mixed solution into the calcium chloride solution at a rate of 2 mL / min; and / or, The calcium chloride solution is a calcium chloride solution with a mass concentration of 2%; and / or, The curing reaction time is 30 minutes; and / or, The drying temperature of the drying process is 60° C.; and / or, The drying time of the drying process is 4 hours.

9. A supported iron oxide, characterized in that The supported iron oxide is prepared by the preparation method of claim 8.

10. Use of the highly crystalline modified iron oxide according to claim 7 or the supported iron oxide according to claim 9 in anaerobic digestion of organic solid waste.

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