A method for preparing wood fiber charcoal and its application in biogas slurry treatment

Abstract

The present invention relates to a method for preparing wood fiber charcoal and its application in biogas slurry treatment. Wood fiber-based waste is used as raw material, and glucose-rich hydrothermal sugar solution and nitrogen-rich hydrothermal charcoal are separated through low-temperature hydrothermal saccharification. The nitrogen-rich hydrothermal charcoal is further subjected to medium-temperature activation modification and low-temperature hydrothermal oxidation to prepare wood fiber charcoal with suitable pore size distribution, rich surface nitrogen-, phosphorus- and oxygen-containing active groups, and high capacitance performance. The wood fiber charcoal can be used as a flocculent carrier to enhance the flocculation effect and obtain solid bacterial fertilizer, and can also be used as a capacitor deionization electrode material to treat the supernatant after flocculation and obtain concentrated liquid fertilizer. This patent proposes a new biogas slurry treatment path, which is of great significance to the deep treatment of biogas slurry and nutrient recovery.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon material preparation and biogas slurry deep treatment, and in particular to a preparation method of wood fiber carbon and application thereof in biogas slurry treatment. Background Art

[0002] In recent years, China's livestock and poultry farming industry has developed rapidly, and the number of large and medium-sized biogas projects using livestock and poultry manure as raw materials has increased. However, the by-product of biogas fermentation, biogas slurry, has brought many environmental problems. For example, the direct discharge of biogas slurry can cause eutrophication of water bodies. Biogas slurry contains a large amount of organic matter, macronutrients and trace nutrients, and has resource utilization value. This patent proposes a new biogas slurry treatment path through flocculation and capacitive deionization technology, which can effectively treat biogas slurry and recover nutrients to obtain solid bacterial fertilizer, concentrated liquid fertilizer and clean water.

[0003] Flocculation can effectively reduce the chemical oxygen demand, biochemical oxygen demand, etc. in wastewater, and is a commonly used technology for the resource utilization of biogas slurry. Microbial flocculant is a special metabolite produced by microorganisms or their secretions. It is a new, green flocculant. However, microorganisms are easily affected by the environment. To solve this problem, microorganisms can be loaded onto a carrier to protect the microorganisms and keep them at a high activity and population density. Biochar is usually made by high-temperature carbonization of biomass under anaerobic conditions. It has a rich pore structure and a good specific surface area, making it an ideal carrier for microorganisms. Chinese patent application CN 115558502 A discloses a preparation method and application of a biochar-fixed microbial soil conditioner, in which a biochar-fixed microbial soil conditioner is obtained by adsorbing microorganisms on biochar. However, this technology is only used for soil improvement, ignoring other uses of biochar-fixed microorganisms.

[0004] Although flocculation can remove most organic matter, ammonia nitrogen, potassium, etc. are still present in the clear liquid, failing to meet the first-level standard of the "Integrated Wastewater Discharge Standard" (GB8978-1996). Capacitive deionization is a new electrochemical desalination technology with the characteristics of easy regeneration, high efficiency, and low energy consumption. Capacitive deionization technology can be used to further deeply treat the supernatant after flocculation of biogas slurry. Electrode materials are an important factor affecting the performance of capacitive deionization, and carbon materials are widely used in electrode materials. Chinese patent application CN 112340728 A discloses a chestnut shell-based biomass carbon material and its preparation method and application. The capacitor deionization electrode material is obtained by acid-base modification of chestnut shell powder and then high-temperature carbonization. Chinese patent application CN113035592A discloses a method for preparing a capacitor deionization electrode using corn straw. The corn straw is pre-carbonized and then doped with potassium hydroxide. After high-temperature activation, it is acid-washed to obtain the electrode material. Chinese patent application CN 113880086 A discloses a method for preparing a nitrogen-phosphorus co-doped biomass-derived capacitive deionizing electrode. The method involves pre-carbonizing soybean straw and mixing it with diammonium hydrogen phosphate. The mixture is then calcined at high temperature, activated with carbon dioxide, and then acid-washed to produce a soybean straw-derived carbon material. While these electrode materials have demonstrated some effectiveness, they still suffer from drawbacks such as underdeveloped porosity and a low number of surface active groups. Plant wood fiber is the most abundant natural multi-molecular material. It is inexpensive, easily degradable, pollution-free, and has a high carbon content and aromatic rings, making it an ideal raw material for biochar.

[0005] However, the direct carbonization of plant wood fiber results in an uneven pore size distribution and an unsatisfactory structure. Chinese patent application CN113830766 A discloses a method for preparing porous activated biochar by fermentation, activation, and oxidation. Forestry waste is activated at medium to high temperatures using a mixed activator of zinc chloride and melamine to produce porous activated biochar. However, melamine, as an organic activator, easily blocks pores in the zinc chloride-activated micropores, thereby reducing the pore structure. The zinc chloride used in this patent works synergistically with phosphoric acid activation, allowing phosphoric acid to enter the zinc chloride-activated micropores, promoting the carbon material to produce more mesoporous structures and surface phosphorus-containing groups. Furthermore, loofah sponge itself is rich in nitrogen, and low-temperature hydrothermal saccharification can extract sugars. The subsequent hydrothermal oxidation can then increase the surface oxygen-containing groups of the carbon material based on the activation. Summary of the Invention

[0006] The present invention provides a preparation method of wood fiber charcoal and its application in biogas slurry treatment. Wood fiber-based waste is used as raw material, and low-temperature hydrothermal saccharification, medium-temperature impregnation activation and hydrothermal oxidation are combined to prepare wood fiber charcoal with suitable pore size distribution, rich surface oxygen, nitrogen and phosphorus active groups, and high capacitance performance. The wood fiber charcoal can not only be used as a carrier for flocculent bacteria to effectively improve the flocculation rate of biogas slurry, but also can be used for capacitive deionization to effectively enrich and recover the nutrients in the supernatant of biogas slurry.

[0007] The present invention solves the above technical problems as follows: A method for preparing wood fiber charcoal comprises the following steps:

[0008] 1) mixing loofah sponge and soybean pods in a certain proportion, adding the mixture to a 10-20 wt% citric acid aqueous solution, and performing low-temperature hydrothermal saccharification to obtain hydrothermal sugar solution and hydrothermal charcoal;

[0009] 2) mixing the hydrothermal carbon obtained in step 1 with a mixed activator consisting of zinc chloride and phosphoric acid, impregnating for 12 hours, and then drying at 105° C. to constant weight to obtain a mixture;

[0010] 3) The mixture obtained in step 2 is subjected to an impregnation activation reaction under an inert atmosphere. After the reaction is completed, the activated solid product is washed with acid at a concentration of 1 to 2 mol / L to obtain activated carbon;

[0011] 4) The activated carbon is mixed with hydrogen peroxide to undergo a hydrothermal oxidation reaction. After the reaction is completed, the product is washed with water and dried to obtain wood fiber charcoal.

[0012] Luffa sponges have a fibrous network structure and are rich in nitrogen. Soybean pods contain zinc and iron, which regulate the pore structure of the sponge. Activation with zinc chloride produces a more microporous structure. Compared to alkaline activators, zinc chloride stabilizes the carbon skeleton and increases charcoal yield. The resulting wood fiber charcoal exhibits a well-developed pore structure and abundant surface groups.

[0013] Preferably, in step 1), the mass ratio of loofah sponge to soybean pod is 1:1-2:1, and the feeding ratio of the sum of the mass of loofah sponge and soybean pod to the citric acid aqueous solution is 1:6-1:8 g / L. Loofah sponge has a fibrous network structure and is rich in nitrogen. The soybean pod contains a certain amount of zinc and iron, which can adjust the pore structure of the loofah sponge. The two products are mixed to form in-situ doping with nitrogen and metal elements, which is not easy to clog pores, and has more advantages than chemical addition.

[0014] Preferably, in step 1), the hydrothermal saccharification reaction temperature is 120-160° C., and the reaction time is 2-3 hours. A lower saccharification temperature is beneficial for saving energy.

[0015] Preferably, in step 2), the mass ratio of zinc chloride to phosphoric acid in the mixed activator is 1:3-3:1, and the mass ratio of hydrothermal carbon to the mixed activator is 1:2-2:1. Phosphoric acid can enter the micropores activated by zinc chloride, promoting the formation of more mesoporous structures and surface phosphorus-containing groups in the carbon material, resulting in a suitable pore size distribution for the wood fiber carbon and promoting the formation of phosphorus-containing active groups on its surface.

[0016] Preferably, in step 3), the activation reaction in step 3 has a heating rate of 5-10°C / min, a reaction temperature of 450-650°C, and a reaction time of 1-2 hours. The activation temperature employed is a moderate activation temperature, which is more energy-efficient than a high activation temperature of 700-900°C.

[0017] Preferably, in step 4), the concentration of hydrogen peroxide is 10-30%, the solid-liquid ratio of activated carbon to hydrogen peroxide is 1:7-1:10 g / L, the hydrothermal oxidation reaction temperature is 220-260° C., and the reaction time is 0.5-1 h.

[0018] The application of the wood fiber charcoal prepared by the above-mentioned preparation method in treating biogas slurry comprises the following steps:

[0019] 1) mixing lignocellulosic charcoal, flocculant strains, urea, and hydrothermal sugar solution, aerating for 48-60 hours, and ultrasonicating to obtain a lignocellulosic charcoal-based microbial flocculant; the flocculant strains are one or both of Bacillus subtilis and Pseudomonas; the hydrothermal sugar solution is the hydrothermal sugar solution obtained in step 1) of the above-mentioned method for preparing lignocellulosic charcoal; the hydrothermal sugar solution serves as a carbon source required for the growth of the flocculant, thereby saving the cost of cultivating the flocculant.

[0020] 2) Mixing the lignocellulosic carbon-based microbial flocculant and the coagulant aid with the biogas slurry, letting it stand for 10-20 minutes, and then filtering to obtain the lignocellulosic carbon-based flocculant solid fertilizer and the biogas slurry supernatant.

[0021] As mentioned above, the well-developed pore structure and abundant surface groups of lignocellulose provide nutrients and a habitat for flocculent bacteria, thereby promoting their growth and reproduction, and thus improving the flocculation effect. The resulting flocculent sediment is rich in nutrients such as carbon, nitrogen, and calcium, as well as polysaccharides, proteins, and beneficial flocculent bacteria, and can be used as a flocculating solid fertilizer.

[0022] Preferably, in step 1), the raw materials include, by mass, 15-25 parts of floc-producing bacteria, 5-15 parts of wood fiber charcoal, 3-6 parts of urea, and 700-800 parts of culture solution; and the aeration rate is 0.1-0.4 L / min.

[0023] Preferably, in step 2), the raw materials include, by mass, 90-100 parts of biogas slurry, 10-15 parts of wood fiber charcoal-based microbial flocculant, and 4-6 parts of coagulant aid; the coagulant aid is 8%-15% calcium chloride solution.

[0024] The application of the wood fiber carbon prepared by the above-mentioned preparation method to prepare a deionized electrode in treating biogas slurry comprises the following steps:

[0025] 1) Wood fiber carbon, polyvinylidene fluoride, and Ketjen black are mixed in N-methylpyrrolidone in a mass ratio of 8:1:1, and ultrasonicated for 3 minutes to obtain a mixed slurry. The mixed slurry is evenly applied to a substrate, and vacuum dried at 60°C to obtain a capacitive deionization electrode, which is assembled into a capacitive deionization module. The substrate can be a titanium plate or graphite paper. The titanium plate is smoother, making it easier to scrape off the enriched material and can be recycled.

[0026] 2) Use a capacitive deionization module to treat the supernatant remaining in the biogas slurry after flocculation and recover the nutrients in the supernatant.

[0027] The capacitive deionization electrodes serve as the cathode and anode of the capacitive deionization module, and are used to treat the biogas slurry supernatant in step 6. The capacitive deionization adopts a circulation mode, with a voltage of 0.6V-1.4V applied across the electrodes and a running time of 4-6h.

[0028] Wood fiber carbon has excellent electrochemical properties and can be used as a capacitor deionization electrode material to treat the supernatant after flocculation. After treatment, the enriched concentrated liquid can be used as liquid fertilizer.

[0029] The beneficial effects of the present invention are:

[0030] (1) The raw materials of wood fiber charcoal are loofah and soybean pods. Both loofah and soybean pods are agricultural wastes with low cost and wide sources. Compared with other agricultural wastes, loofah has a fibrous network structure and is rich in nitrogen. Soybean pods contain a certain amount of zinc and iron, which can regulate the pore structure of loofah. The prepared wood fiber charcoal has a rich nitrogen content and a hollow pore structure.

[0031] (2) The wood fiber carbon is activated by the synergistic impregnation of zinc chloride and phosphoric acid. Phosphoric acid can enter the micropores activated by zinc chloride, promoting the carbon material to produce more mesoporous structures and surface phosphorus-containing groups, so that the wood fiber carbon has a suitable pore size distribution, and at the same time promotes the generation of phosphorus-containing active groups on its surface.

[0032] (3) Hydrothermal oxidation reaction can increase the oxygen content on the surface of wood fiber charcoal and increase its surface oxygen-containing groups.

[0033] (4) In the culture medium for floc-producing bacteria, the hydrothermal sugar solution obtained by low-temperature hydrothermal saccharification is fully utilized. The hydrothermal sugar solution provides the carbon source required for the growth of floc-producing bacteria, saving the cost of cultivating floc-producing bacteria.

[0034] (5) Lignocellulosic carbon-based microbial flocculants are easily degradable and cause no secondary pollution. The flocculants used are also beneficial bacteria. The flocculent sediment produced by flocculation is rich in nutrients such as carbon, nitrogen, and calcium. It also contains rich polysaccharides, proteins, and beneficial flocculants, and can be used as flocculation solid fertilizer.

[0035] (6) Nitrogen-rich, phosphorus-rich and oxygen-rich wood fiber charcoal is used in capacitive deionization technology to treat the supernatant of biogas slurry, which can effectively enrich and recover the nutrients in it. The enriched concentrated liquid can be used as liquid fertilizer.

[0036] (7) A new method for treating biogas slurry is proposed. By using wood fiber carbon-based flocculants and wood fiber carbon capacitive deionization technology, not only can solid bacterial fertilizer and concentrated liquid fertilizer be produced, but also the clean water can meet the discharge standards after the biogas slurry is treated.

[0037] In summary, this patent has developed a wood fiber carbon material with suitable pore size distribution, rich surface nitrogen-rich, phosphorus-rich and oxygen-rich active groups, and high capacitance performance. The raw materials are separated into glucose-rich hydrothermal sugar solution and nitrogen-rich hydrothermal carbon through hydrothermal saccharification. The nitrogen-rich hydrothermal carbon is further activated, modified and hydrothermally oxidized to prepare excellent wood fiber carbon. Wood fiber carbon can be used as a carrier for flocculent bacteria to enhance the flocculation effect, and can also be used as a capacitor deionization electrode material to treat the supernatant after flocculation. At the same time, a new biogas slurry treatment path is proposed, which is of great significance to the deep treatment of biogas slurry and nutrient recovery.

[0038] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and to implement it according to the contents of the description, the following preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. The specific implementation methods of the present invention are given in detail by the following embodiments and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0040] Figure 1 This is a flow chart of the steps of preparing wood fiber charcoal and treating biogas slurry according to the present invention;

[0041] Figure 2 This is the SEM image of the wood fiber charcoal prepared in Example 1;

[0042] Figure 3This is the SEM image of the wood fiber charcoal prepared in Example 2;

[0043] Figure 4 This is the SEM image of the wood fiber charcoal prepared in Example 3;

[0044] Figure 5 This is the SEM image of the wood fiber carbon prepared in Comparative Example 1;

[0045] Figure 6 This is the SEM image of the wood fiber carbon prepared in Comparative Example 2. DETAILED DESCRIPTION

[0046] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0047] The application principle of the present invention is as follows:

[0048] Preparation of flocculants and flocculation principle by wood fiber carbon: by fixing the flocculants on wood fiber carbon, the wood fiber carbon can provide a place for the flocculants to grow and reproduce, and provide certain nutrients, so the flocculants are more developed, and then the bacteria are broken by ultrasound to release a large amount of active substances such as proteins, which react with Ca 2+ Through bridging and charge adsorption, a complex and stable three-dimensional flocculent network structure is formed, which makes the suspended particles in the biogas slurry adhere to each other, and then form flocs and precipitate, thus achieving the purpose of flocculation.

[0049] The principle of capacitive deionization of wood fiber carbon: Under the action of the DC electric field, the anions and cations in the water move toward the electrodes with opposite polarity and are adsorbed on the active sites of the wood fiber carbon, thereby removing the anions and cations in the water. By reversing the power supply, the adsorbed nutrient ions are released into the liquid to obtain a concentrated solution, which can be used as liquid fertilizer.

[0050] Example 1

[0051] like Figure 1 As shown, this embodiment provides a preparation method of wood fiber charcoal (denoted as N-120-Zn-P-600-OB) and the application of the wood fiber charcoal in treating biogas slurry.

[0052] 1. First, loofah sponge and soybean pods are crushed by a crusher and passed through a 20-mesh sieve, and the mixture is mixed in a mass ratio of 2:1 to obtain a mixed raw material. The mixed raw material is mixed with a 10% mass concentration of citric acid aqueous solution at a solid-liquid ratio of 1:7 g / L, and low-temperature hydrothermal saccharification is carried out at 120°C. After saccharification for 2 hours, the mixture is cooled and filtered to obtain hydrothermal sugar solution and nitrogen-rich hydrothermal charcoal.

[0053] 2. Zinc chloride and phosphoric acid are mixed in a mass ratio of 2:1 to obtain a mixed activator, and then nitrogen-rich hydrothermal charcoal and the mixed activator are mixed in a mass ratio of 1:1, and after impregnation for 12 hours, they are dried at 105°C to constant weight to obtain a mixture.

[0054] 3. Place the mixture in an inert gas, heat it to 600°C at a heating rate of 10°C / min, and activate the reaction for 1 hour. After the reaction, the solid product is washed with 1 mol / L nitric acid to obtain nitrogen-rich and phosphorus-rich activated carbon.

[0055] 4. Nitrogen-rich and phosphorus-rich activated carbon was mixed with hydrogen peroxide at a solid-liquid ratio of 1:10 g / L and hydrothermally oxidized at 250 °C for 0.5 h. After the reaction, the solid product was washed with water to obtain wood fiber carbon (N-120-Zn-P-600-OB). The SEM image of the carbon is shown in Figure 4. Figure 2 shown.

[0056] 5. Mix 10 parts of lignocellulosic charcoal (N-120-Zn-P-600-OB), 20 parts of Pseudomonas species, 3 parts of urea, and 800 parts of hydrothermal sugar solution. Incubate at 0.3 L / min aeration for 48 hours. After sonication, a lignocellulosic charcoal-based microbial flocculant is obtained. Then, 12 parts of the lignocellulosic charcoal-based microbial flocculant and 5 parts of a 10% calcium chloride solution are added to 100 parts of biogas slurry. After standing for 15 minutes, filter to obtain a biogas slurry supernatant and a flocculated precipitate, respectively. The flocculated precipitate can be used directly as a lignocellulosic charcoal-based solid biofertilizer.

[0057] 6. Mix wood fiber charcoal (N-120-Zn-P-600-OB) with polyvinylidene fluoride and Ketjen black in a mass ratio of 8:1:1 in N-methylpyrrolidone. After ultrasonic treatment for 3 minutes, the mixture was evenly applied to a titanium plate and vacuum-dried at 60°C to obtain capacitive deionization electrodes, which were then assembled into a capacitive deionization module. The capacitive deionization electrodes served as the cathode and anode of the capacitive deionization module. The capacitive deionization system treated the supernatant of the flocculated biogas slurry in a cyclic mode, applying a voltage of 1.4 V across the electrodes for 6 hours.

[0058] Example 2

[0059] This example provides a method for preparing wood fiber charcoal (denoted as N-150-Zn-P-600-OB) and the application of the wood fiber charcoal in treating biogas slurry. The experimental steps of this example are basically the same as those of Example 1, except that in step 1, the temperature of low-temperature hydrothermal saccharification is 150°C. The SEM image of the wood fiber charcoal (N-150-Zn-P-600-OB) is shown in FIG. Figure 3 shown.

[0060] Example 3

[0061] This embodiment provides a method for preparing wood fiber charcoal (denoted as N-150-Zn-P-450-OB) and the application of the wood fiber charcoal in treating biogas slurry. The experimental steps of this embodiment are basically the same as those of Example 1, except that in step 1, the temperature of low-temperature hydrothermal saccharification is 150°C; in step 3, the activation reaction temperature is 450°C. The SEM image of wood fiber charcoal (N-150-Zn-P-450-OB) is shown in FIG. Figure 4 shown.

[0062] Comparative Example 1

[0063] This comparative example provides a method for preparing wood fiber charcoal (denoted as N-120-Zn-600-OB) and the application of the wood fiber charcoal in treating biogas slurry. The experimental steps of this comparative example are basically the same as those of Example 1, except that in step 2, the activator used is zinc chloride activator. The SEM image of wood fiber charcoal (N-120-Zn-600-OB) is shown in FIG. Figure 5 shown.

[0064] Comparative Example 2

[0065] This comparative example provides a method for preparing wood fiber carbon (N-120-Zn-P-600-B) and the application of the wood fiber carbon in treating biogas slurry. The experimental steps of this comparative example are basically the same as those of Example 1, except that the nitrogen-rich and phosphorus-rich activated carbon prepared in step 3 is not subjected to the hydrothermal oxidation treatment in step 4. The SEM image of the wood fiber carbon (N-120-Zn-P-600-B) is shown in FIG. Figure 6 shown.

[0066] The pore structure data of the wood fiber carbons prepared in each embodiment and comparative example are shown in Table 1, the elemental composition of the wood fiber carbons prepared in each embodiment and comparative example are shown in Table 2, and the flocculation characteristics and capacitive deionization performance of the wood fiber carbons prepared in each embodiment and comparative example in treating biogas slurry are shown in Table 3.

[0067] Table 1 Pore structure data of wood fiber carbon in each embodiment and comparative example

[0068]

[0069] Table 2 Elemental composition of wood fiber carbon in each embodiment and comparative example

[0070]

[0071] Table 3 Flocculation characteristics and capacitive deionization performance of wood fiber carbon in each embodiment and comparative example

[0072]

[0073] The lignocellulosic charcoal (N-120-Zn-600-OB) in Comparative Example 1 was activated using zinc chloride alone. Compared to Comparative Example 1, the lignocellulosic charcoal (N-120-Zn-P-600-OB) in Example 1 was activated using a mixed activator of zinc chloride and phosphoric acid. The effects of this mixed activator on the lignocellulosic charcoal were investigated. Figure 2 and Figure 3 The porous fiber bundle structure can be observed. Table 1 shows that the specific surface area of ​​the N-120-Zn-P-600-OB carbon of Example 1 is 20.56% higher than that of the N-120-Zn-600-OB carbon of Comparative Example 1, and the mesopore volume is greatly increased from 0.17 cm 3 The average pore size of the N-120-Zn-P-600-OB in Example 1 increased from 0.02% to 5.07% compared to Comparative Example 1. The phosphoric acid content of the N-120-Zn-P-600-OB in Example 1 increased significantly from 0.43 cm³ / g to 0.43 cm³ / g. This is because zinc chloride can produce many microporous structures, and phosphoric acid enters the micropores to activate the reaction, further expanding the pores and creating more mesoporous structures, resulting in an increase in the average pore size from 2.27 nm to 4.41 nm. As can be seen in Table 2, compared to Comparative Example 1, the phosphorus (P) content of the N-120-Zn-P-600-OB in Example 1 increased significantly, from 0.02% to 5.07%, indicating that phosphoric acid activation generated a large number of surface phosphorus-containing active groups. Compared to Comparative Example 1, the density of flocculants in N-120-Zn-P-600-OB in Example 1 increased significantly, and the capacitance removal performance of each component of the capacitive deionization was also significantly improved. This indicates that the activation effect of a single zinc chloride activator can be significantly improved by using a mixed activator of zinc chloride and phosphoric acid. This is specifically manifested in increasing the mesoporous structure, increasing the specific surface area, and generating surface phosphorus-containing active groups. The rich pore structure provides a place for the flocculants to grow and reproduce, and the rich P content provides nutrients for the flocculants.

[0074] Compared to Example 1, the wood fiber charcoal (N-120-Zn-P-600-B) of Comparative Example 2 was not subjected to hydrothermal oxidation. Comparing the pore structure and O content of N-120-Zn-P-600-OB and N-120-Zn-P-600-B in Tables 1 and 2, hydrothermal oxidation can further increase the pore structure and O element content of the wood fiber charcoal, thereby increasing its surface oxygen-containing groups and increasing surface activity. As can be seen from Table 3, compared to Comparative Example 2, the density of flocculent bacteria in Example 1 increased, the flocculation rate increased from 84.18% to 87.35%, and the removal performance of each component by capacitive deionization was also significantly increased. This may be because hydrothermal oxidation increases the surface O-active groups of the wood fiber charcoal, which can provide nutrients for the growth and reproduction of flocculent bacteria, and can also improve the wettability of the capacitive deionization electrode, thereby improving the capacitive deionization performance.

[0075] Compared with Example 1, the hydrothermal saccharification temperature of the wood fiber charcoal (N-150-Zn-P-600-OB) in Example 2 was increased from 120°C to 150°C. The N-150-Zn-P-600-OB wood fiber charcoal had a honeycomb structure and had a richer pore structure, larger specific surface area and pore volume than the N-120-Zn-P-600-OB (Table 2, Figure 5 and Figure 3 ), indicating that hydrothermal saccharification at 150°C can improve the pore structure of wood fiber charcoal. At the same time, the flocculent density OD of the wood fiber charcoal (N-150-Zn-P-600-OB) produced in Example 2 was 600 The flocculation rate reached 2.63, and the flocculation rate also reached 89.69%, which was 2.68% higher than that of Example 1, and the performance of capacitive deionization was also improved. + The unit removal amount even reached 63.27 mg / g (Table 3), and the total nutrient recovery rate of biogas slurry concentration was ≥90%. This may be because hydrothermal saccharification at 150°C is more conducive to the decomposition of cellulose and other components in the raw materials into small molecular sugars. A higher saccharification temperature is conducive to increasing the sugar content of the hydrothermal sugar solution, thereby providing more nutrients for floc-producing bacteria. At the same time, it can increase the porosity of the hydrothermal carbon and further improve its capacitive deionization performance.

[0076] Compared to Example 2, the lignocellulosic carbon (N-150-Zn-P-450-OB) of Example 3 was activated at a temperature of 450°C. Compared to Example 2 (N-150-Zn-P-600-OB), the specific surface area of ​​the lignocellulosic carbon (N-150-Zn-P-450-OB) of Example 3 decreased by 21.79% (Table 1), and the carbon content decreased by 5.06% (Table 2). The flocculation rate of N-150-Zn-P-450-OB in Example 3 decreased by 10.99% compared to Example 2, and its capacitive deionization performance decreased (Table 3). This indicates that lower activation temperatures are not conducive to hydrothermal carbon activation, while relatively higher, moderate activation temperatures can promote the development of a fine pore structure in the lignocellulosic carbon, thereby improving its flocculation characteristics and capacitive deionization performance.

[0077] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any ordinary technician in this industry can smoothly implement the present invention as shown in the drawings and described above. However, any equivalent changes, modifications and evolutions made by technicians familiar with this profession without departing from the scope of the technical solution of the present invention using the technical content disclosed above are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing wood fiber charcoal, characterized in that: The following steps are involved: 1) Mixing loofah sponge and soybean pods in a certain proportion, adding the mixture to a citric acid aqueous solution medium, and performing hydrothermal saccharification to obtain hydrothermal sugar solution and hydrothermal charcoal; 2) mixing the hydrothermal carbon obtained in step 1 with a mixed activator consisting of zinc chloride and phosphoric acid, impregnating for a period of time, and drying to a constant weight to obtain a mixture; 3) The mixture obtained in step 2 is heated to 600-650°C under an inert atmosphere for a medium-temperature activation reaction, and then acid-washed after the reaction to obtain activated carbon; 4) The activated carbon is mixed with hydrogen peroxide to undergo a hydrothermal oxidation reaction. After the reaction is completed, the product is washed with water and dried to obtain wood fiber charcoal.

2. The method for preparing wood fiber charcoal according to claim 1, characterized in that: In the step 1), the mass ratio of the loofah sponge to the soybean pod is 1:1-2:1, the concentration of the citric acid aqueous solution is 10%-20%, and the feeding ratio of the sum of the mass of the loofah sponge and the soybean pod to the citric acid aqueous solution is 1:6-1:8 g / L.

3. The method for preparing wood fiber charcoal according to claim 1, characterized in that: In step 1), the hydrothermal saccharification reaction temperature is 120-160° C., and the reaction time is 2-3 h.

4. The method for preparing wood fiber charcoal according to claim 1, characterized in that: In the step 2), the mass ratio of zinc chloride to phosphoric acid in the mixed activator is 1:3-3:1, and the mass ratio of hydrothermal charcoal to the mixed activator is 1:2-2:

1.

5. The method for preparing wood fiber charcoal according to claim 1, characterized in that: In step 3), the activation reaction has a heating rate of 5-10°C / min and a reaction time of 1-2 h.

6. The method for preparing wood fiber charcoal according to claim 1, characterized in that: In step 4), the concentration of hydrogen peroxide is 10-30%, the solid-liquid ratio of activated carbon to hydrogen peroxide is 1:7-1:10 g / L, the hydrothermal oxidation reaction temperature is 220-260° C., and the reaction time is 0.5-1 h.

7. Use of wood fiber charcoal prepared according to the method for preparing wood fiber charcoal according to any one of claims 1 to 6 in treating biogas slurry, characterized in that: The following steps are involved: 1) Mixing wood fiber charcoal, flocculant-producing bacteria, urea, and hydrothermal sugar solution, and then aerating and ultrasonicating to obtain a wood fiber charcoal-based microbial flocculant; 2) The lignocellulosic carbon-based microbial flocculant and the coagulant aid are mixed with the biogas slurry, allowed to stand and then filtered to obtain the lignocellulosic carbon-based flocculant solid fertilizer and the biogas slurry supernatant.

8. Use of the wood fiber charcoal prepared by the method for preparing wood fiber charcoal according to claim 7 in treating biogas slurry, characterized in that: In step 1), the raw materials include, by mass, 15-25 parts of floc-producing bacteria, 5-15 parts of wood fiber charcoal, 3-6 parts of urea, and 700-800 parts of hydrothermal sugar solution; and the aeration rate is 0.1-0.4 L / min.

9. Use of the wood fiber charcoal prepared by the method for preparing wood fiber charcoal according to claim 7 in treating biogas slurry, characterized in that: In step 2), the raw materials include, by mass, 90-100 parts of biogas slurry, 10-15 parts of wood fiber charcoal-based microbial flocculant, and 4-6 parts of a coagulant aid; the coagulant aid is an 8%-15% calcium chloride solution.

10. Use of wood fiber charcoal prepared according to the method for preparing wood fiber charcoal according to any one of claims 1 to 6 in treating biogas slurry, characterized in that: The following steps are involved: 1) Wood fiber carbon, polyvinylidene fluoride, Ketjen black, and N-methylpyrrolidone are uniformly mixed to obtain a mixed slurry, the mixed slurry is evenly applied to a substrate, and after vacuum drying, a capacitive deionization electrode is obtained, which is then assembled into a capacitive deionization module; 2) Use a capacitive deionization module to treat the supernatant remaining from the flocculated biogas slurry, enrich and recover the nutrients in the supernatant to obtain concentrated liquid fertilizer.

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