Preparation method of modified biochar based on CMC-Na and application of modified biochar in ammonia-nitrogen wastewater treatment

Abstract

The invention relates to the technical field of sewage treatment, and discloses a preparation method of CMC-Na-based modified biochar and application of the CMC-Na-based modified biochar in ammonia nitrogen wastewater treatment.According to the preparation method of the CMC-Na-based modified biochar, reed straw is converted into biochar, so that resource utilization of agricultural waste is achieved, and environmental pollution is reduced; the biochar has a porous structure and a relatively large specific surface area, so that adsorption and catalytic reaction are facilitated; due to the introduction of CMC-Na, the surface property and pore structure of the biochar are further improved, and the electron transfer capability is enhanced; besides, the CMC-Na modified charcoal powder CMC-BC is added into the anaerobic iron ammonia oxidation sludge system, so that the removal rate of ammonia nitrogen is approximate to 100%, near zero emission of ammonia nitrogen is realized, the removal efficiency of total nitrogen and the electron transfer rate are remarkably improved, and the method has a positive effect on functional microorganisms of the anaerobic iron ammonia oxidation system.

Description

Technical Field

[0001] The invention belongs to the technical field of sewage treatment, and specifically relates to a preparation method based on CMC-Na modified biochar and its application in ammonia nitrogen wastewater treatment. Background Art

[0002] Nitrogen-containing wastewater leads to eutrophication of water bodies, causing ecosystem disorder, and is toxic and affects human health. Although traditional nitrification, denitrification biological denitrification processes and anaerobic ammonia oxidation processes (Anaerobic Ammonia Oxidation, Anammox) have their own advantages, they also have a series of problems. The former is a mature process that reduces nitrogen pollution in water bodies, but it consumes a lot of energy and has a complex process flow. The latter can reduce energy consumption and carbon sources to a certain extent, but the conditions are harsh and nitrogen removal is unstable. Anaerobic iron ammonia oxidation (Feaammox) is a microbial-mediated ammonia iron reduction phenomenon under anaerobic conditions. Under the autotrophic conditions of a single-stage reactor, NH4 + Converted into N2, NO3 - and NO2 - The process does not require a carbon source, consumes little energy, has a low sludge output, does not require regular sludge discharge, and produces no greenhouse gases.

[0003] However, because the research on Feammox process is still in its infancy, the biggest challenge currently faced is its low nitrogen conversion rate and solid Fe(III) reduction rate. When environmental factors and reaction substrates are uncontrollable, adding some exogenous substances that can improve the efficiency of Feammox becomes an effective method. As a substance with redox activity, electron shuttles can play such a role and have been shown to accelerate the efficiency of Feammox reaction. Although some soluble ESs, such as humus and riboflavin, can significantly accelerate the reaction rate, their continuous addition and secondary pollution still lead to high costs and environmental problems. Therefore, insoluble ESs have the potential to be applied to the extracellular microbial reduction of Fe(III) and the recovery of redox mediators. As an excellent insoluble electron shuttle, biochar is also rich in quinones, hydroquinones and fused aromatic (sub) structures with redox active groups on its surface, which play an important role in electron transfer in biochemical processes. Quinone molecules can improve the ability of electron transfer from ammonium to Fe(III) and promote the functional microbial-mediated NH4 + Therefore, it is of great engineering significance to study and develop an efficient, low-cost and environmentally friendly electron shuttle to provide a green and low-energy new method for the treatment of nitrogen pollution in water bodies. Summary of the invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a preparation method based on CMC-Na modified biochar and its application in ammonia nitrogen wastewater treatment, which solves the problem of limited ammonia nitrogen removal by anaerobic iron ammonia oxidation.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention provides a preparation method of CMC-Na modified biochar and its application in ammonia nitrogen wastewater treatment, comprising the following steps: Step 1, washing the reed straw with distilled water and drying it, then chopping and sieving it to obtain biomass powder; Step 2, weigh a certain amount of biomass powder and evenly place it in a porcelain boat, place the porcelain boat in a tube furnace, evacuate the air in the tube furnace to a near vacuum state, and then introduce N2, repeating 3 to 5 times; Step 3, heating to a preset temperature, maintaining the constant temperature for 100-150 minutes, then cooling down, and taking out the sample after it reaches room temperature; Step 4, washing and drying the sample to obtain unmodified biochar powder BC; Step 5, dissolving CMC-Na powder in deionized water at 40-60° C. and stirring for 15-30 min to obtain CMC-Na hydrogel; Step 6, adding unmodified biochar powder BC to CMC-Na hydrogel, stirring and reacting in a water bath at 50-100° C. for 1-1.5 h to obtain a mixed solution; Step 7: After the reaction, the mixture was allowed to stand, and washed by centrifugation with ultrapure water until the supernatant was neutral and colorless, and then vacuum freeze-dried; Step 8: Grind the vacuum freeze-dried biochar to obtain CMC-Na modified biochar powder CMC-BC.

[0006] A further improvement of the present invention is that in step 1, the dried reed straw is cut into small segments of 1 to 3 cm, then ground and passed through an 80-mesh sieve to obtain biomass powder.

[0007] A further improvement of the present invention is that in step 3, the temperature is raised to a preset temperature at a rate of 3-6°C / min, wherein the preset temperature is 500°C.

[0008] A further improvement of the present invention is that in step 4, the drying temperature is 50-80° C. and the drying time is 10-15 hours.

[0009] A further improvement of the present invention is that the specific method for obtaining the CMC-Na hydrogel in step 5 is: Step 5.1, measure a certain amount of deionized water and place it in a beaker, and heat it in a water bath to 40-60°C; Step 5.2, weigh a certain amount of CMC-Na powder, and dissolve the CMC-Na powder in deionized water while stirring; Step 5.3, place the magnetic stirrer in a water bath, maintain the water bath temperature at 40-60°C, rotate at 500-700 rpm, and stir for 15-30 min. After the reaction is completed, a uniformly mixed CMC-Na hydrogel is obtained.

[0010] A further improvement of the present invention is that in step 5, the concentration of the CMC-Na hydrogel is 10 g / L.

[0011] A further improvement of the present invention is that in step 6, unmodified biochar powder BC is added to the CMC-Na hydrogel, and the mass ratio of CMC-Na / BC is 1:(1-10).

[0012] A further improvement of the present invention is that the specific method for obtaining the mixed solution in step 6 is: Step 6.1, weigh a certain amount of unmodified biochar powder BC and place it into the mixed CMC-Na hydrogel, and perform ultrasonic treatment for 1-5 min until the biochar powder is dissolved; Step 6.2, place the magnetic stirrer in a water bath, set the water bath temperature to 50-100°C, rotate at 500-700 rpm, and stir for 1-1.5 h. The reaction is completed to obtain a fully reacted and uniform mixed solution.

[0013] A further improvement of the present invention is that in step 7, the number of centrifugal washings is 3 to 5 times.

[0014] The present invention also provides an application of CMC-Na modified biochar for treating ammonia nitrogen wastewater.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a preparation method of biochar modified by CMC-Na. By converting reed straw into biochar, not only the resource utilization of agricultural waste is realized, but also the biochar itself has a porous structure and a large specific surface area, which is conducive to adsorption and catalytic reactions; the introduction of CMC-Na further improves the surface properties and pore structure of the biochar, enhances the electron transfer capacity, and the preparation method of the present invention has clear steps, is simple to operate, and is easy to apply and promote on an industrial scale.

[0016] The present invention also provides an application of CMC-Na modified biochar in ammonia nitrogen wastewater treatment. By directly adding CMC-Na modified biochar into the influent of the anaerobic iron ammonia oxidation process, the ammonia nitrogen removal rate can reach approximately 100%, and the ammonia nitrogen emission is near zero. At the same time, the total nitrogen removal efficiency and electron transfer rate are significantly improved, which has a positive effect on the functional microorganisms of the anaerobic iron ammonia oxidation system. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings described herein are only for explanation purposes and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the various components in the drawings are only schematic, used to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the various components of the present invention.

[0018] Figure 1 It is the scanning electron microscope SEM image of the modified biochar CMC-BC with a CMC-Na / BC mass ratio of 1:8 and the unmodified biochar BC; Figure 2 It is a graph showing the nitrogen changes when different proportions of CMC-Na modified biochar were added in Example 1 and Example 2; Figure 3 It is a graph showing the nitrogen changes of modified biochar CMC-BC and unmodified biochar BC added in Example 1 and Comparative Example 1; Figure 4 It is a graph showing changes in cytochrome c (cytc) and INT-ETS content before and after cultivation in the ASBR reactor of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0020] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0021] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are for simplicity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0022] In this document, unless otherwise specified, “includes,” “including,” “contains,” “has,” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0023] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0024] The present invention provides a method for preparing biochar based on CMC-Na modification, comprising the following steps: Step 1, washing the reed straw with distilled water and drying it in an oven, then cutting it into small pieces of 1 to 3 cm, grinding it clockwise, and passing it through an 80-mesh sieve to obtain biomass powder; It should be noted that since biomass powder is very easy to float in the air, the stirring force and speed need to be controlled during the grinding process, and the amount of biomass added should be appropriate.

[0025] Step 2, weigh a certain amount of biomass powder and evenly place it in a porcelain boat, place the porcelain boat in the center of the constant temperature zone of the tube furnace, use a vacuum pump to evacuate the air in the tube furnace to a near vacuum state, and then introduce N2, repeating 3 to 5 times; Step 3, start the heating program, heat up to the preset temperature at a rate of 3-6°C / min, keep the constant temperature for 100-150 minutes, then start cooling, and take out the sample after it reaches room temperature; Step 4, after the sample is fully washed with deionized water and anhydrous ethanol, it is placed in a drying oven at 50-80°C for 10-15 hours to obtain unmodified biochar powder BC; Step 5, measuring a certain amount of deionized water and placing it in a beaker, heating it to 40-60° C. in a water bath, then weighing a certain amount of CMC-Na powder, dissolving the CMC-Na powder in the deionized water while stirring, and then placing a magnetic stirrer in the water bath, maintaining the water bath temperature at 40-60° C., rotating speed at 500-700 rpm, stirring time at 15-30 min, and completing the reaction to obtain a uniformly mixed CMC-Na hydrogel; Step 6, weigh a certain amount of unmodified biochar powder BC and place it in the evenly mixed CMC-Na hydrogel, so that the mass ratio of CMC-Na hydrogel to unmodified biochar powder BC is 1: (1-10), then perform ultrasonic treatment for 1-5 minutes until the biochar powder is dissolved, place a magnetic stirrer in a water bath, the water bath temperature is 50-100°C, the speed is 500-700rpm, the stirring time is 1-1.5 hours, and the reaction is completed to obtain a fully reacted and uniform mixed solution; Step 7: After the reaction is complete, let it stand and wash it with ultrapure water by centrifugation for 3 to 5 times until the supernatant is neutral and colorless, and then perform vacuum freeze drying; Step 8: Grind the vacuum freeze-dried biochar to obtain CMC-Na modified biochar powder CMC-BC.

[0026] The present invention also provides an application of CMC-Na modified biochar in ammonia nitrogen wastewater treatment, and the ammonia nitrogen removal performance and electron transfer rate of the anaerobic iron ammonia oxidation process can be significantly improved by adding CMC-BC modified biochar to the anaerobic iron ammonia oxidation sludge.

[0027] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0028] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples where specific conditions are not specified are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The various raw materials used in the following examples are conventional commercial products unless otherwise specified, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" means weight percentage, "part" means weight part, and ratio means weight ratio.

[0029] Example 1 This embodiment provides a method for preparing CMC-Na modified biochar, comprising the following steps: Step 1: Rinse the reed straw with distilled water, dry it in an oven, cut it into small pieces of 2 cm in length, grind it clockwise, and pass it through an 80-mesh sieve to obtain biomass powder; Step 2: Weigh 2 g of biomass powder and place it evenly in a porcelain boat. Place the porcelain boat in the center of the constant temperature zone of the tube furnace. Use a vacuum pump to extract the air in the tube furnace until the furnace is close to a vacuum state, then introduce N2. Repeat this three times. Step 3: After the temperature rise program is started, the temperature is raised to the preset temperature of 500°C at a rate of 5°C / min. After maintaining the constant temperature for 120 minutes, the temperature is lowered and the sample is taken out after it reaches room temperature. Step 4: After the sample was thoroughly washed with deionized water and anhydrous ethanol, it was placed in an oven and dried at 60°C for 12 h to obtain unmodified biochar powder BC; Step 5: Measure 200 ml of deionized water and place it in a beaker, heat it to 50°C in a water bath, then weigh 2 g of CMC-Na (sodium carboxymethyl cellulose) powder, add the CMC-Na powder into the deionized water while stirring and dissolving, then place a magnetic stirrer in the water bath, keep the water bath temperature at 50°C, rotate at 600 rpm, and stir for 20 min. After the reaction is completed, a 10 g / L CMC-Na hydrogel is obtained. Step 6: Set the mass ratio of CMC-Na / BC to 1:8, weigh 16 g of unmodified biochar powder BC and place it in 200 ml of evenly mixed CMC-Na hydrogel, ultrasonicate for 2 min until the biochar powder is dissolved, place a magnetic stirrer in a water bath, keep the water bath temperature at 90 ° C, rotate at 600 rpm, stir for 1 h, and complete the reaction to obtain a fully reacted and uniform mixed solution; Step 7: After the mixed solution is allowed to settle for 1 hour, it is washed by centrifugation with ultrapure water three times until the supernatant is neutral and colorless, and then the solution is placed in a -80°C refrigerator for 24 hours. The frozen solution is placed in a vacuum freezer and freeze-dried for 48 hours. Step 8: Grind the vacuum freeze-dried biochar to obtain CMC-Na modified biochar powder CMC-BC.

[0030] Embodiment 2: This embodiment provides a method for preparing modified biochar of CMC-Na, wherein the mass ratio of CMC-Na / BC is set to 1:2, 4 g of unmodified biochar powder BC is weighed and placed in 200 ml of uniformly mixed CMC-Na hydrogel, ultrasonically treated for 2 min until the biochar powder is dissolved, a magnetic stirrer is placed in a water bath, the water bath temperature is maintained at 90°C, the speed is 600 rpm, the stirring time is 1 h, and the reaction is completed to obtain a fully reacted and uniform mixed solution. The contents not mentioned in this embodiment are the same as the description of Example 1.

[0031] Example 3 This embodiment provides a method for preparing modified biochar of CMC-Na, wherein the mass ratio of CMC-Na / BC is set to 1:4, 8 g of unmodified biochar powder BC is weighed and placed in 200 ml of uniformly mixed CMC-Na hydrogel, ultrasonically treated for 2 min until the biochar powder is dissolved, a magnetic stirrer is placed in a water bath, the water bath temperature is maintained at 90°C, the speed is 600 rpm, the stirring time is 1 h, and the reaction is completed to obtain a fully reacted and uniform mixed solution. The contents not mentioned in this embodiment are the same as the description of Example 1.

[0032] Example 4 This embodiment provides a method for preparing modified biochar of CMC-Na, wherein the mass ratio of CMC-Na / BC is set to 1:6, 12 g of unmodified biochar powder BC is weighed and placed in 200 ml of uniformly mixed CMC-Na hydrogel, ultrasonically treated for 2 min until the biochar powder is dissolved, a magnetic stirrer is placed in a water bath, the water bath temperature is maintained at 90°C, the speed is 600 rpm, the stirring time is 1 h, and the reaction is completed to obtain a fully reacted and uniform mixed solution. The contents not mentioned in this embodiment are the same as the description of Example 1.

[0033] Example 5 This embodiment provides a method for preparing modified biochar of CMC-Na, wherein the mass ratio of CMC-Na / BC is set to 1:10, 20 g of unmodified biochar powder BC is weighed and placed in 200 ml of uniformly mixed CMC-Na hydrogel, ultrasonically treated for 2 min until the biochar powder is dissolved, a magnetic stirrer is placed in a water bath, the water bath temperature is maintained at 90°C, the speed is 600 rpm, the stirring time is 1 h, and the reaction is completed to obtain a fully reacted and uniform mixed solution. The contents not mentioned in this embodiment are the same as the description of Example 1.

[0034] Comparative Example 1 This comparative example provides a method for preparing powdered biochar, comprising the following steps: Step 1: Rinse the reed straw with distilled water, dry it in an oven, cut it into small pieces of 2 cm in length, grind it clockwise, and pass it through an 80-mesh sieve to obtain biomass powder; Step 2: Weigh 2 g of biomass powder and place it evenly in a porcelain boat. Place the porcelain boat in the center of the constant temperature zone of the tube furnace. Use a vacuum pump to extract the air in the tube furnace until the furnace is close to a vacuum state, then introduce N2. Repeat this three times. Step 3: After the temperature rise program is started, the temperature is raised to the preset temperature of 500°C at a rate of 5°C / min. After maintaining the constant temperature for 120 minutes, the temperature is lowered and the sample is taken out after it reaches room temperature. Step 4: After the sample was thoroughly washed with deionized water and anhydrous ethanol, it was placed in an oven and dried at 60°C for 12 h to obtain unmodified powdered biochar BC.

[0035] The present invention also provides a method for treating ammonia nitrogen wastewater based on CMC-BC modified biochar enhanced anaerobic iron ammonia oxidation, comprising the following steps: Step 1, anaerobic iron-ammonia oxidation sludge pretreatment: using anaerobic iron-ammonia oxidation sludge with long-term stable operation of ASBR and good operation performance, and adjusting the concentration of anaerobic iron-ammonia oxidation sludge to 5200±500mg / L, and maintaining the ammonia nitrogen removal rate at about 40%; Step 2, preparation of simulated wastewater solution containing ammonia nitrogen: including the following components with mass concentrations: 50 mg N / L NH4 + -N, 24mg Fe 3+ / L of FeCl3 concentration, 43mg / L of KH2PO4, 75mg COD / L of glucose, 500mg / L of NaHCO3 and 0.75mL / L of trace element concentrate, wherein the trace element concentrate includes the following subcomponents in mass concentration: 8630mg / L of Na2EDTA, 640mg / L of ZnSO4·7H2O, 360mg / L of CoCl2·6H2O, 490mg / L of MnCl2·4H2O, 130mg / L of CuSO4·5H2O, 330mg / L of Na2MoO4·2H2O, 280mg / L of NiCl2·6H2O, 310mg / L of NaSeO4·10H2O and 14mg / L of H3BO3; Step 3, using an anaerobic sequencing batch reactor ASBR reactor with an effective volume of 3L, artificial water inlet and outlet, a water exchange ratio set to 50%, and a hydraulic retention time of 2d; the ASBR reactor adopts mechanical stirring, and the stirring speed is controlled at 150rpm; the temperature of the ASBR reactor working area is controlled to be about 25-30°C, the dissolved oxygen in the influent is less than 0.2mg / L, and the dissolved oxygen in the reactor is controlled to be less than 0.6mg / L; the ASBR reactor uses a time controller to control a single operation cycle of 24h, and the time of each operation stage is: water inlet 0.3h, stirring 21h, sedimentation 2h, and drainage 0.3h; Step 4, two groups of experiments were set up respectively: in the first group of experiments, 15 g of five modified biochars with different CMC-Na / BC mass ratios (Example 1, Example 2, Example 3, Example 4 and Example 5) were added to five reactors at a dosage of 5 g / L, and no biochar was added as a blank control group. The experiment was run for 36 hours to explore the short-term effects of adding different modified biochars on anaerobic iron-ammonia oxidation; Step 5, the second group of experiments, taking an equal amount of 15g of biochar (Example 1 and Comparative Example 1) and adding it to two different reactors (CMC-BC and BC), the dosage is 5g / L, and no biochar is added as the blank control group, and running for 30 days to explore the long-term effect of adding modified CMC biochar on anaerobic iron-ammonia oxidation; Step 6. After the two groups of experimental reactions are completed, the supernatant is taken and filtered through a 0.45um filter membrane to determine the changes in the reaction matrix (nitrogen: ammonia nitrogen, nitrite nitrogen and nitrate nitrogen), and the sludge mixed liquid is taken to determine the energy metabolism level indicators of the second group of experiments (changes in cytochrome c (cytc) and INT-ETS content).

[0036] like Figure 1 As shown in the figure, the surface of unmodified reed biochar is smooth and tubular, with irregular grooves between the skeletons, and no obvious pore structure is observed. However, the surface of biochar modified by CMC is rough and uneven with many small particles attached, which is the most dense and has multiple pore structures. These rich pore structures and the presence of surface particles provide adsorption sites for biochar, giving it good physical adsorption capacity.

[0037] like Figure 2 As shown in the figure, adding different proportions of CMC-BC makes the residual ammonia nitrogen in the anaerobic iron ammonia oxidation system less than that without adding biochar. Among them, the modified biochar NH4 + -N removal efficiency is the highest, reaching 99.36%.

[0038] like Figure 3 As shown in the figure, compared with the blank control group and the group with unmodified biochar BC, when 5g / L CMC-BC was added to the ASBR reactor, the ammonia nitrogen concentration in the effluent of the ASBR reactor was significantly reduced, with an average NH4 + -N removal rate can reach 95.10%. In addition, it was observed that the effluent nitrate concentration decreased and the total nitrogen removal performance was further improved, with an average TN removal rate of 74.05%, which was significantly higher than that of the control group (54.48%) and the BC group (67.86%).

[0039] like Figure 4 As shown, it is the energy metabolism level index before and after the ASBR reactor is cultivated. When 5g / L CMC-BC is added to the ASBR reactor, compared with the blank control group and the group adding unmodified biochar BC, the cytochrome C (c-Cyts) content is significantly increased, and the electron transfer system activity (INT-ETS) is 68.58% and 21.94% higher than that of the blank group and BC, respectively.

[0040] In summary, the ammonia nitrogen removal performance and electron transfer rate of the anaerobic iron-ammonia oxidation process can be significantly improved by adding 5 g / L CMC-BC modified biochar to the anaerobic iron-ammonia oxidation sludge concentration of 5200±500 mg / L.

[0041] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing biochar based on CMC-Na modification, characterized in that: The following steps are involved: Step 1, washing the reed straw with distilled water and drying it, then chopping and sieving it to obtain biomass powder; Step 2, weigh a certain amount of biomass powder and evenly place it in a porcelain boat, place the porcelain boat in a tube furnace, evacuate the air in the tube furnace to a near vacuum state, and then introduce N2, repeating 3 to 5 times; Step 3, heating to a preset temperature, maintaining the constant temperature for 100-150 minutes, then cooling down, and taking out the sample after it reaches room temperature; Step 4, washing and drying the sample to obtain unmodified biochar powder BC; Step 5, dissolving CMC-Na powder in deionized water at 40-60° C. and stirring for 15-30 min to obtain CMC-Na hydrogel; Step 6, adding unmodified biochar powder BC to CMC-Na hydrogel, stirring and reacting in a water bath at 50-100° C. for 1-1.5 h to obtain a mixed solution; Step 7: After the reaction, the mixture was allowed to stand, and washed by centrifugation with ultrapure water until the supernatant was neutral and colorless, and then vacuum freeze-dried; Step 8: Grind the vacuum freeze-dried biochar to obtain CMC-Na modified biochar powder CMC-BC.

2. The method for preparing CMC-Na modified biochar according to claim 1, characterized in that: In the step 1, the dried reed straw is cut into small pieces of 1-3 cm, and then ground and passed through an 80-mesh sieve to obtain biomass powder.

3. The method for preparing CMC-Na modified biochar according to claim 1, characterized in that: In step 3, the temperature is raised to a preset temperature at a rate of 3-6°C / min, wherein the preset temperature is 500°C.

4. The method for preparing CMC-Na modified biochar according to claim 1, characterized in that: In step 4, the drying temperature is 50-80° C. and the drying time is 10-15 hours.

5. The method for preparing biochar based on CMC-Na modification according to claim 1, characterized in that: The specific method for obtaining the CMC-Na hydrogel in step 5 is: Step 5.1, measure a certain amount of deionized water and place it in a beaker, and heat it in a water bath to 40-60°C; Step 5.2, weigh a certain amount of CMC-Na powder, and dissolve the CMC-Na powder in deionized water while stirring; Step 5.3, place the magnetic stirrer in a water bath, maintain the water bath temperature at 40-60°C, rotate at 500-700 rpm, and stir for 15-30 min. After the reaction is completed, a uniformly mixed CMC-Na hydrogel is obtained.

6. The method for preparing CMC-Na modified biochar according to claim 1, characterized in that: In step 5, the concentration of CMC-Na hydrogel is 10 g / L.

7. The method for preparing CMC-Na modified biochar according to claim 1, characterized in that: In step 6, unmodified biochar powder BC is added to the CMC-Na hydrogel, and the mass ratio of CMC-Na / BC is 1: (1-10).

8. The method for preparing biochar based on CMC-Na modification according to claim 1, characterized in that: The specific method for obtaining the mixed solution in step 6 is: Step 6.1, weigh a certain amount of unmodified biochar powder BC and place it into the mixed CMC-Na hydrogel, and perform ultrasonic treatment for 1-5 min until the biochar powder is dissolved; Step 6.2, place the magnetic stirrer in a water bath, set the water bath temperature to 50-100°C, rotate at 500-700 rpm, and stir for 1-1.5 h. The reaction is completed to obtain a fully reacted and uniform mixed solution.

9. The method for preparing CMC-Na modified biochar according to claim 1, characterized in that: It is characterized in that In step 7, the number of centrifugal washings is 3 to 5 times.

10. An application of CMC-Na modified biochar prepared by the preparation method according to claim 1, characterized in that: CMC-Na modified biochar for ammonia nitrogen wastewater treatment.

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