Magnetic ldh biochar and preparation method and application thereof

Abstract

The application discloses a magnetic LDH biochar as well as a preparation method and application thereof, and belongs to the technical field of pollution prevention and control. The preparation method of the magnetic LDH biochar comprises the following steps: sequentially performing puffing treatment and pyrolysis carbonization treatment on iron-containing biomass to obtain magnetic biochar; mixing a magnesium salt solution and the magnetic biochar, and then performing hydrothermal reaction to obtain magnetic loaded iron-magnesium hydrotalcite biochar; mixing the magnetic loaded iron-magnesium hydrotalcite biochar and a nitrogen source, and then heating and doping to obtain the magnetic LDH biochar. The magnetic LDH biochar prepared by the application can adsorb dissolved oxygen around the carbon material, improve the mass transfer efficiency and dissolved oxygen utilization rate of microbial degradation of pollutants in the artificial wetland, rapidly enrich oxygen, increase the dissolved oxygen concentration of the water body, has the advantages of low energy consumption, difficulty in blocking, high nitrogen and phosphorus removal efficiency and the like, and when the magnetic LDH biochar is applied to the intermittent aeration artificial wetland, the nitrogen and phosphorus treatment performance of the artificial wetland can be effectively strengthened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pollution prevention and control, in particular to a magnetic LDH biochar as well as a preparation method and application thereof. BACKGROUND

[0002] Inorganic pollution such as nitrogen and phosphorus is a difficult problem of global pollution, especially the loss of soil nitrogen and phosphorus caused by agricultural non-point source pollution. Most of the inorganic nitrogen and phosphorus have water solubility and exchangeability, and have strong migration ability, which is the main reason leading to water eutrophication. Agricultural non-point source pollution has the characteristics of diverse and dispersed sources, wide range of involvement, and difficulty in monitoring, which has become a problem in water environmental pollution prevention and control. Direct discharge of farmland runoff can easily cause water eutrophication. Artificial wetlands, as a wastewater treatment technology, mainly rely on substrates, plants and microorganisms to treat wastewater by physical, chemical and biological synergistic effects, effectively removing nitrogen and phosphorus in farmland runoff. However, the characteristics of low carbon-nitrogen ratio (COD / TN) of farmland runoff affect the removal effect of microorganisms on nitrogen and phosphorus.

[0003] Therefore, how to prepare a biochar that can enhance the removal of nitrogen and phosphorus in farmland runoff has become a technical problem that needs to be solved by those skilled in the art. SUMMARY

[0004] The purpose of the present application is to provide a magnetic LDH biochar as well as a preparation method and application thereof, so as to solve the problems existing in the prior art.

[0005] To achieve the above purpose, the present application provides the following solutions:

[0006] One of the technical solutions of the present application is a preparation method of a magnetic LDH biochar, comprising the following steps:

[0007] The iron-containing biomass is sequentially subjected to puffing treatment and pyrolysis carbonization treatment to obtain a magnetic biochar;

[0008] The magnesium salt solution and the magnetic biochar are mixed and subjected to hydrothermal reaction to obtain a magnetic loaded iron-magnesium hydrotalcite biochar;

[0009] The magnetic loaded iron-magnesium hydrotalcite biochar and a nitrogen source are mixed and heated to dope to obtain the magnetic LDH biochar.

[0010] Further, the iron-containing biomass is iron-rich water hyacinth; the iron content of the iron-containing biomass is 1.5-5 wt.%; the puffing treatment specifically comprises: heating to 160-180 DEG C, the pressure is 5-7 atm, and then reducing the pressure to normal pressure within 1-5 s.

[0011] Further, after the puffing treatment and the pyrolysis carbonization treatment, drying and crushing steps are further included; the drying and crushing specifically include: drying at a temperature of 90-100 DEG C until the water content is 10-20%, then drying at a temperature of 80-90 DEG C until dry, and finally crushing to 50-100 mesh.

[0012] The biochar can promote the growth of plant roots, increase the air permeability of the substrate, facilitate the diffusion of oxygen, prevent the clogging of the artificial wetland when the artificial wetland is used for treating sewage, effectively conserve dissolved oxygen, improve the utilization rate of the dissolved oxygen, promote the oxygen secretion of the plant roots, increase the dissolved oxygen content of the water body, and further improve the treatment performance of the artificial wetland.

[0013] Water hyacinth is a kind of plant with iron enrichment ability induced by artificial selection, and the magnetic biochar can be prepared by using the water hyacinth rich in iron as a raw material of the biochar, the magnetic biochar has strong specificity for the absorption of nitrogen and phosphorus elements, especially has a reinforcing effect on the adsorption and fixation of nitrogen, and can be used for constructing a biochar-based reinforced artificial wetland, and the water hyacinth biomass is cheap and easy to obtain.

[0014] In the technical scheme of the present application, the raw material water hyacinth is first subjected to heat puffing treatment, and then subjected to drying and crushing, so that the water hyacinth is dehydrated and puffed, and the prepared magnetic biochar has higher pore structure and better adsorption effect.

[0015] Further, the pyrolysis carbonization treatment is microwave pyrolysis carbonization treatment, the power is 600-1800 W, and the time is 5-40 min; and the pyrolysis carbonization treatment is carried out in a nitrogen atmosphere.

[0016] Further, after the pyrolysis carbonization treatment, a washing and drying step is further included; the washing specifically includes: repeatedly washing 3-5 times with an equal volume of deionized water and 95vol.% anhydrous ethanol; and the drying temperature is 50-90 DEG C.

[0017] Further, the magnetic biochar and the magnesium salt solution are mixed to carry out a hydrothermal reaction to obtain the magnetic iron-magnesium hydrotalcite biochar.

[0018] The magnetic biochar is added into the magnesium salt solution and stirred and mixed uniformly to obtain a solution A;

[0019] Sodium hydroxide and sodium carbonate are dissolved in water to obtain a solution B;

[0020] The solution B is added into the solution A, and after stirring and reaction, hydrothermal reaction, separation and drying, the magnetic iron-magnesium hydrotalcite biochar is obtained.

[0021] Further, the use amount ratio of the magnesium salt in the magnesium salt solution and the magnetic biochar is 0.02-0.04 mol:1.5-2.5 g; the magnesium salt solution is composed of the magnesium salt and water with a use amount ratio of 0.02-0.04 mol:60-100 mL; the use amount ratio of the sodium hydroxide, the sodium carbonate and the water is 0.02-0.03 mol:0.03-0.05 mol:30-50 mL; and the volume ratio of the solution A and the solution B is 6:5-2:1.

[0022] Further, the use amount ratio of the magnesium salt in the magnesium salt solution, the magnetic biochar, the sodium hydroxide and the sodium carbonate is 0.02-0.04 mol:1.5-2.5 g:0.02-0.03 mol:0.03-0.05 mol.

[0023] Further, the magnesium salt is magnesium nitrate; the stirring speed of the mixing is 300-450 r / min; the stirring reaction time is 2-4 h; the hydrothermal reaction temperature is 120-180 DEG C, and the hydrothermal reaction time is 4-6 h; and the separation is centrifugal separation, the centrifugal separation speed is 7000-9000 r / min, and the centrifugal separation time is 25-35 min.

[0024] Mg can promote the adsorption of phosphorus, and loading Mg on the magnetic biochar can strengthen microbial activity and improve the degradation and removal of organic matters such as pesticides. The application further improves the treatment performance of the magnetic biochar used in the treatment of sewage in the artificial wetland, mixes the magnesium salt solution and the magnetic biochar, performs hydrothermal reaction, obtains the magnetic loaded iron-magnesium hydrotalcite biochar, the magnetic loaded iron-magnesium hydrotalcite biochar has the typical layered structure of hydrotalcite, therefore, has developed pore structure and high specific surface area, so that the product has better adsorption performance and improves the nitrogen and phosphorus removal efficiency, thereby having better treatment effect when used in the treatment of sewage in the artificial wetland.

[0025] Loading Mg on the magnetic biochar can strengthen microbial activity and improve the nitrogen and phosphorus removal efficiency, but when the composite amount of the magnesium salt and the magnetic carbon is high (the use amount of the magnetic carbon is large), in the hydrothermal synthesis process, the carbon element enters the crystal lattice, destroys and reconstructs the crystal structure of the hydrotalcite, changes the surface chemical properties and functional groups of the LDH loaded on the biochar, and thus reduces the adsorption capacity for nitrogen and phosphorus.

[0026] The solution B is a mixed alkali solution of sodium hydroxide and sodium carbonate, the sodium hydroxide functions to regulate the alkali environment, and the sodium carbonate functions as an effective precipitant. The solution B is added to the solution A, and by controlling the dropping speed of the mixed alkali solution, the pH value of the solution system is maintained at 9-11.

[0027] Further, the mass ratio of the magnetic loaded iron-magnesium hydrotalcite biochar and the nitrogen source is 1:(0.1-0.4); the heating and doping temperature increasing rate is 1-5 DEG C / min, the temperature is 800-1200 DEG C, and the time is 90-150 min; and the nitrogen source comprises urea.

[0028] The heating and doping temperature increasing rate should not be too fast, which is not conducive to the formation of initial pores. The carbonization temperature should not be too high or too low, and the carbonization time should not be too short, which is not conducive to the complete carbonization of the material.

[0029] Further, the drying temperature is 70-90 DEG C, and the time is 8-10 h. After the magnetic loaded iron-magnesium hydrotalcite biochar is prepared, the magnetic LDH biochar can be further doped with urea by heating, so that the nitrogen content in the magnetic LDH biochar reaches 4-7%.

[0030] The urea has high N content and low price, and its pyrolysis product is non-toxic. The N-doped carbon material (magnetic LDH biochar) has a unique complex structure, high pore volume, large adsorption capacity, fast adsorption kinetics, mechanical and thermal stability, and low cost.

[0031] The second technical scheme of the present application is a magnetic LDH biochar prepared by the above preparation method.

[0032] The third technical scheme of the present application is an application of the above magnetic LDH biochar in the removal of nitrogen and phosphorus in wastewater in a constructed wetland.

[0033] Further, the concentration of COD in the wastewater is 30.0-50.0 mg / L, the concentration of ammonia nitrogen is 8.0-12.5 mg / L, the concentration of total nitrogen is 11.0-20.0 mg / L, the concentration of total phosphorus is 1.2-4.0 mg / L, and the carbon-nitrogen ratio (COD / TN) is 1.5-4.5.

[0034] Further, the method of the application specifically comprises: incorporating the magnetic LDH biochar into a substrate layer of a constructed wetland, and performing intermittent aeration.

[0035] The incorporation ratio of the magnetic LDH biochar is 1.2-2.5 wt.%.

[0036] The intermittent aeration is intermittent oxygen supply, and the frequency is 20-40 min of oxygen supply every 3-6 h.

[0037] The present application discloses the following technical effects:

[0038] (1) The magnetic LDH biochar prepared by the application can adsorb dissolved oxygen around the carbon material, improve the mass transfer efficiency and dissolved oxygen utilization rate of microorganisms in the artificial wetland, rapidly enrich oxygen, increase the dissolved oxygen concentration of the water body, has the advantages of low energy consumption, not easy to block, high denitrification and phosphorus removal efficiency, etc. By applying the magnetic LDH biochar to the intermittent aeration artificial wetland, the nitrogen and phosphorus treatment performance of the artificial wetland can be effectively strengthened, the cost of the artificial wetland ecosystem can be reduced, the artificial wetland can be operated efficiently and stably, and the application is suitable for farmland runoff treatment engineering or existing engineering improvement.

[0039] The magnetic LDH biochar of the application is applied to the artificial wetland, and the magnetic LDH biochar can be used as an external carbon source to regulate the system electron supply. The magnetic biochar has a strengthening effect on the adsorption and fixation of nitrogen, Mg promotes the adsorption of phosphorus, and the intermittent aeration optimizes the redox environment of the system, so that the efficient microbial degradation and removal of nitrogen and phosphorus in the low-carbon-nitrogen ratio farmland runoff can be realized.

[0040] (2) The magnetic LDH biochar prepared by the application has a high specific surface area (225.1 m 2 / g) and surface potential, has a large number of nitrogen and phosphorus adsorption sites, can efficiently adsorb nitrogen and phosphorus, and has a magnetic response characteristic, so that the material can be recovered by an external magnetic field, solving the problems of difficult recovery, difficult separation and large loss of the material.

[0041] (3) The magnetic LDH biochar is prepared by using the waste biomass water hyacinth as a raw material through thermal expansion and pyrolysis carbonization. The magnetic LDH biochar has a high specific surface area and surface potential, a more stable layered structure, a higher loading capacity and a stronger magnetic response strength. At the same time, the magnetic LDH biochar can form a micro-battery and an electron conduction pipeline in wastewater, has a more significant potential difference and redox activity, makes up for the lack of electron acceptors and drives the electron transfer activity, promotes the enrichment of electroactive microorganisms, and further strengthens the microbial denitrification and phosphorus removal performance. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0043] Figure 1 The electron microscope graph of the magnetic LDH biochar prepared in Example 1 of the application;

[0044] Figure 2 The electron microscope graph of the ordinary biochar material prepared in Comparative Example 1 of the application;

[0045] Figure 3 Figure 6 is a graph of the total nitrogen removal performance results for a constructed wetland without biochar, a constructed wetland incorporating magnetic LDH biochar prepared according to Example 1 of the present application, and a constructed wetland incorporating conventional biochar material prepared according to Comparative Example 1, on farmland runoff water;

[0046] Figure 4 Figure 7 is a graph of the total phosphorus removal performance results for a constructed wetland without biochar, a constructed wetland incorporating magnetic LDH biochar prepared according to Example 1 of the present application, and a constructed wetland incorporating conventional biochar material prepared according to Comparative Example 1, on farmland runoff water;

[0047] Figure 5 Figure 8 is a graph of the reoxygenation capacity results for a constructed wetland without biochar, a constructed wetland incorporating magnetic LDH biochar prepared according to Example 1 of the present application, and a constructed wetland incorporating conventional biochar material prepared according to Comparative Example 1. DETAILED DESCRIPTION

[0048] Various illustrative embodiments of the present application are described in detail herein, with reference to which like numerals indicate like elements throughout the various figures. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, the use of "including," "comprising," "having," "containing," and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "comprising," "including," "containing," and "having," and variations thereof, are intended to be open-ended and mean that other components, materials, acts, and / or steps not specifically recited are optional and can be added.

[0049] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. In addition, for a range of values of a parameter, unless otherwise indicated, each intervening value by each intervening value, as well as any other stated or intervening value in that stated range is encompassed. In addition, any combination of the above ranges, as well as any other stated or intervening value in that stated range, is encompassed. Unless otherwise stated, the above ranges are inclusive of the recited endpoints. Any numerical values recited herein include all values between the lower and upper values, unless the context clearly dictates otherwise. The dimensions and other such numerical values are not to be understood as being strictly limited to the exact numerical values recited.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All publications mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present application is not entitled to antedate such publication by virtue of prior application.

[0051] Various modifications and variations can be made to the specific embodiments of the application described herein without departing from the scope or spirit of the application. Other embodiments of the application will be apparent from consideration of the specification and practice of the application disclosed herein. For example, the specification can be used to design equivalent processes and equivalent structures to those described herein. The specification can also be used to design other processes and structures that do not depart from the spirit and scope of the present application. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the application being indicated by the following claims.

[0052] With respect to the use of "comprising", "including", "containing", "having" and "by" herein, these terms are used in their open, conventional sense and should be read to mean "including, but not limited to".

[0053] The "parts" described in the following examples are "parts by weight".

[0054] Example 1

[0055] A method for preparing a magnetic LDH biochar:

[0056] (1) Selecting a plant water hyacinth with high iron content as a raw material (iron content of 2.5 wt.%), washing the water hyacinth with deionized water and then placing it in an expander, heating to an expansion temperature of 160°C, and a pressure of 5 atmospheres, then instantaneously reducing the pressure to normal pressure within 3s to make the water hyacinth expand; drying the expanded water hyacinth at a temperature of 100°C until the water content is 20%, and then drying it to dryness at a temperature of 80°C, crushing and passing through a 100 mesh sieve to obtain expanded water hyacinth powder.

[0057] The expanded water hyacinth powder is placed in a microwave pyrolysis carbonization device and subjected to microwave pyrolysis carbonization treatment under a nitrogen atmosphere (power 1100W, time 20min). After carbonization, it is cooled to room temperature and removed, and then repeatedly washed with an equal volume of deionized water and 95vol.% anhydrous ethanol in the order of first deionized water and then anhydrous ethanol, for 3 times, and then dried at a temperature of 70°C to obtain a magnetic biochar.

[0058] (2) Weigh 0.03 mol of magnesium nitrate and dissolve it in 80 mL of deionized water, add 1.5 g of magnetic biochar and stir uniformly (rotational speed of 350 r / min) to obtain solution A; weigh 0.02 mol of sodium hydroxide and 0.04 mol of sodium carbonate and dissolve them in 50 mL of deionized water to obtain solution B; slowly add solution B to solution A and stir at room temperature for 3h, then transfer it to a polytetrafluoroethylene high-pressure reaction kettle, and hydrothermally react at a temperature of 120°C for 6h, then remove it, separate the solid phase by centrifuge, the centrifugal speed is 7000r / min, the centrifugal time is 25min, wash with clean water, and dry at a temperature of 90°C for 8h to obtain a magnetic iron-magnesium hydrotalcite-loaded biochar.

[0059] (3) Mix the magnetic iron-magnesium hydrotalcite-loaded biochar and urea in a mass ratio of 1:0.4, then heat to 1000°C at a heating rate of 3°C / min, and dope at 1000°C for 100min to obtain a magnetic LDH biochar (see Figure 1 ).

[0060] Comparative Example 1

[0061] Common biochar material:

[0062] Select the water hyacinth containing 0.3wt.% of iron as raw material, wash the water hyacinth with deionized water, then place it in the expander, heat to the expansion temperature of 160℃, the pressure is 5 atmospheres, then instantaneously reduce the pressure to normal pressure within 3s, so that the water hyacinth is expanded; the expanded water hyacinth is dried at a temperature of 100℃ until the water content is 20%, then dried at a temperature of 80℃ until dry, crushed and passed through a 100 mesh sieve to obtain expanded water hyacinth powder.

[0063] The expanded water hyacinth powder is placed in a microwave pyrolysis carbonization device, and microwave pyrolysis carbonization treatment is carried out under a nitrogen atmosphere (power 1100W, time 20min). After carbonization, cool to room temperature and take out, then repeatedly wash with equal volume of deionized water and 95vol.% anhydrous ethanol in the order of first and then second, and then dry at a temperature of 70℃ to obtain ordinary biochar material (see electron microscope image Figure 2 ).

[0064] Comparative Example 2

[0065] The same as Example 1, the difference is only that step (2) of preparing the magnetic loaded iron-magnesium hydrotalcite biochar is omitted, and the magnetic biochar prepared in step (1) is directly used as raw material for step (3) of urea doping.

[0066] Comparative Example 3

[0067] The same as Example 1, the difference is only that step (3) of urea doping of the magnetic loaded iron-magnesium hydrotalcite biochar is omitted, and the magnetic loaded iron-magnesium hydrotalcite biochar obtained in step (2) is directly used as the product.

[0068] Comparative Example 4

[0069] The same as Example 1, the difference is only that the high-iron-content plant water hyacinth in step (1) is replaced by water hyacinth without iron to prepare LDH biochar without magnetism.

[0070] Effect Example 1

[0071] The electron microscope images of the magnetic LDH biochar prepared in Example 1 and the ordinary biochar material prepared in Comparative Example 1 are shown in Figure 1 and Figure 2 .

[0072] From Figure 1 and Figure 2 , it can be seen that the magnetic LDH biochar prepared in Example 1 has a developed pore structure, while the ordinary biochar material prepared in Comparative Example 1 has fewer pores.

[0073] The magnetic LDH biochar prepared in Example 1 has a higher specific surface area (225.1m 2 / g), the common biochar material prepared in Comparative Example 1 has poor dispersibility and small specific surface area (35.7 m 2 / g).

[0074] The XPS full spectrum of the magnetic LDH biochar prepared in Example 1 of the present application was determined.

[0075] As can be seen from the XPS full spectrum of the magnetic LDH biochar prepared in Example 1 of the present application, the elements Fe and Mg are successfully loaded in the magnetic LDH biochar, and the proportions of Fe and Mg are 5.52 wt.% and 1.43 wt.% respectively.

[0076] Example 2

[0077] The magnetic LDH biochar prepared in Example 1 and the common biochar material prepared in Comparative Example 1 were incorporated into the substrate layer of a horizontal subsurface flow constructed wetland at a proportion of 1.5 wt.%, and a conventional constructed wetland without adding biochar was set as a control, the oxygen supply mode was intermittent, the frequency was 25 min of oxygen supply every 5 h, and the hydraulic retention time was 24 h. Farmland return water (the average concentration of COD was 52.2 mg / L, the average concentration of total nitrogen was 15.6 mg / L, the average concentration of total phosphorus was 2.3 mg / L, and the carbon-nitrogen ratio (COD / TN) was about 3.35) was used as the influent of the constructed wetland. After a 15-day adaptation period, the system was stably operated for 30 days, and water quality test was performed every day to analyze the removal effect of nitrogen and phosphorus, and the results are shown in Figure 3 and Figure 4 .

[0078] Figure 3 and Figure 4 Treatment 1 in Figure 3 and Figure 4 is a constructed wetland without adding biochar; treatment 2 is a constructed wetland adding the common biochar material prepared in Comparative Example 1; and treatment 3 is a constructed wetland adding the magnetic LDH biochar prepared in Example 1.

[0079] As can be seen from Figure 3 and Figure 4 , the total nitrogen and total phosphorus removal rates of the conventional constructed wetland (treatment 1) on farmland return water are averagely 41.5% and 32.5%, which has a certain removal efficiency. The common biochar material (treatment 2) as a substrate has a certain promoting effect on the removal rate, but it is not obvious, and only increases by 10% or less. The magnetic LDH biochar (treatment 3) as a substrate, compared with the conventional constructed wetland, the removal rates of total nitrogen and total phosphorus are increased by more than 1 times, reaching 89.6% and 67.7%, and the improvement effect is very significant.

[0080] Using the same method as in this example, the wastewater treatment capacity of the materials prepared in Comparative Examples 2-4 of the present invention was measured. The results showed that adding the materials prepared in Comparative Examples 2-4 of the present invention to the constructed wetland could enhance the wastewater treatment effect of the constructed wetland, with the removal rates of total nitrogen and total phosphorus increasing by 15-31% and 12-23%, respectively, which was far lower than the enhancement effect of the magnetic LDH biochar prepared in Example 1.

[0081] The oxygen enrichment capacity of constructed wetlands without biochar (Treatment 1), constructed wetlands with conventional biochar material prepared in Comparative Example 1 (Treatment 2), and constructed wetlands with magnetic LDH biochar prepared in Example 1 (Treatment 3) were measured during operation. The results are shown in [Figure 1]. Figure 5 .

[0082] from Figure 5 As can be seen, the dissolved oxygen concentration of constructed wetlands with added magnetic LDH biochar is 2.13 times that of conventional constructed wetlands, and the improvement effect of ordinary biochar materials is not obvious.

[0083] Using the same method as in this example, the oxygen enrichment capacity of the materials prepared in Comparative Examples 2-4 was measured. The results showed that the materials prepared in Comparative Examples 2-4 of this invention have a certain effect on the recovery of dissolved oxygen in constructed wetlands, and the dissolved oxygen concentration is increased by 27-41% compared with conventional constructed wetlands, but it is significantly worse than the oxygen enrichment effect of the magnetic LDH biochar prepared in Example 1 of this invention.

[0084] In subsequent experiments of this invention, the biomass raw materials were screened for the preparation method of magnetic LDH biochar. The results showed that when the iron-containing biomass was iron-containing water hyacinth and the iron content of the iron-containing biomass was 1.5 to 5 wt.%, magnetic LDH biochar was prepared. It had a strong specificity for the absorption of nitrogen and phosphorus nutrients, especially for the enhanced adsorption and fixation of nitrogen.

[0085] The preparation method of magnetic LDH biochar was experimentally verified by examining the expansion treatment conditions and pyrolysis carbonization treatment conditions of iron-containing biomass. The results showed that the magnetic LDH biochar prepared by the following conditions was obtained: expansion treatment was carried out at a temperature of 160-180℃ and a pressure of 5-7 atmospheres, followed by depressurization to atmospheric pressure within 1-5 seconds. The pyrolysis carbonization treatment was controlled as follows: microwave pyrolysis carbonization treatment was carried out under a nitrogen atmosphere with a power of 600-1800W and a time of 5-40min. This resulted in a magnetic biochar with higher pore structure, specific surface area and surface potential, more stable layered structure and better adsorption effect.

[0086] The raw material dosage and preparation conditions of the magnetic LDH biochar are verified, and the results show that when the dosage ratio of magnesium salt, magnetic biochar, sodium hydroxide and sodium carbonate is 0.02-0.04 mol:1.5-2.5 g:0.02-0.03 mol:0.03-0.05 mol, the hydrothermal reaction temperature is 120-180 DEG C, and the reaction time is 4-6 h, the obtained magnetic LDH biochar loaded with iron-magnesium hydrotalcite has a hydrotalcite structure, and the magnetic LDH biochar prepared by using the magnetic LDH biochar loaded with iron-magnesium hydrotalcite has a better promoting effect on the adsorption of phosphorus, and has the best effect of strengthening microbial activity and degrading and removing organic matter.

[0087] The conditions of heating and doping nitrogen are verified, and the results show that when the mass ratio of the magnetic LDH biochar loaded with iron-magnesium hydrotalcite and urea is 1:(0.1-0.4), the heating and doping temperature rising rate is 1-5 DEG C / min, the temperature is 800-1200 DEG C, and the time is 90-150 min, the obtained magnetic LDH biochar has a better nitrogen and phosphorus treatment performance of artificial wetland.

[0088] The above-described embodiments are only used to describe the preferred modes of the present application, and do not limit the scope of the present application, and various modifications and improvements of the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application.

Claims

1. A method for preparing magnetic LDH biochar for enhancing nitrogen and phosphorus removal from wastewater in constructed wetlands, characterized in that, Includes the following steps: Magnetic biochar was obtained by sequentially puffing and pyrolytic carbonization of iron-containing biomass. Magnetic biochar was added to a magnesium salt solution and stirred until homogeneous to obtain solution A; sodium hydroxide and sodium carbonate were dissolved in water to obtain solution B; solution B was added to solution A, stirred and reacted, followed by hydrothermal reaction, separation, and drying to obtain magnetically loaded iron-magnesium hydrotalcite biochar. The magnetic LDH biochar was obtained by mixing magnetically loaded iron-magnesium hydrotalcite biochar with a nitrogen source and then heating and doping it. The iron-containing biomass is iron-containing water hyacinth; the iron content of the iron-containing biomass is 1.5–5 wt.%; the puffing treatment specifically involves heating to 160–180°C, applying a pressure of 5–7 atmospheres, and then depressurizing to atmospheric pressure within 1–5 seconds; the pyrolysis carbonization treatment is microwave pyrolysis carbonization treatment with a power of 600–1800W and a time of 5–40 minutes; the pyrolysis carbonization treatment is carried out under a nitrogen atmosphere; The magnesium salt solution contains magnesium salt and magnetic biochar in a ratio of 0.02–0.04 mol: 1.5–2.5 g; the magnesium salt solution is composed of magnesium salt and water in a ratio of 0.02–0.04 mol: 60–100 mL; the sodium hydroxide, sodium carbonate and water are in a ratio of 0.02–0.03 mol: 0.03–0.05 mol: 30–50 mL; the volume ratio of solution A to solution B is 6:5–2:

1. The magnesium salt is magnesium nitrate; the stirring speed is 300-450 r / min; the stirring reaction time is 2-4 h; the hydrothermal reaction temperature is 120-180℃ and the time is 4-6 h; the separation is centrifugal separation, the centrifugation speed is 7000-9000 r / min and the time is 25-35 min; The mass ratio of the magnetically loaded iron-magnesium hydrotalcite biochar to the nitrogen source is 1:(0.1-0.4); the heating doping rate is 1-5℃ / min, the temperature is 800-1200℃, and the time is 90-150min; the nitrogen source includes urea.

2. A magnetic LDH biochar prepared by the preparation method of claim 1.

3. The application of the magnetic LDH biochar of claim 2 in enhancing the removal of nitrogen and phosphorus from wastewater in constructed wetlands.

4. The application according to claim 3, characterized in that, The wastewater has a COD concentration of 30.0–50.0 mg / L, an ammonia nitrogen concentration of 8.0–12.5 mg / L, a total nitrogen concentration of 11.0–20.0 mg / L, a total phosphorus concentration of 1.2–4.0 mg / L, and a carbon-to-nitrogen ratio of 1.5–4.

5.

5. The application according to claim 3, characterized in that, The specific method of application includes: incorporating the magnetic LDH biochar into the substrate layer of the constructed wetland and performing intermittent aeration; The magnetic LDH biochar is incorporated at a ratio of 1.2 to 2.5% of the matrix layer mass. The intermittent aeration refers to intermittent oxygen supply, with a frequency of 20-40 minutes every 3-6 hours.

Images