Magnesium oxide modified surface-enhanced charcoal adsorbent material and preparation method thereof

Abstract

The invention relates to a magnesium oxide modified surface-enhanced charcoal adsorbent material and a preparation method thereof. The method comprises the following steps: pre-carbonizing a biomass material to obtain biochar; the preparation method comprises the following steps: stirring and dipping biochar in a hot gallic acid solution to obtain a dipping solution, and then filtering the dipping solution to obtain biochar coated with a gallic acid layer; stirring and dipping the biochar coated with the gallic acid layer and a magnesium salt solution, and then drying to obtain a mixture; the mixture is sequentially subjected to heating treatment at 120-150 DEG C and pyrolysis treatment at 400-600 DEG C in inert gas, and the magnesium oxide modified surface-enhanced charcoal adsorbent material is prepared. The biochar adsorbent material obtained by the invention can efficiently and quickly treat low-concentration heavy metal wastewater, ammonia-nitrogen wastewater and phosphorus-containing wastewater, has excellent adsorption performance, reduces the use of auxiliary reagents such as a flocculating agent and the like, has low mud yield, and can optimize the separation of pollutants and the reuse of heavy metals.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of adsorbent materials, in particular to a magnesium oxide modified surface reinforced biochar adsorbent material and a preparation method thereof. BACKGROUND

[0002] Adsorption is a common method in wastewater treatment, which has a wide range of applications, is easy to operate, has low effluent concentration, less secondary pollution, and less hazardous solid waste production, and is one of the main methods for treating heavy metal wastewater, ammonia-nitrogen wastewater and phosphorus-containing wastewater.

[0003] MgO as an adsorbent has a relatively green and environmentally friendly characteristic, and can fix heavy metal ions through ion exchange and precipitation, and due to its low solubility and slow hydroxyl release rate, it can avoid the precipitation dissolution caused by excessive alkalinity. In addition, MgO also has strong removal capacity for pollutants such as ammonia-nitrogen and phosphate in wastewater. However, the slow reaction speed leads to a longer time for using MgO to treat heavy metal and nitrogen-phosphorus wastewater, and a larger dosage is required. MgO-based adsorbents usually need to be activated before use to increase the specific surface area and improve the adsorption efficiency. However, the general activation method has high cost and energy consumption, and the preparation temperature is more than 1000℃; or there are too many attached preparation processes and equipment (such as spray drying method, sol-gel method, etc.) and too high reagent consumption (such as precipitation method, sol-gel method, etc.) in the preparation process, so the cost is high. At the same time, due to the small size of active MgO adsorbent particles and the particles generated by the precipitation, the dependence on auxiliary reagents such as flocculants and floatation agents is large, the sludge production is further increased, the subsequent treatment is difficult, and the adsorbent is easy to coagulate and lose effectiveness during use. When MgO is loaded on a porous carbon material as a substrate, the coagulation problem of the adsorbent can be reduced to some extent, and the adsorption activity of magnesium oxide can be improved, but when the unactivated substrate is used for loading, the activation degree of magnesium oxide particles is still low, and the adsorption rate is still insufficient. Therefore, when preparing magnesium oxide / biochar adsorbent material, the surface properties of biochar material have high requirements, and the original biochar has insufficient support and dispersion effect on magnesium oxide, and the activation degree of magnesium oxide is also low, so it is usually necessary to improve the activation degree of magnesium oxide to further improve the adsorption performance of magnesium oxide / biochar adsorbent material on heavy metal wastewater, ammonia-nitrogen wastewater and phosphorus-containing wastewater.

[0004] In summary, in order to solve one or more of the above technical problems, it is necessary to provide a magnesium oxide modified surface reinforced biochar adsorbent material and a preparation method thereof. SUMMARY

[0005] In order to solve one or more technical problems existing in the prior art, the present application provides a magnesium oxide modified surface reinforced biochar adsorbent material and a preparation method thereof. The biochar adsorbent material obtained by the present application can efficiently and quickly treat low-concentration heavy metal wastewater, ammonia-nitrogen wastewater and phosphorus-containing wastewater, has excellent adsorption performance, reduces the use of auxiliary reagents such as flocculants, has low sludge production, and can optimize the separation of pollutants and the reuse of heavy metals.

[0006] The present application provides, in a first aspect, a preparation method of a magnesium oxide modified surface reinforced biochar adsorbent material, comprising the following steps:

[0007] (1) pre-carbonizing a biomass material to obtain biochar;

[0008] (2) stirring and impregnating the biochar in a hot gallic acid solution to obtain an impregnation solution, and then filtering the impregnation solution to obtain biochar coated with a gallic acid layer;

[0009] (3) stirring and impregnating the biochar coated with the gallic acid layer in a magnesium salt solution, and then drying to obtain a mixture;

[0010] (4) sequentially performing 120-150℃ heating treatment and 400-600℃ pyrolysis treatment on the mixture in an inert gas to obtain the magnesium oxide modified surface reinforced biochar adsorbent material.

[0011] Preferably, in step (1), the biomass material is a lignocellulosic biomass material, preferably, the lignocellulosic biomass material is one or more of agricultural and forestry waste, grass leaves and wood materials, more preferably, the lignocellulosic biomass material is one or more of corn cob, straw, grass leaves, fruit shells and wood materials, and further preferably, the lignocellulosic biomass material is corn cob.

[0012] Preferably, in step (1), the biomass material is pretreated before pre-carbonization. The pretreatment of the biomass material comprises: washing the biomass material, cutting it into blocks, drying, crushing and sieving to obtain biomass material with a particle size of less than 18 mesh.

[0013] Preferably, in step (1), the pre-carbonization is performed under the protection of an inert gas, preferably in a nitrogen atmosphere, or the pre-carbonization is performed in a closed space by pre-removing oxygen through limited combustion; and / or the temperature of the pre-carbonization is 300-600℃, and the time is 1-3h, preferably, the temperature of the pre-carbonization is 400℃, and the time is 2h.

[0014] Preferably, in step (2): the gallic acid concentration of the hot gallic acid solution is 50-150 g / L, the temperature of the hot gallic acid solution is 70-90°C, preferably 80-90°C; the ratio of the amount of the biochar to the hot gallic acid solution is (25-75) g: 1 L; the mass ratio of the biochar to the gallic acid in the hot gallic acid solution is 1:(1-3); and / or the time for the stirring and impregnation is 12-24 h.

[0015] Preferably, in step (2): the temperature of the impregnation solution is not lower than 70°C before filtration, and the filtration time is not more than 5 min; preferably, the filtrate after filtration is recovered and reused until no solid is precipitated when cooled to 40°C.

[0016] Preferably, in step (3): the magnesium salt in the magnesium salt solution is one or more of magnesium nitrate, magnesium sulfate and magnesium chloride, preferably magnesium chloride; the temperature of the magnesium salt solution is not more than 30°C, preferably not more than 20°C; the concentration of the magnesium salt solution is 200-500 g / L; the mass ratio of the magnesium in the magnesium salt solution to the biochar in step (2) is (0.1-0.3):1; and / or the time for the stirring and impregnation is 1-2 h.

[0017] Preferably, in step (3): the temperature for the drying is 60-75°C; and / or the drying is performed until a slurry-like mixture with a water content of 20-40% is obtained.

[0018] Preferably, in step (4): the heating treatment is performed at 120-150°C for 1-2 h; and / or the pyrolysis treatment is performed at 400-600°C for 1-2 h.

[0019] In a second aspect, the present application provides a magnesium oxide modified surface reinforced biochar adsorbent material prepared by the preparation method described in the first aspect of the present application.

[0020] Compared with the prior art, the present application has at least the following beneficial effects:

[0021] (1) The application provides a preparation method of a surface-strengthened biochar adsorbent material in which magnesium oxide is highly activated and dispersed and loaded on the surface of biochar coated with a gallic acid layer containing a large amount of carboxyl and phenolic hydroxyl groups, for rapid and efficient removal of lead and cadmium ions, ammonia nitrogen and phosphate in heavy metal wastewater; the method provided by the application uses biomass materials as raw materials, utilizes the different solubilities of gallic acid in hot water and cold water and the directional surface affinity of the aromatic structure of gallic acid and biochar, adopts a hot solution impregnation method to uniformly coat gallic acid (a surface-strengthening agent) on the surface of biochar, and then utilizes the interaction between the surface of the strengthened biochar and a magnesium salt solution, that is, the phenolic hydroxyl groups and carboxyl groups contained in the surface of the biochar, to enable magnesium ions to be highly and uniformly dispersed on the surface of biochar during magnesium salt impregnation, and to convert the magnesium salt highly dispersed on the surface of biomass carbon into magnesium oxide with good dispersity and high activity through pyrolysis in an inert atmosphere, so as to achieve high dispersity and high activity of the loaded magnesium oxide, and finally obtain a surface-strengthened biochar adsorbent material modified by magnesium oxide; when the surface-strengthened biochar adsorbent material modified by magnesium oxide in the application is used to adsorb lead and cadmium ions, the phenolic hydroxyl groups and carboxyl groups uniformly distributed on the surface of biochar are utilized to provide complexation and concentrate and fix heavy metal ions, and enable the heavy metal ions to react with active magnesium oxide dispersed on the surface of the adsorbent to produce precipitates, and the precipitates are captured through the pores of biochar, the hydrophobic surface of biochar and carboxyl / phenolic hydroxyl groups, so that large-size heavy metal precipitate particles easy to separate can be obtained; the biochar adsorbent material obtained by the application can efficiently and rapidly treat low-concentration heavy metal wastewater, and can reduce the amount of auxiliary reagents such as flocculants, and has low sludge production.

[0022] (2) Compared with the traditional preparation method of active magnesium oxide, the heating temperature required by the method of the application is lower (for example, it is reduced from about 1000 DEG C required for calcination preparation from magnesite to about 400-600 DEG C), and the energy consumption is less; at the same time, compared with the methods such as the precipitation method, the sol-gel method and the spray drying method, the method has a simple process and fewer procedures, the method provided by the application is relatively simple, has low energy consumption, good adsorption effect and a wide application market.

[0023] (3) Compared with modification using only magnesium oxide, the gallic acid layer is coated on the surface of biochar in advance in the application, so that the dispersity of magnesium ions on the surface of biochar during the impregnation process is improved, the activation effect of biochar is improved, and the distribution of surface pores is more uniform; the utilization rate of magnesium oxide in the activation process is improved, and the adsorption performance of the biochar adsorbent material on low-concentration heavy metal wastewater, ammonia nitrogen wastewater and phosphorus-containing wastewater is also improved, which is beneficial to rapid and efficient removal of lead and cadmium ions, ammonia nitrogen and phosphate in heavy metal wastewater.

[0024] (4) Compared with the biochar adsorbent modified by magnesium oxide only, the magnesium oxide modified surface reinforced biochar adsorbent material in the application has obvious improvement in adsorption capacity and adsorption rate when treating nitrogen and phosphorus wastewater; when treating lead and cadmium wastewater, the lead and cadmium precipitation produces faster, and the adsorption rate is significantly improved, under the joint action of surface carboxyl and phenolic hydroxyl and active magnesium oxide, the adsorption capacity of the adsorbent exceeds the sum of the biochar loaded with magnesium oxide only and the biochar surface reinforced by gallic acid only, indicating that the gallic acid surface reinforcement and the magnesium oxide modification achieve effective coupling and good synergistic effect.

[0025] (5) The application finds that the biochar material coated with a surface reinforcing agent (gallic acid) and then immersed in a magnesium salt solution has stronger adsorption performance improvement effect than that of the biochar material coated with a surface reinforcing agent and immersed in a magnesium salt solution at the same time; after the biochar adsorbent material prepared by the application is adsorbed, the heavy metal precipitation particle size increases significantly and is attached to the adsorbent substrate, so that larger particle products are obtained, the recovery difficulty is reduced, and the cost and energy consumption are reduced. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a product real object diagram of the magnesium oxide modified surface reinforced biochar adsorbent material prepared in Example 1 of the application;

[0027] Figure 2 is an SEM diagram of the magnesium oxide modified surface reinforced biochar adsorbent material prepared in Example 1 of the application under different magnifications;

[0028] Figure 3 is an XRD spectrum diagram of the magnesium oxide modified surface reinforced biochar adsorbent material prepared in Example 1 of the application;

[0029] Figure 4 is an SEM diagram of the adsorption of lead and cadmium by the magnesium oxide adsorbent directly and the magnesium oxide modified surface reinforced biochar adsorbent material prepared in Example 1; in the diagram, a is an SEM diagram of the adsorption of lead by the magnesium oxide adsorbent directly, b is an SEM diagram of the adsorption of cadmium by the magnesium oxide adsorbent directly, c is an SEM diagram of the adsorption of lead by the magnesium oxide modified surface reinforced biochar adsorbent material prepared in Example 1, and d is an SEM diagram of the adsorption of cadmium by the magnesium oxide modified surface reinforced biochar adsorbent material prepared in Example 1. DETAILED DESCRIPTION

[0030] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the protection scope of the present application.

[0031] The present application provides, in a first aspect, a method for preparing a magnesium oxide modified surface-enhanced biochar adsorbent material, the method comprising the following steps:

[0032] (1) pre-carbonizing a biomass material to obtain a biochar; in the present application, the biochar is also referred to as an original biomass carbon material; in the present application, for example, the pre-treated biomass material is pre-carbonized under a limited oxygen condition (for example, a nitrogen atmosphere) to obtain the original biomass carbon material (i.e. the biochar);

[0033] (2) stirring and impregnating the biochar in a hot gallic acid solution to obtain an impregnation solution, and then filtering the impregnation solution to obtain the biochar coated with a gallic acid layer (a gallic acid thin layer) (also referred to as a surface-enhanced biochar); in the present application, the hot solution impregnation method is used to coat the biochar with the gallic acid layer (also referred to as the gallic acid thin layer), for example, the biochar is stirred and impregnated in a hot solution of gallic acid with a certain concentration for a certain time, and then filtered in a room temperature environment (for example, a room temperature environment with a temperature not higher than 30°C), so as to make the gallic acid precipitate on the surface of the biochar during the filtering process by using the different solubility of gallic acid in hot water and cold water, thereby achieving a uniform and dense coating effect; in step (2), the filtering is performed by using a filtering device, and the pore size of the filter membrane, filter paper, filter cloth or filter screen used by the filtering device needs to ensure that the biochar particles can be intercepted; in the present application, the hot gallic acid solution uses water as a solvent and gallic acid as a solute;

[0034] (3) stirring and impregnating the biochar coated with the gallic acid layer in a magnesium salt solution (a cold magnesium salt solution), and then drying to obtain a mixture; in the present application, the stirring and impregnation in step (3) makes the biochar coated with the gallic acid layer fully mixed with the magnesium salt solution; in the present application, for example, drying at a proper temperature to obtain a slurry-like mixture; in the present application, the magnesium salt solution uses water as a solvent and a magnesium salt as a solute; the present application does not specifically limit the stirring and impregnation speed in step (2) and / or step (3), which can be routinely selected by those skilled in the art, for example, can be 200-1000 r / min;

[0035] (4) the mixture is subjected to 120-150°C (for example 120°C, 130°C, 140°C or 150°C) heat treatment and 400-600°C (for example 400°C, 500°C or 600°C) pyrolysis treatment in inert gas in sequence, to obtain a magnesium oxide modified surface reinforced biochar adsorbent material; in the present application, both the 120-150°C heat treatment and the 400-600°C pyrolysis treatment are carried out in inert gas; in the present application, the inert gas can be, for example, nitrogen and / or argon.

[0036] Unlike the existing magnesium salt impregnation technology, the present application uses gallic acid as a surface reinforcing agent, utilizes the change of the solubility of the surface reinforcing agent in water with temperature (the different solubility of gallic acid in hot water and cold water) and the characteristic of the aromatic structure of the surface reinforcing agent being similar to the direction of the surface of biochar, to uniformly coat the surface of biochar, so that a uniform gallic acid layer is formed on the surface of biochar, and the phenolic hydroxyl and carboxyl groups contained in the gallic acid layer enable the magnesium ions to be highly dispersed and distributed on the surface of biochar during the magnesium salt impregnation process, thereby achieving a good improvement effect on the dispersibility of magnesium oxide on the surface of biochar, and achieving a high dispersibility and activity of the loaded magnesium oxide; when adsorbing heavy metal ions, the delocalized π bond formed by the aromatic structure of gallic acid can produce conjugation with heavy metal ions, thereby achieving the effect of quickly adsorbing heavy metal ions; in addition, through the complexation of phenolic hydroxyl and carboxyl groups, the effect of quickly adsorbing heavy metal ions is achieved; after the surface functional groups are strengthened, the heavy metal ions are concentrated near the surface of biochar, and under the action of ion exchange and precipitation promotion provided by the magnesium oxide on the surface of biochar, the heavy metal ions are converted into solid precipitated particles, which are captured by the pore structure and surface functional groups of biochar, increasing the particle size, reducing the difficulty of recovery, reducing the use of auxiliary reagents such as flocculants, and reducing the amount of sludge produced.

[0037] The present application is to utilize the difference in solubility of gallic acid in hot water and cold water, by dissolving gallic acid in hot water and then adding biochar into the system, and then cooling to precipitate on the surface of biochar, so as to form a uniform layer of gallic acid on the surface of biochar. Compared with the traditional method of not controlling the temperature of impregnation and loading, but directly coating the surface of biochar with organic acid by stirring, the present application can significantly improve the uniformity and compactness of the coating by utilizing the precipitation process formed by the difference in hot and cold solubility, and avoid the problem of uneven deposition of polyphenol hydroxyl compounds on the surface of biochar. The gallic acid with high solubility in hot water can fully dissolve and penetrate into the surface of biochar, and then precipitate on the surface of biochar to form a uniform and compact thin layer by cooling. In addition, the present application can improve the utilization rate of gallic acid and effectively reduce the waste of raw materials by alternating hot and cold to induce precipitation, so that gallic acid preferentially deposits on the surface of biochar rather than remaining in the solution. The thin layer formed by cooling after hot water dissolution usually has better crystallinity or agglomeration structure, which enhances its adhesion on the surface of biochar. The combination of the gallic acid layer and the surface of biochar is more firm, which gives it better stability and elution resistance. Ultimately, the adsorption performance of the prepared magnesium oxide modified surface reinforced biochar adsorbent material can be significantly improved.

[0038] Compared with only using magnesium oxide for modification, the present application coats a layer of gallic acid on the surface of biochar in advance, thereby improving the dispersibility of magnesium ions on the surface of biochar during the impregnation process, so that the surface of biochar is more fully and uniformly activated, and the activation effect of biochar is improved. The surface pore distribution is more uniform, which improves the distribution state of the effective adsorption sites of the prepared surface reinforced biochar adsorbent material, and increases the surface that can participate in the adsorption reaction. While improving the utilization rate of magnesium oxide in the activation process, the adsorption performance of the biochar adsorbent material on low concentration heavy metal wastewater, ammonia nitrogen wastewater and phosphorus-containing wastewater is also improved. Compared with biochar adsorbent modified only by magnesium oxide, the magnesium oxide modified surface reinforced biochar adsorbent material in the present application has significantly improved adsorption capacity and adsorption rate when treating nitrogen and phosphorus wastewater; when treating lead and cadmium wastewater, the precipitation of lead and cadmium occurs faster, and the adsorption rate is significantly improved. Under the joint action of surface carboxyl and phenolic hydroxyl groups and active magnesium oxide, the adsorption capacity of the adsorbent exceeds the sum of biochar loaded with only magnesium oxide and biochar surface reinforced with only gallic acid, indicating that the surface reinforcement of gallic acid and the modification of magnesium oxide have achieved effective coupling and good synergistic effect. In the present application, when the dosage of magnesium oxide modified surface reinforced biochar adsorbent material is 300 ppm, the removal rate is more than 99.9% when treating 200 mg / L of lead wastewater, and the effluent concentration is less than 0.1 mg / L; when the dosage is 600 ppm, the effluent concentration is less than 0.02 mg / L when treating 100 mg / L of cadmium wastewater.

[0039] According to some preferred embodiments, in step (1), the biomass material is a lignocellulosic biomass material, preferably, the lignocellulosic biomass material is one or more of agricultural and forestry wastes, grass leaves and wood materials, more preferably, the lignocellulosic biomass material is one or more of corn cob, straw, grass leaves, fruit shells and wood materials, and further preferably, the lignocellulosic biomass material (agricultural waste) is corn cob.

[0040] According to some preferred embodiments, in step (1), the biomass material is pretreated before pre-carbonization, and the pretreatment of the biomass material is: the biomass material is washed and cut into blocks, dried, crushed and sieved to obtain biomass material with a particle size of less than 18 mesh; specifically, the pretreatment is, for example, first washing the biomass material to remove excess surface dirt and the like, then cutting the biomass material into small pieces, drying to remove most of the water, crushing with a crusher, and sieving to select biomass material with a size (particle size) of not more than 18 mesh, and naturally air-drying for use.

[0041] According to some preferred embodiments, in step (1), the pre-carbonization is carried out under inert gas protection, preferably in a nitrogen atmosphere, or the pre-carbonization is carried out in a closed space by pre-removing oxygen through limited combustion; and / or the temperature of the pre-carbonization is 300-600°C (for example, 300°C, 400°C, 500°C or 600°C), and the time is 1-3 hours, preferably 1-2 hours, and preferably, the temperature of the pre-carbonization is 400°C, and the time is 2 hours; in step (1), the pre-carbonization can be carried out in static or dynamic inert gas, and when carried out in dynamic inert gas, the inert gas flow rate is not more than 10 L / h, preferably 5-10 L / h.

[0042] According to some specific embodiments, step (1) is: the pretreated biomass material powder can pass through an 18-mesh sieve, and the material that can pass through the 18-mesh sieve is pre-carbonized at 300-600°C (for example, 300°C, 400°C, 500°C or 600°C) for 1-3 hours under limited oxygen conditions to obtain raw biomass carbon material (biochar); in the present application, the limited oxygen condition can be obtained by introducing inert gas such as nitrogen into a tubular furnace or a high-temperature reaction furnace, or pyrolysis is carried out in a closed environment, and the limited oxygen effect is achieved by the consumption of a limited amount of oxygen by an excess amount of biomass raw material; the biochar prepared under these different limited oxygen pre-carbonization conditions does not have significant differences in properties and does not have a significant impact on the performance of the subsequent activated biochar.

[0043] According to some preferred embodiments, in step (2): the gallic acid concentration of the hot gallic acid solution is 50-150 g / L (e.g. 50, 60, 70, 80, 90, 100, 110, 120, 130, 140 or 150 g / L), and the temperature of the hot gallic acid solution is 70-90°C (e.g. 70°C, 75°C, 80°C, 85°C or 90°C), preferably 80-90°C (e.g. 80°C, 85°C or 90°C); in the present application, even if there is a slight precipitation in the hot gallic acid solution, the performance of the material prepared is not greatly affected, as long as the concentration of the hot gallic acid solution is in the range of 50-150 g / L. It is found in the present application that, in the process of stirring and soaking in the hot solution, the temperature of the hot solution is 70-90°C, preferably 80-90°C; if the temperature is too high, the boiling of the hot solution will affect the coating effect; if the temperature is too low, the surface strengthening agent will be precipitated in excess, which will eventually lead to the difficulty of the magnesium oxide to adhere to the surface of the biochar; the ratio of the amount of the biochar to the amount of the hot gallic acid solution is (25-75) g: 1 L (e.g. 25 g: 1 L, 30 g: 1 L, 35 g: 1 L, 40 g: 1 L, 45 g: 1 L, 50 g: 1 L, 55 g: 1 L, 60 g: 1 L, 65 g: 1 L, 70 g: 1 L or 75 g: 1 L), preferably (25-50) g: 1 L (e.g. 25 g: 1 L, 30 g: 1 L, 35 g: 1 L, 40 g: 1 L, 45 g: 1 L or 50 g: 1 L); the mass ratio of the biochar to the gallic acid in the hot gallic acid solution is 1:(1-3) (e.g. 1:1, 1:1.5, 1:2, 1:2.5 or 1:3), preferably 1:(1.2-3); in the present application, it is preferred that the mass ratio of the biochar to the gallic acid is 1:(1-3); if the gallic acid is too much, it will be precipitated in excess on the surface of the biochar, which will lead to the difficulty of the magnesium oxide to adhere to the surface of the biochar; if the gallic acid is too little, it will not be able to achieve sufficient effect; and / or the time of the stirring and soaking is 12-24 h (e.g. 12, 14, 16, 18, 20, 22 or 24 h).

[0044] According to some preferred embodiments, in step (2): the temperature of the impregnation solution is not less than 70℃, preferably 70-90℃, more preferably 80-90℃, before the impregnation solution is filtered, i.e. when the impregnation solution contacts the filter layer of the filter device, and the filtration time is not more than 5 min. Preferably, the filtrate after filtration is recovered and reused until no solid precipitates when cooled to 40℃. In the present application, the process of separating the biochar coated with the gallic acid layer from the impregnation solution by filtration requires the use of a filter device at room temperature not higher than 30℃, and the time for all the impregnation solution to pass through the filter device should be controlled within 5 min to control the amount of gallic acid precipitated on the surface of the biochar and ensure the appropriate thickness of the coating layer. However, the temperature of the impregnation solution should be controlled not less than 70℃ before filtration, otherwise, the excessive precipitation of gallic acid may occur, which makes it difficult for magnesium oxide to adhere to the surface of the biochar.

[0045] In order to ensure the modification of magnesium oxide and the surface strengthening effect of the biochar, the present application requires the coating of gallic acid under the above appropriate conditions and process parameters, so that the biochar surface is uniformly coated with a gallic acid layer with appropriate thickness, thereby making the subsequent distribution of magnesium oxide on the surface of the biochar more uniform. The present application finds that both too low or too high loading (thickness) of gallic acid (thin layer) can easily lead to a decrease in surface activation effect, thereby resulting in a decrease in adsorption capacity. If the loading is too low, it cannot ensure that enough surface is fully activated, thereby resulting in a decrease in adsorption capacity. If the loading is too high, gallic acid is easily detached from the surface of the biochar during the impregnation process of the magnesium salt solution, which makes it difficult for magnesium oxide to adhere to the surface of the biochar, thereby also leading to a decrease in activation effect and a decrease in adsorption capacity. In order to ensure the coating effect of gallic acid, the temperature of the impregnation solution, the concentration of gallic acid, the mass ratio of biochar to gallic acid, the impregnation time, the filtration time and the filtration temperature during the coating process need to be controlled, so that a gallic acid thin layer with appropriate thickness can be uniformly coated on the surface of the biochar.

[0046] According to some preferred embodiments, in step (3): the magnesium salt in the magnesium salt solution is one or more of magnesium nitrate, magnesium sulfate and magnesium chloride, preferably magnesium chloride. In the present application, the magnesium salt may, for example, be magnesium nitrate, magnesium sulfate and magnesium chloride, etc. It can be a mixture of multiple magnesium salts. In some other embodiments, if the magnesium salt also contains non-magnesium ions, for example, the molar proportion of magnesium in the mixed salt cations is not less than 80%, and if non-magnesium ion cations are contained, the non-magnesium ion cations are one or several of sodium, potassium and calcium.

[0047] According to some preferred embodiments, the temperature of the magnesium salt solution is not more than 30°C, preferably not more than 20°C; it is preferred in the present application that the temperature of the magnesium salt solution is not more than 30°C, it is found in the present application that too high temperature of the magnesium salt solution is easy to cause the gallic acid to dissolve and separate from the surface of the biochar; the concentration of the magnesium salt solution is 200-500 g / L (e.g. 200, 250, 300, 350, 400, 450 or 500 g / L), preferably 200-300 g / L; the mass ratio of magnesium contained in the magnesium salt solution to the biochar in step (2) is (0.1-0.3): 1 (e.g. 0.1:1, 0.15:1, 0.2:1, 0.25:1 or 0.3:1); and / or the time for stirring and impregnation is 1-2 h; it is preferred in the present application that the temperature of the magnesium salt solution is not more than 30°C, the concentration of the magnesium salt solution is 200-500 g / L, and the time for stirring and impregnation in the magnesium salt solution is 1-2 h, it is found in the present application that too high temperature of the magnesium salt solution is easy to cause the gallic acid to dissolve and separate from the surface of the biochar, too low concentration of the magnesium salt solution corresponds to a large solution volume, which is also easy to cause the gallic acid to dissolve and separate from the surface of the biochar and increase the time for subsequent drying, and too high concentration of the magnesium salt solution corresponds to a small solution volume, which is easy to cause the biochar to be insufficiently dispersed in the solution and finally cause the magnesium oxide to be unevenly dispersed on the surface of the biochar; in addition, too long stirring time is also easy to cause the gallic acid to dissolve and separate from the surface of the biochar, and too short stirring time is easy to cause the gallic acid and the magnesium oxide to be insufficiently reacted.

[0048] According to some preferred embodiments, in step (3): the temperature for drying is 60-75°C (e.g. 60°C, 65°C, 70°C or 75°C), more preferably 60-70°C, the time for drying is 12-24 h (e.g. 12, 18 or 24 h); and / or the drying is performed until a mud-like mixture with a water content (relative water content) of 20-40% is obtained; it is preferred in the present application that the temperature for drying is 60-75°C and the drying is performed until a mud-like mixture with a water content (relative water content) of 20-40% is obtained, it is found in the present application that too high temperature for drying is easy to cause the gallic acid to excessively dissolve and separate from the surface of the biochar, which causes the gallic acid to be ineffective, too low temperature for drying is easy to cause the time for drying to be too long, which also causes the gallic acid to excessively dissolve and separate from the surface of the biochar, which also causes the gallic acid to be ineffective, and too low water content after drying is easy to cause the mixture to be too dense during the pyrolysis process, which affects the pore-forming process during the pyrolysis process, and too high water content is easy to cause the water to not be timely removed during the pyrolysis process, which causes the surface strengthening agent to separate from the surface of the biochar, which causes the surface strengthening effect on the biochar to be insufficient.

[0049] According to some preferred embodiments, in step (4): the heating treatment is at 120-150°C for 1-2h; and / or the pyrolysis treatment is at 400-600°C for 1-2h; in step (4), the heating treatment and pyrolysis treatment can be performed in static or dynamic inert gas, when performed in dynamic inert gas, the inert gas flow rate is no more than 10 L / h, preferably 5-10 L / h (e.g. 5, 6, 7, 8, 9 or 10 L / h).

[0050] The present application provides, in a second aspect, a magnesium oxide modified surface enhanced biochar adsorbent material made by the preparation method provided in the first aspect of the present application.

[0051] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some, but not all, of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application. In the absence of specific description, each raw material used in the embodiments and comparative examples of the present application can be commercially purchased or synthesized by existing methods.

[0052] Embodiment 1

[0053] ①First, the biomass material (corn cob) is washed to remove excess dirt and other attachments on the surface, then the corn cob is cut into small pieces, dried to remove most of the water, and then crushed by a crusher. The material with a size (particle size) less than 18 mesh is selected by sieving, and the pretreated biomass material is obtained and naturally air-dried for use.

[0054] ②The pretreated biomass material obtained in step ① is moved to a heating device, and the temperature is raised to 400°C under the condition of passing in inert gas for 2h to prepare biochar; wherein the inert gas is nitrogen, and the flow rate of the inert gas is 8 L / h.

[0055] ③The biochar obtained in step ② is added to a hot gallic acid solution with a temperature of 90°C and a concentration of 60 g / L, and stirred for 12 hours under the condition of keeping the solution temperature at 90°C to obtain an impregnation solution, wherein the dosage ratio of the biochar to the hot gallic acid solution is 30 g: 1 L, that is, the addition amount of the biochar is 30 g per liter of the hot gallic acid solution; the obtained impregnation solution (temperature is 90°C before filtration) is poured into a filtration device (suction filtration device) containing a water filter membrane (pore size 0.45 μm) while hot, and filtered at room temperature of 25°C, and the filtration time is controlled to be no more than 5 min, and the solid is separated to obtain biochar coated with a gallic acid layer.

[0056] IV. The gallic acid-coated biochar obtained in step III is stirred in a magnesium salt cold solution (temperature 20℃) for 1 h, and then dried at 70℃ to obtain a slurry-like mixture with a water content of 25%; wherein the magnesium salt in the magnesium salt cold solution is magnesium chloride hexahydrate, the concentration of magnesium chloride hexahydrate in the magnesium salt cold solution is 200 g / L, and the amount of magnesium salt used is 1 times the mass of the biochar added in step III.

[0057] V. The slurry-like mixture obtained in step IV is moved to a heating device and heated under the condition of passing an inert gas, first increasing the temperature to 135℃ at a rate of 3℃ / min, then continuing to pass the inert gas and increasing the temperature to 600℃ at a rate of 3℃ / min for pyrolysis for 1.5 h, and finally washing the obtained material with clean water and drying at 70℃ to constant weight to obtain a magnesium oxide-modified surface-strengthened biochar adsorbent material; wherein the inert gas is nitrogen, and the flow rate of the inert gas is 8 L / h.

[0058] Example 2

[0059] Example 2 is basically the same as Example 1, except that:

[0060] The biomass material corn cob used in step I is replaced by wheat straw.

[0061] Example 3

[0062] Example 3 is basically the same as Example 1, except that:

[0063] In step IV, the magnesium salt is replaced by magnesium nitrate hexahydrate with the same magnesium content.

[0064] Example 4

[0065] Example 4 is basically the same as Example 1, except that:

[0066] In step V, the pyrolysis at 600℃ for 1.5 h is replaced by pyrolysis at 400℃ for 2 h.

[0067] Example 5

[0068] Example 5 is basically the same as Example 1, except that:

[0069] ③The biochar obtained in step 2 is added to a hot gallic acid solution with a temperature of 80℃ and a concentration of 150g / L, and stirred and immersed for 24 hours under the condition that the temperature of the solution is kept at 80℃, to obtain an immersion solution, wherein the ratio of the use amount of biochar to the hot gallic acid solution is 75g:1L, that is, the addition amount of biochar is 75g per liter of hot gallic acid solution; the obtained immersion solution (temperature is 80℃ before filtration) is poured into a filtering device (suction filtering device) containing a water filter membrane (pore size is 0.45μm) while hot, filtered at room temperature of 25℃, and the filtering time is controlled to be not more than 5 minutes, to separate the solid and obtain the biochar coated with a gallic acid layer.

[0070] ④The biochar coated with a gallic acid layer obtained in step 3 is stirred and immersed (stirring and mixing) in a magnesium salt cold solution (temperature is 20℃) for 2 hours, and then dried at a temperature of 70℃ to obtain a slurry-like mixture with a water content of 40%; wherein the magnesium salt in the magnesium salt cold solution is magnesium chloride hexahydrate, the concentration of magnesium chloride hexahydrate contained is 300g / L, and the use amount of magnesium salt is 2.5 times the mass of the biochar added in step 3.

[0071] Comparative Example 1

[0072] ①First, the biomass material (corn stalks) is washed to remove excess dirt and other attachments on the surface, and then the corn stalks are cut into small block-shaped objects, dried to remove most of the water, and then crushed using a crusher, and the materials with a size (particle size) less than 18 mesh are selected by sieving to obtain pretreated biomass materials, which are naturally air-dried for use.

[0073] ②The pretreated biomass material obtained in step 1 is moved to a heating device, and the temperature is increased to 400℃ under the condition that inert gas is introduced, and pre-carbonization is carried out for 2 hours to obtain biochar (adsorbent material); wherein the inert gas is nitrogen, and the flow rate of the introduced nitrogen is 8L / h.

[0074] Comparative Example 2

[0075] ①First, the biomass material (corn stalks) is washed to remove excess dirt and other attachments on the surface, and then the corn stalks are cut into small block-shaped objects, dried to remove most of the water, and then crushed using a crusher, and the materials with a size (particle size) less than 18 mesh are selected by sieving to obtain pretreated biomass materials, which are naturally air-dried for use.

[0076] ②The pretreated biomass material obtained in step 1 is moved to a heating device, and the temperature is increased to 400℃ under the condition that inert gas is introduced, and pre-carbonization is carried out for 2 hours to obtain biochar; wherein the inert gas is nitrogen, and the flow rate of the introduced nitrogen is 8L / h.

[0077] ③ The biochar obtained in step ② is stirred and immersed (stirring and mixing) in a magnesium salt solution (temperature is 20℃) for 1h, and then dried at 70℃ to obtain a slurry-like mixture with a water content of 25%; wherein the magnesium salt in the magnesium salt solution is magnesium chloride hexahydrate, the concentration of magnesium chloride hexahydrate contained is 200g / L, and the amount of magnesium salt used is 1 times the mass of the added biochar.

[0078] ④ The slurry-like mixture obtained in step ③ is moved to a heating device and subjected to heating treatment under the condition of passing in inert gas, first increasing the temperature to 135℃ at a rate of 3℃ / min, continuing to pass in inert gas after heating treatment for 1.5h, and then increasing the temperature to 600℃ at a rate of 3℃ / min for pyrolysis for 1.5h, finally washing the obtained material with clean water and drying at 70℃ to constant weight to obtain a magnesium oxide modified biochar adsorbent material; wherein the inert gas is nitrogen, and the flow rate of the inert gas is 8L / h.

[0079] Comparative Example 3

[0080] ① First, the biomass material (corn stalks) is washed to remove excess dirt and other attachments on the surface, then the corn stalks are cut into small pieces, dried to remove most of the water, and then crushed using a crusher, and materials with a size (particle size) less than 18 mesh are selected by sieving to obtain pretreated biomass material, which is naturally air-dried for use.

[0081] ② The pretreated biomass material obtained in step ① is moved to a heating device, and the temperature is increased to 400℃ under the condition of passing in inert gas for 2h to obtain biochar; wherein the inert gas is nitrogen, and the flow rate of the inert gas is 8L / h.

[0082] ③ The biochar obtained in step ② is added to a hot gallic acid solution with a temperature of 90℃ and a concentration of 60g / L, and stirred and immersed for 12h while keeping the solution temperature at 90℃ to obtain an immersion solution, wherein the ratio of the use amount of biochar to hot gallic acid solution is 30g:1L, i.e. the addition amount of biochar is 30g per liter of hot gallic acid solution; the obtained immersion solution (temperature is 90℃ before filtration) is poured into a filtration device (suction filtration device) containing a water filter membrane (pore size 0.45μm) while hot, filtered at room temperature of 25℃, the filtration time is controlled to be not more than 5min, and the solid is separated, and then dried at 70℃ to constant weight to obtain biochar coated with a gallic acid layer (surface strengthened biochar) as a biochar adsorbent material.

[0083] Comparative Example 4

[0084]

[0085]

[0086]

[0087]

[0088] Comparative Example 5

[0089]

[0090]

[0091] ​​​​​​③ The biochar obtained in step ② is stirred and immersed (stirring and mixing) in a magnesium salt solution (temperature is 20℃) for 1h, and then dried at 70℃ to obtain a slurry-like mixture with a water content of 25%; wherein the magnesium salt in the magnesium salt solution is magnesium chloride hexahydrate, the concentration of magnesium chloride hexahydrate contained is 200g / L, and the amount of magnesium salt used is 1 times the mass of the added biochar.

[0092] ④ The slurry-like mixture obtained in step ③ is moved to a heating device and subjected to heating treatment under the condition of passing in inert gas, first increasing the temperature to 135℃ at a rate of 3℃ / min, continuing to pass in inert gas after heating treatment for 1.5h, and then increasing the temperature to 600℃ at a rate of 3℃ / min for pyrolysis for 1.5h, finally washing the obtained material with clean water and drying at 70℃ to constant weight to obtain magnesium oxide modified biochar; wherein the inert gas is nitrogen, and the flow rate of the passing in is 8L / h.

[0093] ⑤ The magnesium oxide modified biochar obtained in step ④ is added to a hot gallic acid solution with a temperature of 90℃ and a concentration of 60g / L, and stirred and immersed for 12 hours under the condition of keeping the solution temperature at 90℃ to obtain an immersion solution, wherein the amount ratio of the hot gallic acid solution to the biochar added in step ② is 1L:30g; the obtained immersion solution (temperature is 90℃ before filtration) is poured into a filtering device (suction filtering device) containing a water filter membrane (pore size is 0.45μm) while hot, filtered at room temperature of 25℃, the filtering time is controlled to be not more than 5min, and the solid is separated out, and then dried at 70℃ to constant weight to obtain a magnesium oxide loaded biochar material with surface strengthened oxidation.

[0094] Comparative Example 6

[0095] Comparative Example 6 is basically the same as Example 1, except that:

[0096] In step ③, a hot gallic acid solution with a concentration of 25g / L is used.

[0097] Comparative Example 7

[0098] Comparative Example 7 is basically the same as Example 1, except that:

[0099] In step ③, a hot gallic acid solution with a concentration of 200g / L is used.

[0100] Comparative Example 8

[0101] Comparative Example 8 is basically the same as Example 1, except that:

[0102] In step ③, the filtering device is pre-heated to 55℃ before filtering, and the filtering is carried out at 55℃.

[0103] Comparative Example 9

[0104] Comparative Example 9 is basically the same as Example 1, except that:

[0105] In step IV, drying is performed to obtain a slurry-like mixture with a water content of 5%.

[0106] Comparative Example 10

[0107] Comparative Example 10 is basically the same as Example 1, except that:

[0108] In step IV, drying is performed to obtain a slurry-like mixture with a water content of 50%.

[0109] Comparative Example 11

[0110] Comparative Example 11 is basically the same as Example 1, except that:

[0111] IV. The biochar coated with the gallic acid layer obtained in step III is subjected to stirring impregnation (stirring mixing) in a magnesium salt solution (temperature: 70°C) for 1 h, and then is dried at 70°C to obtain a slurry-like mixture with a water content of 25%; wherein the magnesium salt in the magnesium salt solution is magnesium chloride hexahydrate, the concentration of the magnesium chloride hexahydrate is 200 g / L, and the amount of the magnesium salt used is 1 times the mass of the biochar added in step III.

[0112] Comparative Example 12

[0113] Comparative Example 12 is basically the same as Example 1, except that:

[0114] In step III, the obtained impregnation solution is cooled to 25°C, and then is poured into a filtering device (suction filtering device) provided with a water filter membrane (pore size: 0.45 μm) and is filtered at room temperature of 25°C, with the filtering time being controlled to be no more than 5 min, so as to separate the solid and obtain the biochar coated with gallic acid.

[0115] Comparative Example 13

[0116] Comparative Example 13 is basically the same as Example 1, except that:

[0117] III. The biochar obtained in step II is added into a gallic acid solution with a concentration of 1 g / L (the temperature of the gallic acid solution is room temperature of 25°C), and is subjected to stirring impregnation at room temperature of 25°C for 18 h to obtain an impregnation solution, wherein the amount ratio of the biochar to the gallic acid solution is 2 g: 1 L, i.e. the amount of the biochar added is 2 g per liter of the gallic acid solution; the obtained impregnation solution (temperature: 25°C before filtering) is poured into a filtering device (suction filtering device) provided with a water filter membrane (pore size: 0.45 μm) and is filtered at room temperature of 25°C, with the filtering time being controlled to be no more than 5 min, so as to separate the solid and obtain the biochar coated with gallic acid.

[0118] IV. The gallic acid-coated biochar obtained in step III was stirred in a magnesium salt solution (temperature 25°C) for 1 h, and then dried at 70°C to obtain a slurry-like mixture with a water content of 25%; wherein the magnesium salt in the magnesium salt solution was magnesium chloride hexahydrate, the concentration of magnesium chloride hexahydrate was 200 g / L, and the amount of magnesium salt used was 1 times the mass of the biochar added in step III.

[0119] Comparative Example 14

[0120] I. First, the biomass material (corn cob) was washed to remove excess dirt and other attachments on the surface, and then the corn cob was cut into small pieces, dried to remove most of the water, and then crushed using a crusher. The material with a size (particle size) less than 18 mesh was selected by sieving, and a pretreated biomass material was obtained. The pretreated biomass material was naturally air-dried for use.

[0121] II. The pretreated biomass material obtained in step I was moved to a heating device, and the temperature was increased to 400°C under the condition of passing inert gas for 2 h to prepare biochar; wherein the inert gas was nitrogen, and the flow rate of the inert gas was 8 L / h.

[0122] III. Gallic acid was prepared into a 1.0 g / L gallic acid aqueous solution at room temperature 25°C, and the pH was adjusted to 5.0 using 0.1 mol / L hydrochloric acid and 0.1 mol / L sodium hydroxide.

[0123] IV. The biochar obtained in step II was added to the gallic acid aqueous solution to obtain a mixed solution with a biochar mass concentration of 2 g / L, and the mixed solution was stirred at room temperature 25°C for 7 days.

[0124] V. After stirring, the liquid phase was removed by filtration at 25°C, and the residual solid was freeze-dried to obtain gallic acid-impregnated biochar.

[0125] VI. The gallic acid-impregnated biochar obtained in step V was stirred in a magnesium salt solution (temperature 25°C) for 1 h, and then dried at 70°C to obtain a slurry-like mixture with a water content of 25%; wherein the magnesium salt in the magnesium salt solution was magnesium chloride hexahydrate, the concentration of magnesium chloride hexahydrate was 200 g / L, and the amount of magnesium salt used was 1 times the mass of the biochar added in step IV.

[0126] ⑦ The slurry mixture obtained in step ⑥ was transferred to a heating device and heated under the condition of passing inert gas. First, the temperature was raised to 135°C at a rate of 3°C / min. After heating for 1.5 hours, the inert gas was continued to be passed and the temperature was raised to 600°C at a rate of 3°C / min for pyrolysis for 1.5 hours. Finally, the obtained material was rinsed with clean water and dried at 70°C to constant weight to obtain magnesium oxide-modified biochar; wherein the inert gas was nitrogen, and the flow rate was 8L / h.

[0127] The present invention tested the adsorption performance of the biochar adsorbents finally prepared in each embodiment and each comparative example. The test method is as follows: treating 500 mg / L lead wastewater (dosage 200 ppm), 500 mg / L cadmium wastewater (dosage 600 ppm), 100 mg / L NH3-N wastewater (dosage 500 ppm) and 200 mg / L PO4-P wastewater (dosage 500 ppm) at 25°C. After adsorption equilibrium, the adsorption capacity (maximum adsorption capacity) is measured and the results are shown in Table 1.

[0128] Table 1

[0129]

[0130]

[0131] The product picture of the surface-enhanced biochar adsorbent material modified with magnesium oxide prepared in Example 1 of the present invention is as follows: Figure 1 As shown; SEM images of the surface-enhanced biochar adsorbent material modified with magnesium oxide prepared in Example 1 of the present invention at different magnifications, as shown Figure 2 shown; from Figure 2 The results show that magnesium salt impregnation after surface functional group strengthening can effectively improve the activation degree and load dispersibility of magnesium oxide; the XRD spectrum of the surface-enhanced biochar adsorbent material modified with magnesium oxide prepared in Example 1 of the present invention is as follows: Figure 3 As shown in FIG. 1 , the XRD pattern proves that the surface of the magnesium oxide-modified surface-enhanced biochar adsorbent material is loaded with magnesium oxide; the SEM images of the magnesium oxide adsorbent directly used in the present invention and the magnesium oxide-modified surface-enhanced biochar adsorbent material prepared in Example 1 after adsorbing lead and cadmium are shown in FIG. Figure 4 shown; from Figure 4 The results show that after the adsorption is completed, compared with the direct use of magnesium oxide adsorbent, the lead and cadmium precipitates can be attached to the surface of the magnesium oxide-modified surface-enhanced biochar adsorbent in the method of the present invention after lead and cadmium removal, and the solid size is significantly increased, making separation and removal simpler. Figure 3 and Figure 4The magnesium oxide in the magnesium oxide-containing material is prepared as follows: magnesium nitrate is added to water (mass / volume ratio of magnesium nitrate to water is 1 g: 15 mL), the mixture is stirred to be uniform, and then dried at 70°C for 8 hours to form a mud-like mixture. The mud-like mixture is heated and treated in a nitrogen atmosphere (inlet flow rate is 8 L / h) at a temperature of 135°C for 1.5 hours, and then pyrolyzed in a nitrogen atmosphere (inlet flow rate is 8 L / h) at a temperature of 600°C for 1.5 hours. The prepared material is washed with clean water and dried to constant weight to obtain the magnesium oxide.

[0132] The parts of the present application not described in detail are known to those skilled in the art.

[0133] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent replacements to some of the technical features, without departing from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a surface-enhanced biochar adsorbent material modified with magnesium oxide, characterized in that: The method comprises the following steps: (1) Pre-carbonizing the biomass material to obtain biochar; (2) stirring and impregnating the biochar in a hot gallic acid solution to obtain an impregnation solution, and then filtering the impregnation solution to obtain a biochar coated with a gallic acid layer; (3) stirring and immersing the biochar coated with the gallic acid layer in a magnesium salt solution, and then drying the solution to obtain a mixture; (4) The mixture is subjected to a heating treatment at 120-150° C. and a pyrolysis treatment at 400-600° C. in an inert gas to obtain a magnesium oxide-modified surface-enhanced biochar adsorbent material.

2. The preparation method according to claim 1, characterized in that In step (1): The biomass material is a lignocellulosic biomass material. Preferably, the lignocellulosic biomass material is one or more of agricultural and forestry waste, grass leaves and wood materials. More preferably, the lignocellulosic biomass material is one or more of corn cobs, straw, grass leaves, fruit shells and wood materials. Further preferably, the lignocellulosic biomass material is corn cobs.

3. The preparation method according to claim 1, characterized in that In step (1): Before pre-carbonization, the biomass material is pretreated. The pretreatment of the biomass material is as follows: the biomass material is cleaned and cut into blocks, and then dried, crushed and sieved to obtain a biomass material with a particle size of less than 18 mesh.

4. The preparation method according to claim 1, characterized in that In step (1): The pre-carbonization is carried out under the protection of an inert gas, preferably in a nitrogen atmosphere, or the pre-carbonization is carried out in a closed space after pre-removal of oxygen by limited combustion; and / or The pre-carbonization temperature is 300-600° C., and the time is 1-3 hours. Preferably, the pre-carbonization temperature is 400° C., and the time is 2 hours.

5. The preparation method according to claim 1, characterized in that In step (2): The gallic acid concentration of the hot gallic acid solution is 50-150 g / L, and the temperature of the hot gallic acid solution is 70-90° C., preferably 80-90° C.; The ratio of the biochar to the hot gallic acid solution is (25-75) g:1 L; The mass ratio of the biochar to the gallic acid in the hot gallic acid solution is 1:(1-3); and / or The stirring and soaking time is 12 to 24 hours.

6. The preparation method according to claim 1, characterized in that In step (2): The temperature of the impregnation solution before filtering is not lower than 70°C, and the filtration time is not more than 5 minutes. Preferably, the filtrate after filtration is recovered and reused until no solid is precipitated when it is cooled to 40°C.

7. The preparation method according to claim 1, characterized in that In step (3): The magnesium salt in the magnesium salt solution is one or more of magnesium nitrate, magnesium sulfate and magnesium chloride, preferably magnesium chloride; The temperature of the magnesium salt solution does not exceed 30°C, preferably does not exceed 20°C; The concentration of the magnesium salt solution is 200-500 g / L; The mass ratio of the magnesium contained in the magnesium salt solution to the biochar in step (2) is (0.1-0.3): 1; and / or The stirring and soaking time is 1 to 2 hours.

8. The preparation method according to claim 1, characterized in that In step (3): The drying temperature is 60-75°C; and / or Drying is performed to obtain a slurry mixture having a moisture content of 20 to 40%.

9. The preparation method according to claim 1, characterized in that In step (4): Heating at 120-150°C for 1-2 hours; and / or Pyrolysis treatment at 400-600℃ for 1-2h.

10. A magnesium oxide-modified surface-enhanced biochar adsorbent material prepared by the preparation method according to any one of claims 1 to 9.

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