Mulberry branch biochar with high specific surface area and preparation method thereof

Abstract

The application relates to the technical field of biochar preparation, in particular to mulberry branch biochar with high specific surface area and a preparation method thereof, which is prepared by compounding the following components in parts by weight: 50-70 parts of mulberry branches, 20-30 parts of blue algae, 20-45 parts of polyaniline and 5-10 parts of bio-oil. The mulberry branch waste and the blue algae waste are compounded, the natural nitrogen source and the porous structure of the blue algae are utilized to improve the performance of the biochar, the preparation process is combined, the shortcomings in the existing biochar preparation technology are overcome, and the quality of the biochar is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of biochar preparation, in particular to a mulberry branch biochar with high specific surface area and a preparation method thereof. BACKGROUND

[0002] With the rapid development of modern agriculture, large-scale agricultural and forestry industries will produce waste while harvesting fruits and timber. It is reported that most of the agricultural and forestry waste is directly burned, causing direct or indirect environmental pollution. Mulberry branch waste is a typical agricultural and forestry waste. At present, most of the mulberry branch waste has not been fully utilized. How to effectively utilize mulberry branch waste and make it better resource recovery has become one of the technical problems to be solved in recent years. At present, the recycling of mulberry branch waste mainly includes medicine extraction, edible fungus cultivation and flocculant development. However, the treatment amount of mulberry branch by medicine extraction and edible fungus cultivation is very limited, and most of the mulberry branch waste is still directly burned, and the above processes still have the problem of waste treatment. Therefore, it is urgent to develop a treatment method which can more effectively realize the reduction and resource utilization of mulberry branch waste.

[0003] The prior art Chinese patent CN113293001B proposes a preparation and application of phosphorus-rich biochar based on mulberry branch waste. The present application prepares phosphorus-rich biochar by continuous pyrolysis. The prepared phosphorus-rich biochar is applied to the pollution control of heavy metals in farmland soil, which can effectively passivate soil heavy metals, inhibit the process of heavy metal absorption and accumulation by crops, and promote the nutrient supply of soil to crops. It can be used as an efficient carbon-based soil conditioner for soil remediation. The present application can greatly reduce mulberry branch waste and effectively utilize it.

[0004] Referring to the above-mentioned scheme of preparing biochar from mulberry branches, this is still a relatively rare technical research direction at present. Therefore, how to improve the utilization rate of mulberry branches and how to improve the performance of biochar are still a direction worthy of further research.

[0005] Through our research, we found that biochar is one of the effective carbon adsorption materials in modern social life, industrial production and environmental protection governance. With the rapid development of industry, the use of biochar is continuously expanding, and the demand for biochar is increasing, especially the demand for biochar with high specific surface area and total pore volume, and large average pore size. Traditional biochar preparation processes (such as direct carbonization and KOH activation) have problems such as low porosity (specific surface area ≤ 1500 m² / g), insufficient nitrogen doping (≤ 5 wt%), and high energy consumption (≥ 60 kW·h / kg); while the emerging technology adopts molten salt activation method, which also has technical bottlenecks such as low molten salt recovery rate (≤ 70%) and uneven pore distribution (proportion of micropores / mesopores is out of balance).

[0006] To overcome the above technical problems, we provide a new scheme for preparing biochar from mulberry branches. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a mulberry branch biochar with high specific surface area and a preparation method thereof. The mulberry branch waste and blue-green algae waste are compounded, the natural nitrogen source and porous structure of blue-green algae are utilized to improve the performance of the biochar, and the preparation process overcomes the shortcomings in the existing biochar preparation technology, thereby improving the quality of the biochar.

[0008] To solve the above technical problems, the present application adopts the following technical solutions:

[0009] A mulberry branch biochar with high specific surface area is prepared by compounding the following components in parts by weight: 50-70 parts of mulberry branches, 20-30 parts of blue-green algae, 20-45 parts of polyaniline, and 5-10 parts of bio-oil.

[0010] Preferably, the bio-oil is wheat straw bio-oil.

[0011] A preparation method of a mulberry branch biochar with high specific surface area includes the following steps:

[0012] S1, raw material pretreatment: mulberry branch modification and waste blue-green algae phosphoric acid activation;

[0013] S2, the modified mulberry branches, activated blue-green algae and polyaniline are put into a double screw extrusion granulator with a die hole diameter of 2-4 mm according to the formula ratio, and granulated after mixing uniformly at 90℃, to obtain mixed particles, wherein the modified mulberry branches and activated blue-green algae are dried to a water content of ≤5%;

[0014] S3, the mixed particles obtained in step S2 are put into an enzymatic hydrolysis reaction tank, and a composite enzyme is added to react at a temperature of 85℃ for 30min, and then the temperature is lowered to 70℃ for continuous enzymatic hydrolysis reaction for 3-4h, to obtain a mixed material;

[0015] S4, the mixed material after enzymatic hydrolysis in step S3 is subjected to steam flash explosion treatment;

[0016] S5, the mixed material after step S4 is put into a molten salt reaction tank, and LiCl-KCl-CaCl2 ternary molten salt is added, and nano-SiO2 is added for molten salt confinement treatment;

[0017] S6, the mixed material after molten salt confinement in step S5 is put into a staged temperature control tube furnace for gradient activation;

[0018] S7, the mixed material after activation in step S6 is subjected to acid pickling, and then dried to obtain a biochar.

[0019] Preferably, the specific operation of the mulberry branch modification treatment in step S1 is as follows: the mulberry branches are crushed to 40 mesh, mixed with polyaniline in proportion, sprayed with 15% bio-oil, stirred at 60℃ for 2h to form a coating structure; wherein the mass ratio of mulberry branches to polyaniline is 5-8:1.

[0020] Preferably, the specific operation of the waste blue-green algae pretreatment in step S1 is as follows: the waste blue-green algae is put into an ultrasonic reactor, 70% phosphoric acid is added, and after stirring and mixing, it is activated at 120℃ for 2h, and then ultrasonic oscillation is performed at 40 kHz and 200 W for 30-45 min; wherein the solid-liquid ratio of waste blue-green algae to phosphoric acid is 2-4:1.

[0021] Preferably, the composite enzyme in step S3 is a mixture of 50 U / g of laccase and 40 U / g of cellulase, which is prepared by compounding with citric acid-phosphate buffer at pH=5.5±0.2; wherein the laccase is derived from thermophilic bacteria, and the cellulase is derived from Thermomyces lanuginosus cellulase.

[0022] Preferably, the steam flash explosion treatment of the mixed material in step S4 is carried out using a steam flash explosion tank, and the specific operation includes the following three stages:

[0023] First stage: steam permeation at 2.0 MPa for 10 min;

[0024] Second stage: instant pressure relief to 1.8 MPa to tear the fibers;

[0025] Third stage: pressure reduction to 0.5 MPa for steam recovery, wherein the temperature is raised to 120-150℃.

[0026] Preferably, in step S5, the mass ratio of LiCl:KCl:CaCl2 in the LiCl-KCl-CaCl2 ternary molten salt is 3:5:2; the mass ratio of LiCl-KCl-CaCl2 ternary molten salt to mixed material is 1-2:1; nano-SiO2 is pre-mixed with the molten salt by ball milling, the ball-to-material ratio is 10:1, 300 rpm, 2 hours, to ensure uniform dispersion.

[0027] Preferably, the specific operation of the gradient activation in step S6 includes the following steps:

[0028] S61, the low-temperature section is 200-500℃, the temperature rising rate is controlled at 3℃ / min, and N2 protection is performed;

[0029] S62, the medium-temperature section is 500-800℃, the flow rate is 100 mL / min, and CO2 is pulsed injected at an interval of 10 s;

[0030] S63, the high-temperature section is 800-900 DEG C, KOH is added, the mass ratio of KOH to the mixture is 0.5:1, reaction is carried out under N2 protection for 30 min, KOH is stopped to be added and N2 is blown, CO2 is introduced at a flow rate of 200 mL / min, pore expansion is selectively carried out through the Boudouard reaction (C + CO2→ 2CO), and constant temperature is kept for 1 h.

[0031] Preferably, the specific operation of the acid washing and drying in step S7 is as follows:

[0032] S71, acid washing: in the first section, 0.5 mol / L HCl is used for cleaning (ultrasonic cleaning at room temperature for 20 min), soluble salts such as LiCl and KCl are removed, in the second section, 0.1 mol / L HNO3 is used for cleaning (ultrasonic cleaning at 60 DEG C for 30 min), residual metal oxides and KOH are dissolved, and 0.1 mol / L NaHCO3 is added for soaking for 10 min, and residual acid is neutralized to pH=6-7; in the third section, deionized water is used for cleaning, and ash is removed;

[0033] S72, drying: after cleaning, the material is centrifuged at 3500 rpm, and dehydrated twice for 10 min to remove surface moisture; then transferred to a vacuum drying oven for treatment at -0.1 MPa and 100 DEG C for 6 hours to completely remove internal bound water and stabilize the pore structure.

[0034] Advantages of the present application:

[0035] 1. The present application uses mulberry branches as the main raw material to ensure the compressive strength and specific surface area contribution rate of the biochar. Mulberry branches are rich in cellulose (40-50%) and lignin (20-30%), and the fibrous network structure of the mulberry branches provides mechanical support skeleton for the biochar, and forms a stable microporous structure through high-temperature carbonization. Compared with other plant fibers, the lignin content of mulberry branches is high, and the high-temperature condensation characteristics of lignin can enhance the stability of the carbon skeleton, which can avoid the problem of pore collapse caused by the difference in lignin content. The discarded blue-green algae is used as an auxiliary material, the blue-green algae contains natural nitrogen source (protein content 10-15%) and polysaccharide structure, and after high-temperature activation, pyridine / pyrrole nitrogen doping sites are generated, and the pore size is expanded to 50-200 nm through phosphoric acid activation, which improves the adsorption capacity. Compared with other nitrogen sources (such as urea), the pyrolysis process of organic nitrogen of blue-green algae is more controllable, the nitrogen doping efficiency is improved by 50% (up to 8.5-10.2 wt%), and there is no by-product residue. The use of the two together can significantly improve the adsorption effect of heavy metals compared with pure mulberry branch biochar.

[0036] The polyphenylamine is added in the present application, and is mainly used for enhancing the interface compatibility of mulberry branch fibers in the pretreatment stage, improving the combination ability of lignocellulose and subsequent additives through chemical grafting or physical coating, and improving the electrical conductivity and oxidation stability of the raw material; in the composite granulation stage, the polyphenylamine is used as a functional binder and a conductive filler to promote the bonding strength between the biomass particles, and endow the final product with antistatic or electromagnetic shielding performance, and strengthen the mechanical properties and functional characteristics of the composite material.

[0037] In the present application, the wheat straw bio-oil is selected as the adhesive-lubricating composite component, and is prepared through fast pyrolysis. The high carbon and hydrogen content of the wheat straw bio-oil can be directly used as fuel or lubricating medium, and can improve the thermal stability and processing fluidity of the material, and is compatible with natural raw materials such as mulberry branches and blue algae, and can reduce the interface reaction resistance. Compared with corn straw / rice husk bio-oil or other mixed bio-oil, the wheat straw bio-oil has higher calorific value and smaller amount. Therefore, the selection of the wheat straw bio-oil has cost advantage and functional composite property.

[0038] In the preparation process of the present application, a composite enzyme is used for enzymolysis reaction to release the micropore channels of the raw material, and then a steam flash explosion gradient pressure relief method is used to tear the fibers through steam permeation and instantaneous pressure relief, so that the porosity is improved by 60%, which provides a pore expansion basis for subsequent molten salt confinement; then, the molten salt confinement and gradient activation are used in cooperation, the molten salt penetrates into the biomass at low temperature, and forms a nano-confinement space when carbonized at high temperature, so as to inhibit the shrinkage of the carbon skeleton, and the nano-SiO2 is embedded into the carbon skeleton after being blended with the molten salt, so as to improve the compressive strength (≥50 MPa) and avoid the structure collapse in the adsorption process; the pulse CO2 in the activation stage selectively etches the micropores through the Boudouard reaction, and the KOH etching generates mesopores (2-50 nm), so that the specific surface area reaches 1400 m² / g or more, which is significantly higher than that of the conventional biochar; finally, the acid washing and pore stabilization are performed, the gradient acid washing (HCl→HNO3) is used to remove the molten salt residues (LiCl / KCl) and metal oxides (such as CaO) step by step, the ash content is reduced (≤0.3%), the pore channel is prevented from being blocked, vacuum drying (-0.1 MPa, 100℃) is performed to remove the bound water through negative pressure, the pore structure is stabilized, and the service life is prolonged.

[0039] The biochar prepared in the present application has excellent specific surface area and strong adsorption capacity for heavy metals. DETAILED DESCRIPTION

[0040] In order to facilitate the understanding of those skilled in the art, the present application will be further described below in combination with examples, and the content mentioned in the embodiments is not a limitation on the present application.

[0041] Preparation of biochar

[0042] S1, raw material pretreatment:

[0043] Mulberry branch modification treatment: The mulberry branches are crushed to 40 mesh, and the mulberry branches are mixed with polyaniline at a mass ratio of 5-8:1. During the mixing process, 15% bio-oil is sprayed (here "15%" means the mass concentration of wheat straw bio-oil in the spraying liquid, that is, 15g of wheat straw bio-oil and 85g of other components-water or other solvents per 100g of spraying liquid). Stirring at 60°C for 2h forms a coating structure.

[0044] The yield and calorific value of the wheat straw bio-oil are significantly higher than those of corn straw and rice husk (+7.5-13.7 wt%, +2.4-3.6 MJ / kg), the hydrocarbon content is high (+5.2-8.7%), it is closer to the performance of petroleum-based lubricants, the unit cost is 10-15% and 25-30% lower than that of corn straw and rice husk, respectively, the potential for large-scale production is clear, and the interface compatibility with natural fibers (such as mulberry branches and blue algae) is best, the friction coefficient is reduced to 86% of that of petroleum-based products, and the high-temperature stability requirements can be met without additional additives. The wheat straw bio-oil is superior to corn straw and rice husk bio-oil in yield, calorific value, lubricity and cost, and is the best choice for this scheme.

[0045] Waste blue algae pretreatment: The waste blue algae is put into an ultrasonic reactor, 70% phosphoric acid is added (the solid-liquid ratio of waste blue algae to phosphoric acid is 2-4:1), stirred and mixed, then activated at 120°C for 2h, and then ultrasonically oscillated at 40 kHz, 200 W for 30-45 min.

[0046] S2, according to the formula, the modified mulberry branches, activated blue algae and polyaniline are put into a double screw extrusion granulator with a die aperture of 2-4 mm in proportion, mixed uniformly at 90°C, and then granulated to obtain mixed particles. The modified mulberry branches and activated blue algae are dried to a water content of ≤5%.

[0047] S3, 50 U / g of laccase (derived from thermophilic bacteria) and 40 U / g of cellulase (derived from Thermomyces lanuginosus cellulase) are weighed and compounded by citric acid-phosphate buffer solution at pH=5.5 ±0.2 to obtain a composite enzyme. The obtained mixed particles are put into an enzymatic hydrolysis reaction tank, the composite enzyme is added, and the temperature is raised to 85°C for 30 min, and then the temperature is lowered to 70°C for 3-4h of enzymatic hydrolysis reaction to obtain a mixed material. The water temperature is controlled at 30-40°C when the citric acid-phosphate buffer solution is dissolved, the heat-resistant laccase is dissolved preferentially, and then the cellulase is gradually added. The stirring speed is controlled at 200-300 rpm to avoid denaturation of enzyme protein caused by vigorous stirring. 0.3% polyethylene glycol (PEG) or glycerol can also be added as a stabilizer to reduce the risk of enzyme molecule aggregation and inactivation at high temperature.

[0048] S4, the mixture in which the enzymatic hydrolysis is completed in step S3 is subjected to steam flash explosion treatment; first stage: steam penetration at 2.0 MPa for 10 min; second stage: instant pressure relief to 1.8 MPa, tearing the fibers; third stage: pressure reduction to 0.5 MPa, temperature increase to 120~150℃ steam recovery.

[0049] S5, the mixture after step S4 is put into a molten salt reaction tank, LiCl-KCl-CaCl2 ternary molten salt is added, and nano-SiO2 is added for molten salt confinement treatment; the mass ratio of LiCl:KCl:CaCl2 is 3:5:2; the mass ratio of LiCl-KCl-CaCl2 ternary molten salt to the mixture (excluding nano-SiO2) is 1~2:1; the nano-SiO2 is pre-mixed with the molten salt by ball milling, the ball-to-material ratio is 10:1, 300 rpm, 2 hours, to ensure uniform dispersion.

[0050] S6, the mixture after molten salt confinement in step S5 is put into a staged temperature control tube furnace for gradient activation: the low temperature section is 200~500℃, the temperature increase rate is controlled at 3℃ / min, and N2 is introduced for protection; the medium temperature section is 500~800℃, the flow rate is 100 mL / min, and CO2 is pulsed injected at an interval of 10s; the high temperature section is 800~900℃, KOH is added, the mass ratio of KOH to the mixture is 0.5:1, and the reaction is carried out under N2 protection for 30 min, then KOH is stopped and N2 is blown, CO2 is introduced at a flow rate of 200 mL / min, and selective hole expansion is carried out through Boudouard reaction (C + CO2 → 2CO), and the temperature is kept constant for 1 h.

[0051] S7, the mixture after activation in step S6 is subjected to acid washing: first section, 0.5 mol / L HCl is used for cleaning (ultrasonic at room temperature for 20 min), to remove LiCl, KCl and other soluble salts; second section, 0.1 mol / L HNO3 is used for cleaning (ultrasonic at 60℃ for 30 min), to dissolve residual metal oxides and KOH, and 0.1 mol / L NaHCO3 is added for soaking for 10 min, to neutralize the residual acid to pH=6~7; third section, deionized water is used for cleaning, to remove ash;

[0052] After acid washing, drying is carried out: the cleaned material is centrifuged at 3500 rpm, twice dehydrated for 10 min, to remove surface moisture; then transferred to a vacuum drying oven, treated at-0.1 MPa, 100℃ for 6 hours, to completely remove internal bound water and stabilize the pore structure, to obtain the biochar.

[0053] The application provides a preparation method of mulberry branch biochar with high specific surface area, comprising steps S1-S7.

[0054] In some examples, the mulberry branches and polyaniline are mixed in a mass ratio of 5-8:1 in step S1.

[0055] It can be understood that the mass ratio of mulberry branches and polyaniline includes but is not limited to 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, and 8:1.

[0056] The polyaniline is mainly used to enhance the interfacial compatibility of the mulberry branches, improve the bonding capacity of the lignocellulose and the subsequent additives through chemical grafting or physical coating, and improve the electrical conductivity and antioxidant stability of the raw materials.

[0057] In some examples, the waste blue-green algae is pretreated in step S1: the waste blue-green algae is put into an ultrasonic reactor, 70% phosphoric acid is added (the solid-liquid ratio of the waste blue-green algae to the phosphoric acid is 2-4:1), stirred and mixed, activated at 120°C for 2h, and then ultrasonically oscillated at 40 kHz and 200 W for 30-45 min.

[0058] It can be understood that the solid-liquid ratio of the waste blue-green algae to the phosphoric acid includes but is not limited to 2:1, 2.5:1, 3:1, 3.5:1, and 4:1.

[0059] It can be understood that the ultrasonic oscillation time includes but is not limited to 30 min, 32 min, 34 min, 36 min, 38 min, 40 min, 42 min, 44 min, and 45 min.

[0060] In some examples, the mass ratio of the LiCl-KCl-CaCl2 ternary molten salt to the mixed material is 1-2:1 in step S5.

[0061] It can be understood that the mass ratio of the ternary molten salt to the mixed material includes but is not limited to 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, and 2:1. Specific embodiments:

[0063] The following examples illustrate the biochar and the preparation method thereof according to the present application, and it can be understood that the biochar and the preparation method thereof according to the present application are not limited to the following examples.

[0064] The specific amounts of the raw materials in each example are shown in Table 1.

[0065] Table 1: Raw material usage

[0066] The specific amount of the raw material in each example is shown in Table 1.

[0067] S1, raw material pretreatment:

[0068] Mulberry branch modification treatment: 63 kg of mulberry branches were crushed to 40 mesh, and the mulberry branches were mixed with polyaniline at a mass ratio of 7:1. During the mixing process, 8 kg of 15% bio-oil was sprayed. (Here "15%" means the mass concentration of wheat straw bio-oil in the spraying liquid, i.e. 15 g of wheat straw bio-oil and 85 g of other components - water or other solvents per 100 g of spraying liquid), stirred at 60°C for 2h, forming a coating structure.

[0069] Waste blue-green algae pretreatment: 25 kg of waste blue-green algae was put into an ultrasonic reactor, 50 L of 70% phosphoric acid was added, stirred and mixed, then activated at 120°C for 2h, and then ultrasonically oscillated at 40 kHz, 200 W for 40 min.

[0070] S2, the modified mulberry branches, activated blue-green algae were dried in advance to a water content of ≤5%, and the modified mulberry branches, activated blue-green algae and 25 kg of polyaniline were put into a double screw extruder with a die aperture of 2~4 mm according to the formula ratio, mixed uniformly at 90°C, and then granulated to obtain mixed particles.

[0071] S3, 50 U / g of laccase (derived from thermophilic bacteria) and 40 U / g of cellulase (derived from Thermomyces lanuginosus cellulase) were mixed by pH = 5.5 ± 0.2 citric acid-phosphate buffer to obtain a composite enzyme. The obtained mixed particles were put into an enzymatic hydrolysis reactor, and the composite enzyme was added. The temperature was raised to 85°C and reacted for 30 min, then the temperature was lowered to 70°C and the enzymatic hydrolysis reaction was continued for 3.5h, to obtain a mixed material.

[0072] S4, the mixed material after enzymatic hydrolysis in step S3 was subjected to steam flash explosion treatment; first stage: steam permeation at 2.0 MPa for 10 min; second stage: instant pressure relief to 1.8 MPa, fiber tearing; third stage: pressure reduction to 0.5 MPa, temperature increase to 130°C, steam recovery.

[0073] S5, the mixed material after step S4 was put into a molten salt reaction tank, 160 kg of LiCl-KCl-CaCl2 ternary molten salt was added, and nano-SiO2 was added for molten salt confinement treatment; the mass ratio of LiCl: KCl: CaCl2 is 3:5:2; the mass ratio of LiCl-KCl-CaCl2 ternary molten salt to mixed material is 1.6:1; nano-SiO2 was pre-mixed with molten salt, ball-to-material ratio 10:1, 300 rpm, 2h, to ensure uniform dispersion.

[0074] S6, the mixed material in step S5 was put into a segmented temperature control tube furnace for gradient activation: the low temperature section was 200-500℃, the control heating rate was 3℃ / min, and N2 was protected; the medium temperature section was 500-800℃, the flow rate was 100 mL / min, and CO2 was pulsed injected at intervals of 10 s; the high temperature section was 800-900℃, KOH was added, the mass ratio of KOH to the mixed material was 0.5:1, and the reaction was carried out under N2 protection for 30 min; then the KOH was stopped and N2 was blown; CO2 was introduced at a flow rate of 200 mL / min to selectively expand the hole through the Boudouard reaction (C + CO2 → 2CO), and the temperature was kept constant for 1 h.

[0075] S7, the activated mixed material in step S6 was acid washed: in the first section, 0.5 mol / L HCl was used for cleaning (ultrasonic at room temperature for 20 min) to remove LiCl, KCl and other soluble salts; in the second section, 0.1 mol / L HNO3 was used for cleaning (ultrasonic at 60℃ for 30 min) to dissolve residual metal oxides and KOH, and 0.1 mol / L NaHCO3 was added for 10 min to neutralize the residual acid to pH=6-7; in the third section, deionized water was used for cleaning to remove ash;

[0076] After acid washing, drying was carried out: the cleaned material was centrifuged at 3500 rpm for 10 min to remove surface water; then it was transferred to a vacuum drying oven and treated at -0.1 MPa and 100℃ for 6 hours to completely remove internal bound water and stabilize the pore structure, and finally the biochar was obtained.

[0077] Comparative Example 1

[0078] Comparative Example 1 is basically the same as Example 2, except that only mulberry branches are used as the raw material for preparing biochar.

[0079] Comparative Example 2

[0080] Comparative Example 2 is basically the same as Example 2, except that only discarded blue-green algae is used as the raw material for preparing biochar.

[0081] Comparative Example 3

[0082] Comparative Example 3 is different from Example 2 in that the mulberry branches are directly crushed after drying, without modification and pretreatment.

[0083] Comparative Example 4

[0084] Comparative Example 4 is basically the same as Example 2, except that the raw material in Comparative Example 4 does not undergo enzymatic reaction treatment, but is directly subjected to steam flash explosion and subsequent treatment.

[0085] Comparative Example 5

[0086] The same as Example 2, except that Comparative Example 5 is not subjected to molten salt confinement treatment, and is directly subjected to activation and subsequent treatment.

[0087] The yield, specific surface area, total pore volume and average pore diameter of the biochar prepared in each example and comparative example are shown in Table 2; wherein the yield is the mass ratio of the biochar to the raw material, the specific surface area, total pore volume and average pore diameter are measured by nitrogen adsorption-desorption isotherm (BET method): a Micromeritics ASAP 2460 analyzer is used, and the sample is tested after vacuum degassing at 150°C for 6 hours. The test results are shown in Table 2.

[0088] Table 2: Test results of indicators of each example and comparative example

[0089]

[0090] The above test results show that the preparation method of the present scheme has excellent performance, and the molten salt confinement treatment (LiCl-KCl-CaCl2) and gradient activation (CO2 pore expansion) help to significantly improve the porosity and increase the specific surface area. The low temperature stage (200-500°C) helps to generate micropores, which is more conducive to the adsorption of heavy metals, and the medium-high temperature stage (CO2 pulse + KOH activation) forms mesopores and macropores, which synergistically enhances the mass transfer efficiency.

[0091] A heavy metal solution is configured to test the heavy metal adsorption capacity of the biochar prepared in each example.

[0092] Pb²⁺ stock solution: 1.5984 g of lead nitrate (Pb(NO3)2, analytical pure) was accurately weighed and dissolved in 1 L of deionized water to prepare a 1000 mg / L Pb²⁺ solution.

[0093] Cd²⁺ stock solution: 2.0318 g of cadmium nitrate (Cd(NO3)2·4H2O, analytical pure) was accurately weighed and dissolved in 1 L of deionized water to prepare a 1000 mg / L Cd²⁺ solution.

[0094] Working solution: dilute the stock solution with deionized water to the target concentration (10-500 mg / L), and add 0.01 M NaNO3 as a background electrolyte.

[0095] pH adjustment buffer:

[0096] pH = 3-5: 0.1 M HNO3 / NaOH adjustment.

[0097] pH = 5-7: 0.1 M citric acid-phosphate buffer.

[0098] pH = 7~9: 0.1 M Tris-HCl buffer.

[0099] Adsorbent preparation: The prepared mulberry branch biochar was ground to 100 mesh (150 μm) and dried at 105 ℃ to constant weight.

[0100] Adsorption isotherm experiments were performed on the samples to determine the maximum adsorption capacity of the biochar for Pb²⁺ and Cd²⁺.

[0101] Adsorption kinetics experiments were performed on the samples to determine the adsorption rate and equilibrium time of the biochar.

[0102] pH effect experiments were performed on the samples to evaluate the effect of solution pH on adsorption efficiency.

[0103] Data fitting:

[0104] Langmuir and Freundlich isotherm models were used to fit the data and calculate Q max .

[0105] The test results are shown in Table 3

[0106] Table 3 Adsorption capacity determination results of the biochar prepared in Examples 1-7

[0107] All the technical features in the present embodiment can be modified in appearance according to actual needs.

[0108] The above embodiments are the preferred implementation of the present application, in addition to this, the present application can be implemented in other ways, without departing from the technical solution concept, any obvious replacement within the protection scope of the present application.

Claims

1. A method for preparing mulberry branch biochar with high specific surface area, characterized in that: the mulberry branch biochar is prepared by compounding the following components in parts by weight: 50-70 parts of mulberry branches, 20-30 parts of blue algae, 20-45 parts of polyaniline, and 5-10 parts of bio-oil; the bio-oil is wheat straw bio-oil; the preparation of the mulberry branch biochar comprises the following steps: S1, raw material pretreatment: modification of mulberry branches and activation of discarded blue algae by phosphoric acid; S2, the modified mulberry branches, the activated blue algae, and the polyaniline are put into a double-screw extrusion granulator with a die aperture of 2-4 mm in the formula proportion, mixed uniformly at 90℃, and then granulated to obtain mixed particles, wherein the modified mulberry branches and the activated blue algae are dried to a water content of ≤5%; S3, the mixed particles obtained in step S2 are put into an enzymatic hydrolysis reactor, a composite enzyme is added, and the temperature is raised to 85℃ for 30 min, then the temperature is lowered to 70℃ for 3-4 h of continuous enzymatic hydrolysis to obtain a mixed material; S4, the mixed material after enzymatic hydrolysis in step S3 is subjected to steam flash explosion treatment; S5, the mixed material after the treatment in step S4 is put into a molten salt reaction tank, LiCl-KCl-CaCl2 ternary molten salt is added, and nano-SiO2 is added for molten salt confinement treatment; S6, the mixed material after molten salt confinement in step S5 is put into a staged temperature control tubular furnace for gradient activation; S7, the mixed material after activation in step S6 is subjected to acid pickling, and then dried to obtain biochar. In step S1, the specific operation of the mulberry branch modification treatment is as follows: the mulberry branches are crushed to 40 mesh, mixed with polyaniline in proportion, sprayed with 15% bio-oil, stirred at 60℃ for 2 h to form a coating structure; wherein the mass ratio of mulberry branches to polyaniline is 5-8:

1. In step S1, the specific operation of the discarded blue algae pretreatment is as follows: the discarded blue algae is put into an ultrasonic reaction kettle, 70% phosphoric acid is added, stirred and mixed, then activated at 120℃ for 2 h, and then ultrasonically oscillated at 40 kHz, 200 W for 30-45 min; wherein the solid-liquid ratio of discarded blue algae to phosphoric acid is 2-4:

1. In step S3, the composite enzyme is a mixture of 50 U / g laccase and 40 U / g cellulase, which is compounded by citric acid-phosphate buffer with pH=5.5±0.2; wherein the laccase is derived from thermophilic bacteria, and the cellulase is derived from Thermomyces lanuginosus cellulase. In step S4, the steam flash explosion treatment of the mixed material is carried out in a steam flash explosion tank, and the specific operation includes the following three stages: First stage: steam permeation at 2.0 MPa for 10 min; Second stage: instant pressure relief to 1.8 MPa to tear the fibers; Third stage: pressure reduction to 0.5 MPa for steam recovery, wherein the temperature is raised to 120-150℃. ​ ​ ​ 2. The method according to claim 1, wherein the method is characterized by: ​ 3. The method according to claim 1, wherein the method is characterized by: ​ 4. The method according to claim 1, wherein the method is characterized by: ​ 5. The method of claim 1, wherein the method of preparing a high specific surface area mulberry branch biochar is characterized by: ​ ​ ​ ​ 6. The method of claim 1, wherein the method of preparing a high specific surface area mulberry branch biochar is characterized by: The mass ratio of LiCl:KCl:CaCl2 in the LiCl-KCl-CaCl2 ternary molten salt in step S5 is 3:5:2; the mass ratio of the LiCl-KCl-CaCl2 ternary molten salt to the mixture is 1-2:1; the nano-SiO2 is pre-mixed with the molten salt by ball milling, the ball-to-material ratio is 10:1, 300 rpm, 2 hours, to ensure uniform dispersion.

7. The method according to claim 1, wherein the method is characterized by: The specific operation of the gradient activation in step S6 includes the following steps: S61, the low-temperature section is 200-500°C, the control heating rate is 3°C / min, and N2 is introduced for protection; S62, the medium-temperature section is 500-800°C, the flow rate is 100 mL / min, and CO2 is pulsed injected at intervals of 10 s; S63, the high-temperature section is 800-900°C, KOH is added, the mass ratio of KOH to the mixture is 0.5:1, the reaction is carried out under N2 protection for 30 min, the KOH is stopped and N2 is introduced for purging, CO2 is introduced at a flow rate of 200 mL / min, the pore is selectively expanded through the Boudouard reaction, and the temperature is kept constant for 1 h.

8. The method according to claim 1, wherein the method is characterized by: The specific operation of the acid washing and drying in step S7 is as follows: S71, acid washing: first section, 0.5 mol / L HCl is used for cleaning, room temperature ultrasonic for 20 min, to remove LiCl, KCl soluble salt; second section, 0.1 mol / L HNO3 is used for cleaning, 60°C ultrasonic for 30 min, to dissolve residual metal oxides and KOH, and 0.1 mol / L NaHCO3 is added for soaking for 10 min, to neutralize residual acid to pH=6-7; third section, deionized water is used for cleaning, to remove ash; S72, drying: the cleaned material is centrifuged at 3500 rpm, twice dehydrated for 10 min, to remove surface moisture; then transferred to a vacuum drying oven, treated at-0.1 MPa, 100°C for 6 hours, to completely remove internal bound water and stabilize the pore structure.

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