Method for simultaneously preparing indigo and low-release biochar

Abstract

The invention relates to a method for simultaneously preparing indigo and low-release biochar, which comprises the following specific steps: S1, extracting indigo glycoside from bluegrass biomass to prepare an indigo glycoside solution and solid residues; s2, enzyme is added into the indigoside solution obtained in the step S1 for hydrolysis, and an indoxyl solution is obtained; s3, oxidizing the indol solution obtained in the step S2 to obtain a suspension; s4, carrying out solid-liquid separation on the suspension obtained in the step S2 to obtain an indigo solid; and S5, drying the solid residues obtained in the step S1, and performing pyrolysis to obtain the low-release biochar. Compared with the prior art, the method has the advantages that non-structural substances in the biomass are removed in the process of extracting the isatin, generation of released substances in the biochar preparation process is reduced, preparation of the low-released-substance biochar is achieved, the utilization rate of the biomass is increased, zero emission of waste and full utilization of resources are achieved, and the like.

Description

Technical Field

[0001] The invention relates to the field of natural product extraction and low-release biochar preparation, and in particular to a method for simultaneously preparing indigo and low-release biochar. Background Art

[0002] Biochar is mainly composed of amorphous carbon, aromatic carbon and ash. Biochar contains a high carbon content, mainly in the form of aromatic carbon, with a porous structure and a huge specific surface area. There are pores of varying sizes on the surface of biochar, ranging from nanometers to micrometers. These micropores determine that biochar has a high specific surface area. Biochar can be obtained by pyrolysis of lignocellulosic biomass. However, during the pyrolysis process, a large amount of biologically active substances in the biomass are thermally decomposed and eventually converted into carbon dioxide and water.

[0003] Indigo is an important dye in the denim clothing industry. According to forecasts, the demand for indigo will increase from 86,000 tons / year to 127,000 tons / year from 2022 to 2028. As the concept of green environmental protection has become popular, plant indigo has regained people's attention due to its biodegradability, environmental compatibility and antibacterial and antiviral health benefits. However, the marketization process of plant indigo is limited by a series of problems, such as inefficient production, low raw material utilization and high production costs. In the current plant indigo production process, the stems and leaves of the blue grass plant enter the fermentation container together for fermentation. However, the main component of the stem is lignocellulose, and the dense structure will make the release rate of indoxyl from the stem during the fermentation process significantly slower than the release rate from the leaves. The stems after fermentation are not effectively utilized, and the residues are usually discarded directly, resulting in a large loss of additional products. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of low biomass utilization rate in plant indigo production and provide a method for simultaneously preparing indigo and low-release biochar. The present invention uses stems that are difficult to use in indigo production as raw materials, extracts indigo in the stems through pretreatment, prepares indigo using exogenous enzyme hydrolysis, and prepares biochar through pyrolysis of the remaining solid residue, thereby improving the utilization rate of biomass.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for simultaneously preparing indigo and low-release biochar, the specific steps are as follows:

[0007] S1. extracting indigoside from blue grass biomass to prepare an indigoside solution and a solid residue;

[0008] S2, adding an enzyme to the indigoside solution obtained in step S1 for hydrolysis to obtain an indoxyl solution;

[0009] S3, oxidizing the indoxyl solution obtained in step S2 to obtain a suspension;

[0010] S4, performing solid-liquid separation on the suspension obtained in step S2 to obtain an indigo solid;

[0011] S5. Dry the solid residue obtained in step S1 and then pyrolyze it to obtain low-release biochar.

[0012] Furthermore, in step S1, the method for extracting indigoside from blue grass biomass is selected from a hydrothermal method or an organic solvent extraction method.

[0013] In the above further aspect, the hydrothermal temperature is 60-100°C.

[0014] In the above, further, the organic solvent is selected from methanol, ethanol or acetone, and the extraction conditions are temperature 60-100° C., time 10 minutes to 1.5 hours, and solid-liquid ratio 5%-30% (w / v).

[0015] Furthermore, in step S1, the blue grass biomass includes indigo biomass, polygonum tinctorium biomass and isatis indigotica biomass, etc.

[0016] Furthermore, in step S1, the bluegrass biomass is pretreated bluegrass biomass, and the pretreatment method is: separating the stems and leaves of the bluegrass biomass, microwave drying the stems and leaves respectively, and crushing and mixing to obtain the pretreated bluegrass biomass.

[0017] Furthermore, the enzyme in step S2 includes β-glucosidase and a protein mixture having β-glucosidase activity, the hydrolysis temperature is 30-50° C., the pH value is 4-10, and the stirring speed is 0-1000 rpm.

[0018] Furthermore, in step S3, the oxidation temperature is 10-80° C., the oxidation time is 10 minutes to 120 minutes, the pH value is 2-13, and the stirring speed is 0-1000 rpm.

[0019] Furthermore, in step S4, the solid-liquid separation method is selected from sedimentation, suction filtration or filter press.

[0020] Furthermore, in step S5, the drying method is selected from shade drying or heat drying.

[0021] Furthermore, in step S5, the pyrolysis temperature is 400-800° C., and the pyrolysis time is 1-6 hours.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] (1) In the preparation method of the present invention, non-structural substances in the biomass are removed during the extraction of indigoside, thereby reducing the generation of release substances during the preparation of biochar, thereby achieving the preparation of low-release biochar.

[0024] (2) Indigoside can be converted into indigo under the action of exogenous enzymes, solving the industry problem that the stems of blue grass biomass cannot be converted into indigo and reducing the cost of indigo production.

[0025] (3) The coupled preparation of indigo and biochar can improve the utilization rate of biomass, achieve zero waste emissions and full utilization of resources.

[0026] (4) The present invention utilizes the characteristics of the stem's lignocellulosic biomass components to achieve an integrated biorefining process for indigo production and biochar preparation, thereby forming a production model with full resource utilization and zero waste emissions of biomass, providing a theoretical basis and methodological basis for the efficient utilization of biomass. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the variable response surface diagram of indigoside extraction rate in Experiment 1, where A is the extraction temperature, °C; B is the ethanol concentration, % v / v; C is the extraction time, min;

[0028] Figure 2 The graph of the changes of indoxyl and indole glycosides during the fermentation process and the exogenous enzyme conversion process of Experimental Example 2, wherein A is the fermentation group and B is the exogenous enzyme conversion group;

[0029] Figure 3 This is a schematic diagram showing the material balance comparison between the traditional process and the "two-step process" process for producing indigo in Experimental Example 3;

[0030] Figure 4 Schematic diagram of the effect of indigoside extraction on the preparation of low-release biochar in Experimental Example 4, where A is TDS; B is conductivity; C is TOC; D is biochar yield; and E is indigo conversion rate;

[0031] Figure 5 This is the yield graph of indigo and low-release biochar in Experimental Example 4. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0033] In conjunction with the accompanying drawings, some embodiments of the present invention are described in detail below. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0034] Example 1

[0035] This embodiment provides a method for simultaneously preparing indigo and low-release biochar, and the specific steps are as follows:

[0036] S1. The stems and leaves were separated by a stem-leaf separator, and the stems and leaves were microwave-dried separately using a laboratory microwave oven with a maximum output power of 700 W (G70D20CN1P-D2(S0), Guangdong Galanz Group Co., Ltd., China); when microwave drying the leaves, the leaves were spread on a glass turntable in a single layer and dried at a microwave power of 700 W for 3 min until the moisture content of the sample dropped below 10%.

[0037] S2, using a pulverizer to grind the dried Indigofera tinctoria leaves and stems for 3 minutes to obtain Indigofera tinctoria leaves and stems powder, and passing the powder through a 30-mesh sieve to obtain pretreated Indigofera tinctoria biomass;

[0038] S3, adding the pretreated indigo biomass into a boiling water bath (100°C) and boiling for 60 minutes to extract indigo in the biomass, with a solid-liquid ratio of 1:10, and after cooling in a cold water bath, using a vacuum pump to filter the solution through a 40 μm medium-speed filter paper to achieve solid-liquid separation, and preparing an indigo solution and a solid residue;

[0039] S4, adjusting the indigoside solution obtained in step S3 to an indigoside concentration of 1 g / L and a solution pH of 5, adding Acremonium cellulase to the adjusted indigoside solution to obtain a mixed solution, and hydrolyzing the mixed solution under N2 conditions at 45° C. and 200 rpm for 1 h to obtain an indoxyphenol solution;

[0040] S5, adjusting the pH value of the indoxyl solution obtained in step S4 to 10, condensing and oxidizing at room temperature for 2 h at 200 rpm to obtain a suspension;

[0041] S6. The pH value of the suspension obtained in step S5 is adjusted to 2 with 72% (v:v) sulfuric acid, and then placed in a refrigerator at 4°C for precipitation overnight. After the precipitation is completed, centrifuge at 11000 rpm for 5 minutes, and dry the solid obtained by centrifugation at 105°C overnight to obtain a crude indigo solid;

[0042] S7. Dry the solid residue obtained in step S3 and then pyrolyze it, heat it to 600°C at a constant heating rate of 5°C / min, and keep it at this temperature for 2 hours during anaerobic pyrolysis using a programmable tubular furnace, and pass the pyrolyzed biochar through a 100-mesh sieve to obtain low-release biochar.

[0043] Embodiment 2:

[0044] This embodiment provides a method for simultaneously preparing indigo and low-release biochar, and the specific steps are as follows:

[0045] S1. The stems and leaves were separated by a stem-leaf separator, and the stems and leaves were microwave-dried separately using a laboratory microwave oven with a maximum output power of 700 W (G70D20CN1P-D2(S0), Guangdong Galanz Group Co., Ltd., China); when microwave drying the leaves, the leaves were spread on a glass turntable in a single layer and dried at a microwave power of 700 W for 3 min until the moisture content of the sample dropped below 10%.

[0046] S2, using a pulverizer to grind the dried Indigofera tinctoria leaves and stems for 3 minutes to obtain Indigofera tinctoria leaves and stems powder, and passing the powder through a 30-mesh sieve to obtain pretreated Indigofera tinctoria biomass;

[0047] S3, 50% ethanol reflux extraction of indigo in the pretreated indigo biomass, solid-liquid ratio of 1:10, using a long catheter for condensation reflux for 1h, after cold water bath, using a vacuum pump to filter the solution through a 40μm medium-speed filter paper to achieve solid-liquid separation, and preparing indigo solution and solid residue;

[0048] S4, after removing ethanol from the indigoside solution obtained in step S3 by rotary evaporation using a rotary evaporator, the indigoside solution is adjusted to an indigoside concentration of 1 g / L and a solution pH of 5, Acremonium cellulase is added to the adjusted indigoside solution to obtain a mixed solution, and the mixed solution is hydrolyzed under N2 conditions at 45° C. and 200 rpm for 1 h to obtain an indoxyphenol solution;

[0049] S5, adjusting the pH value of the indoxyl solution obtained in step S4 to 10, condensing and oxidizing at room temperature for 2 h at 200 rpm to obtain a suspension;

[0050] S6. The pH value of the suspension obtained in step S5 is adjusted to 2 with 72% (v:v) sulfuric acid, and then placed in a refrigerator at 4°C for precipitation overnight. After the precipitation is completed, centrifuge at 11000 rpm for 5 minutes, and dry the solid obtained by centrifugation at 105°C overnight to obtain a crude indigo solid;

[0051] S7. Dry the solid residue obtained in step S3 and then pyrolyze it, heat it to 600°C at a constant heating rate of 5°C / min, and keep it at this temperature for 2 hours during anaerobic pyrolysis using a programmable tubular furnace, and pass the pyrolyzed biochar through a 100-mesh sieve to obtain low-release biochar.

[0052] Comparative Example 1

[0053] This comparative example provides a method for preparing common biochar, and the specific steps are as follows:

[0054] S1, first use a laboratory microwave oven with a maximum output power of 700 W (G70D20CN1P-D2 (S0), Guangdong Galanz Group Co., Ltd., China) to dry the stems and leaves of fresh Indigofera indigofera biomass, and then use a grinder to grind for 3 min to obtain Indigofera indigofera leaf and Indigofera indigofera stem powder;

[0055] S2. Dry the indigofera leaf and indigofera stem powder obtained in step S1 and then pyrolyze them. Heat them to 600°C at a constant heating rate of 5°C / min, and maintain them at this temperature for 2 hours during anaerobic pyrolysis using a programmable tubular furnace. Pass the pyrolyzed biochar through a 100-mesh sieve to obtain low-release biochar.

[0056] Test Example 1

[0057] This test example provides a method for extracting indigoside from the stems of Indigofera ovata, and the specific steps are as follows:

[0058] The stems of fresh indigofera biomass were dried using a laboratory microwave oven with a maximum output power of 700 W (G70D20CN1P-D2(S0), Guangdong Galanz Group Co., Ltd., China), and then crushed using a grinder for 3 min to obtain indigofera stem powder. Depending on the experimental requirements, the stems were added to a boiling water bath (100°C) and boiled for 60 min at a solid-liquid ratio of 1:10, or added to preheated ethanol solutions of different concentrations at a solid-liquid ratio of 1:10 for heating extraction.

[0059] like Figure 1 As shown in A, there is an interaction between extraction temperature and ethanol concentration. As the temperature increases or the ethanol concentration decreases, the extraction efficiency increases. The effect of increasing ethanol concentration may be related to the decreased polarity of the solvent mixture.

[0060] like Figure 1 As shown in Figure B, there was little interaction between extraction temperature and time, and the yield of indigoside hardly changed with changes in these two parameters. Therefore, high temperature and long extraction time were not required for the extraction of indigoside from microwave-dried stems.

[0061] like Figure 1 As shown in Figure C, there is an interaction between ethanol concentration and extraction time. The optimal extraction conditions were finally determined to be: extraction temperature, 80°C; ethanol concentration, 50%; extraction time, 1.5h. Under these conditions, the predicted maximum yield of indigoside is 0.35%. In actual operation, the extraction was performed with 50% ethanol aqueous solution at 80°C for 1.5h, and the indigoside yield result was 0.33±0.01%, which is close to the predicted yield.

[0062] Comparative Example 2

[0063] This comparative example provides a method for preparing indigo using indigo leaves as raw materials by fermentation, and the specific steps are as follows:

[0064] S1, using a stem-leaf separator to separate the stems and leaves, using a laboratory microwave oven with a maximum output power of 700 W (G70D20CN1P-D2 (S0), Guangdong Galanz Group Co., Ltd., China) to dry the fresh indigofera leaves biomass, and then using a pulverizer to grind for 3 min to obtain indigofera leaf powder;

[0065] S2. Dry the Indigofera lantana powder obtained in step S1 and ferment it for 2-3 days to obtain a crude indigo solid (fermentation group).

[0066] Test Example 2

[0067] This test example provides a method for preparing indigo using exogenous enzymes with indigo leaves as raw materials, and the specific steps are as follows:

[0068] The stems and leaves were separated by a stem-leaf separator, and microwave drying was performed on the stems and leaves using a laboratory microwave oven with a maximum output power of 700 W (G70D20CN1P-D2 (S0), Guangdong Galanz Group Co., Ltd., China). When microwave drying the leaves, the leaves were spread on a glass turntable in a single layer and dried at a microwave power of 700 W. The drying lasted for 3 min until the moisture content of the sample dropped below 10%. After microwave drying, 20 g of fresh leaves were ground in a mortar and added to 200 mL of preheated water, and then immediately placed in an ice bath after a boiling water bath for 10 min. The above solution was filtered to obtain an aqueous solution containing indole glycosides, which was filled to 300 mL and adjusted to pH = 5 and transferred to a bioreactor. Subsequently, 0.3 g of Acremonium cellulase was added, and the reaction was carried out at 45 ° C and 200 rpm under N2 for 1 h. After the enzymatic hydrolysis reaction was completed, the pH value of the solution was adjusted to 10, and condensation was carried out at room temperature for 2 h at 200 rpm. After the condensation, the pH value of the fermentation broth was adjusted to 2 with 72% (v:v) sulfuric acid, and then placed in a refrigerator at 4°C for precipitation overnight. After the precipitation, the broth was centrifuged at 11000 rpm for 5 min, and the solid obtained by centrifugation was dried at 105°C overnight to obtain the crude indigo solid (exogenous enzyme conversion group).

[0069] Since the indole glycosides in microwave-dried leaves can be preserved for a long time, this experimental example developed a process for producing plant indigo using microwave-dried leaves. The main process is microwave drying leaves → indole glycoside extraction → exogenous enzyme hydrolysis of indole glycosides → indigo production. To further verify the feasibility of this process, this study compared the yield and purity of indigo obtained by this process with fermentation production. Figure 2As shown in A, the indole content of the fresh leaf fermentation group gradually increased with the extension of fermentation time, reaching a maximum of 0.42 mg / g fresh leaf at 36 h; at 48 h, the indole content slightly decreased to 0.37 mg / g fresh leaf. Microwave dried leaves can be extracted with water to obtain an indole glycoside aqueous solution, to which exogenous enzymes are added under nitrogen conditions to hydrolyze indole glycosides. The indole glycosides in the exogenous enzyme conversion group were basically consumed within 1 h, and the final amount of indole generated was 1.02 mg / g fresh leaf ( Figure 2 B), the amount of indoxyl generated in the exogenous enzyme conversion group is 2.4 times the amount of indoxyl obtained by fermentation. The reason why the yield of indoxyl in the exogenous enzyme conversion group is much greater than that of the fermentation group is that the indoxyl generated during the fermentation process may be converted into other substances by various enzymes present in the leaves, such as polyphenol oxidase. However, the instantaneous high temperature of the exogenous enzyme conversion group during the drying process not only curbs the decomposition of indoxyl glycosides by β-glucosidase, but also limits the influence of other enzymes on indoxyl, avoiding the occurrence of side reactions.

[0070] As shown in Table 1, the purity of indigo in the fermentation group is 25.7% and the purity of indigo red is 7.9%. The purity of indigo and indigo red obtained by microwave drying leaves and water extraction of indole glycosides and adding exogenous enzymes for conversion are 20.7% and 4.9%, respectively. The purity of indigo and indigo red in the fermentation group is slightly higher than that of the exogenous enzyme conversion group. However, in terms of yield, the indigo yields of the fermentation group and the exogenous enzyme conversion group are 0.17% and 0.40%, respectively, and the indigo yield of the exogenous enzyme conversion group is 2.4 times that of the indigo yield of the fermentation group. This ratio is the same as the ratio of indoxyl yield, indicating that the indigo yield is directly related to the indoxyl yield. The results of this experimental example show that the process of producing plant indigo using microwave dried leaves after water extraction and conversion is completely feasible. At present, the most common method for enzymatic hydrolysis of precursors to generate indoxyl is short-term fermentation, which basically takes 2-3 days, but a large amount of indoxyl can be obtained within 1 hour by using microwave drying coupled with exogenous enzyme hydrolysis process. Compared with traditional fermentation, this process not only improves the indigo yield and significantly shortens the reaction time, but also solves the industry problem of seasonal indigo production.

[0071] Table 1 Indigo purity and yield of fermentation group and exogenous enzyme conversion group in Experimental Example 2

[0072]

[0073] Test Example 3

[0074] This test example provides a method for producing indigo using indigo stems as raw materials, and the specific steps are as follows:

[0075] The microwave stems were extracted with 50% ethanol at a solid-liquid ratio of 1:10 and a temperature of 80°C for 1.5 h. At the end of the reaction, the stem residue was separated and then the ethanol was removed by rotary evaporation to obtain an indigo solution. 0.05% Acremonium cellulase was added to the indigo solution, and the solution was enzymatically hydrolyzed at 45°C in a bioreactor for 2 h. The pH of the solution was adjusted to 10 and stirred for another 2 h to convert indigo into indigo.

[0076] After extracting indigo from the stems, indigo conversion was performed with the addition of exogenous β-glucosidase to determine the feasibility of producing indigo from the stems of Indigofera serrata. In order to more intuitively show the advantages of this process, the traditional indigo production process of soaking and fermentation was referenced to compare the material changes of the two processes.

[0077] like Figure 3 As shown, in 100kg of Indigofera indigo biomass, the stems and leaves contain a total of 684.0g indigo (453.0g in the leaves and 231.0g in the stems). During the soaking and fermentation process, the obtained indoxyphenol is oxidized to indigo, and the final yield of pure indigo obtained after separation is 85.3g, the total conversion rate is 28.1%, the stem conversion rate is 1.6%, and the leaf conversion rate is 41.6%, that is, 98% of the plant indigo is produced by leaves. Therefore, the traditional fermentation process hardly produces indigo from Indigofera indigo stems. This means that almost all of the indigo in the stems are not effectively converted into indigo, resulting in a large amount of biomass waste.

[0078] like Figure 3 As shown, in the "two-step" process for indigo production proposed in this study, the leaves and stems are separated before fermentation, and the leaves are used for conventional fermentation to produce indigo. The "two-step" process produced a total of 115.7g of pure indigo from 100kg of indigo biomass, which is 1.4 times that of conventional fermentation. Of the indigo produced by the "two-step" process, 27.7% was produced from stems and 72.3% was produced from leaves. The total conversion rate of indigo to indigo increased to 38.1%, which was 1.4 times higher than that of conventional fermentation. The conversion rates of stems and leaves were 31.2% and 41.6%, respectively. The indigo conversion rate of indigo in the stems was 20 times higher than that of untreated stems, so the "two-step" process significantly increased the indigo yield from stems as raw materials.

[0079] It is worth noting that compared with the traditional process using stems as raw materials, the two-step process also significantly improves the purity of indigo. The purity of indigo obtained by the two-step process is 28.2%, while the purity of indigo obtained by the traditional process is only 0.6%. This is because indole glycosides are effectively released during the two-step process, the yield of indigo is increased, and the purity can also be improved accordingly. In addition, the purity of indigo in the two-step process is close to that of indigo produced from leaves as raw materials in the traditional process, which shows that the quality of plant indigo produced from stems is close to that of indigo produced from leaves as raw materials. Considering the yield and purity, it is feasible to use stems for plant indigo production. Therefore, the "two-step" process will be a new method to improve the utilization of biomass in indigo production.

[0080] Test Example 4

[0081] This test example provides a method for preparing low-release biochar, and the specific steps are as follows:

[0082] The measurement method of TDS, conductivity and TOC is as follows: weigh 0.03g biochar, put it into a 100ml conical flask, add 60ml UP water (water filtered through an ultrafiltration membrane) to prepare a 0.5g / L mixed solution. Put it into a shaker, stir it at 30℃, 200r / min for one hour, and then measure TDS, conductivity and TOC.

[0083] The TDS, conductivity and TOC of the ordinary biochar prepared in Comparative Example 1 after stirring for one hour were 90.67 mg / L, 181.37 μS / cm and 2.66 mg / L respectively. Figure 4 ). The TDS, conductivity and TOC of the low-release biochar prepared after hydrothermal treatment at 100°C in Example 1 decreased to 34.56 mg / L, 69.11 μS / cm and 1.30 mg / L, respectively. The TDS, conductivity and TOC of the low-release biochar prepared after 50% ethanol reflux treatment in Example 2 decreased to 16.37 mg / L, 32.83 μS / cm and 1.87 mg / L, respectively. This is because the hydrothermal or ethanol reflux treatment removes most of the water-soluble organic matter, resulting in a reduction in non-structural substances in the solid residue. These substances include indigoside, which can be used to generate indigo in the subsequent use of β-glucosidase, thereby forming a coupled preparation of plant indigo and low-release biochar.

[0084] Under these conditions, 19.84 g of low-release biochar and 332.5 mg of indigo ( Figure 5 ). Currently, the price of biochar is between $150-200 / ton, while the price of plant indigo (purity: 30%) is around $200 / kg. Figure 4The data shown in the figure indicate that plant indigo can bring an added value of more than US$200 when producing 1 ton of biochar, thus increasing the utilization value of indigo biomass.

[0085] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for simultaneously preparing indigo and low-release biochar, characterized in that: The specific steps are as follows: S1. extracting indigoside from blue grass biomass to prepare an indigoside solution and a solid residue; S2, adding an enzyme to the indigoside solution obtained in step S1 for hydrolysis to obtain an indoxyl solution; S3, oxidizing the indoxyl solution obtained in step S2 to obtain a suspension; S4, performing solid-liquid separation on the suspension obtained in step S2 to obtain an indigo solid; S5. Dry the solid residue obtained in step S1 and then pyrolyze it to obtain low-release biochar.

2. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: In step S1, the method for extracting indigoside from blue grass biomass is selected from a hydrothermal method or an organic solvent extraction method.

3. The method for simultaneously preparing indigo and low-release biochar according to claim 2, characterized in that: The hydrothermal temperature is 60-100°C.

4. The method for simultaneously preparing indigo and low-release biochar according to claim 2, characterized in that: The organic solvent is selected from methanol, ethanol or acetone, and the extraction conditions are temperature 60-100° C., time 10 minutes to 1.5 hours, and solid-liquid ratio 5%-30% (w / v).

5. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: In step S1, the blue grass biomass includes indigofera tinctoria biomass, polygonum tinctoria biomass and isatis indigotica biomass.

6. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: In step S1, the bluegrass biomass is pretreated bluegrass biomass, and the pretreatment method is: separating the stems and leaves of the bluegrass biomass, microwave drying the stems and leaves respectively, and crushing and mixing to obtain the pretreated bluegrass biomass.

7. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: The enzyme in step S2 includes β-glucosidase and a protein mixture having β-glucosidase activity, the hydrolysis temperature is 30-50° C., the pH value is 4-10, and the stirring speed is 0-1000 rpm.

8. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: In step S3, the oxidation temperature is 10-80° C., the oxidation time is 10 to 120 minutes, the pH value is 2-13, and the stirring speed is 0-1000 rpm.

9. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: In step S4, the solid-liquid separation method is selected from sedimentation, suction filtration or filter press method.

10. The method for simultaneously preparing indigo and low-release biochar according to claim 1, characterized in that: In step S5, the drying method is selected from shade drying or heat drying; In step S5, the pyrolysis temperature is 400-800° C., and the pyrolysis time is 1-6 hours.

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