A method for treating yellow slurry water based on modified chitosan

Abstract

The present invention provides a method for treating yellow slurry water based on modified chitosan. First, the yellow slurry water is subjected to flocculation and clarification pretreatment with modified chitosan to obtain a pretreatment liquid; the pretreatment liquid is subjected to enzymatic hydrolysis treatment with a composite enzyme to obtain an enzymatic hydrolysate; the enzymatic hydrolysate is centrifuged and then ultrafiltered to obtain an ultrafiltrate; then the ultrafiltrate is subjected to desalination and purification treatment with an ion exchange resin to obtain a clarified and transparent desalted liquid; finally, the desalted liquid is vacuum freeze-dried to obtain a yellow slurry water treatment product. The yellow slurry water treatment product has a high content of soybean oligosaccharides and good water solubility, and can be applied to functional foods.

Description

Technical Field

[0001] The present invention belongs to the technical field of yellow slurry water treatment, and relates to a method for treating yellow slurry water based on modified chitosan. Background Art

[0002] Yellow slurry water is a by-product generated during the processing of soybean products. Enterprises generally discharge and treat yellow slurry water as waste, which not only wastes resources but also pollutes the environment. After testing, yellow slurry water contains raffinose, stachyose, sucrose, and also contains low-molecular-weight proteins, crude soybean saponins, polypeptides, inorganic salts, etc. If the tofu yellow slurry water is directly discharged, the rich organic matter in it will cause a significant increase in the COD and BOD values of the water quality, and the easily decomposable macromolecular proteins and salt substances will all cause water pollution.

[0003] Soybean oligosaccharides is the general term for soluble oligosaccharides such as sucrose, raffinose, and stachyose in mature soybeans, with a content of about 10%, mainly distributed in the hypocotyl. It is widely used in food processing because of its high viscosity, moderate sweetness, and good stability to heat and acid. Soybean oligosaccharides are functional oligosaccharides. Although they are difficult to be directly digested and absorbed by the human body, they can be utilized by probiotics - Bifidobacterium in the intestine. Based on this, soybean oligosaccharides indirectly inhibit the growth and metabolism of intestinal saprophytic bacteria and reduce the damage caused by toxic metabolites to the human body. Soybean oligosaccharides have the effects of lowering blood pressure, promoting calcium absorption, improving skin allergies, preventing and assisting in the treatment of lactose intolerance, and protecting the liver. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for treating yellow slurry water based on modified chitosan. The modified chitosan is used to flocculate and clarify the yellow slurry water. The clarified liquid is treated by enzymatic hydrolysis, centrifugation, ultrafiltration, and ion exchange resin to remove salts, and then vacuum freeze-dried to obtain a yellow slurry water treatment product with a high oligosaccharide content. The present invention can effectively extract the functional components in yellow slurry water, save resources, protect the environment, and increase economic benefits.

[0005] To solve the above technical problems, the technical solution of the present invention is as follows:

[0006] A method for treating yellow slurry water based on modified chitosan, comprising the following steps:

[0007] S1. Add 2.5G PAMAM to ultrapure water, stir for 10 minutes, then dropwise add hydrochloric acid aqueous solution to adjust the pH value to 3, and stir and react at room temperature for 48 hours to obtain a reaction solution. Evaporate the ultrapure water in the reaction solution to obtain acidified PAMAM, where 2.5GPAMAM is 2.5-generation polyamide-amine dendritic polymer;

[0008] S2. Add EDC·HCl, NHS, chitosan, and the acidified PAMAM obtained in step S1 into ultrapure water. After stirring for 10 minutes, adjust the pH value to 4 with an aqueous hydrochloric acid solution, and stir and react at 40 °C for 48 hours to obtain a reaction solution. Filter the reaction solution to obtain a solid. Wash the solid 3 times with distilled water and then vacuum dry it at 40 °C until constant weight to obtain modified chitosan; where EDC·HCl is 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and NHS is N-hydroxysuccinimide.

[0009] S3. Add the modified chitosan obtained in step S2 into the yellow slurry water, adjust the pH value to 5 with an aqueous hydrochloric acid solution, stir at room temperature for 20 minutes, let it stand for 1 - 2 hours, and then centrifuge for 20 minutes. ， Obtain the supernatant, which is the pretreatment solution.

[0010] S4. Add the pretreatment solution obtained in step S3 into pure water, stir until evenly mixed, then adjust the pH value to 5 with an aqueous hydrochloric acid solution, add a composite enzyme, and enzymatically hydrolyze at 50 - 60 °C for 5 - 6 hours. After the enzymatic hydrolysis ends, raise the temperature to 100 °C and keep it warm for 5 minutes to inactivate the enzyme to obtain an enzymatic hydrolysate.

[0011] S5. Centrifuge the enzymatic hydrolysate obtained in step S4 for 20 minutes to obtain the supernatant, and perform ultrafiltration treatment on the supernatant using an ultrafiltration membrane to obtain an ultrafiltrate.

[0012] S6. Use 001×7 strong acidic cation exchange resin and D301 weak basic anion resin to perform desalting treatment on the ultrafiltrate obtained in step S6 to obtain a desalted solution, and vacuum freeze-dry the desalted solution until constant weight to obtain a yellow slurry water treatment product.

[0013] Further, in step S1 of the present invention, the ratio of 2.5G PAMAM to ultrapure water is 3g:50 mL, and the mass concentration of the aqueous hydrochloric acid solution is 25%.

[0014] Further, in step S2 of the present invention, the ratio of EDC·HCl, NHS, chitosan, the acidified PAMAM obtained in step S1, and ultrapure water is 19mg:11mg:200mg:500mg:100 mL, and the mass concentration of the aqueous hydrochloric acid solution is 10%.

[0015] Further, in step S3 of the present invention, the ratio of the modified chitosan obtained in step S2 to the yellow slurry water is 1g:1L, the centrifugation speed is 5000 rpm, and the mass concentration of the aqueous hydrochloric acid solution is 10%.

[0016] Further, in step S4 of the present invention, the composite enzyme is composed of papain and endoglucanase with the same enzyme activity, and the ratio of the pretreatment solution obtained in step S3, pure water, and the composite enzyme is 1g:500 mL:5000 U, and the mass concentration of the aqueous hydrochloric acid solution is 10%.

[0017] Furthermore, in step S5 of the present invention, the centrifugal speed is 5000 rpm, the ultrafiltration membrane is a polyethersulfone membrane with a molecular weight cutoff of 5000 Da, and the ultrafiltration treatment conditions are: temperature of 40-50°C, pressure of 50 KPa, concentration multiple of 4 times, and pH value of 7.

[0018] Furthermore, in step S6 of the present invention, 001×7 strongly acidic cation exchange resin and D301 weakly basic anion resin are connected in series at a volume ratio of 1:1, and the conditions for desalination treatment are: temperature of 30-40° C. and flow rate of 12 μs / cm.

[0019] Furthermore, in step S6 of the present invention, the temperature of vacuum freeze drying is -35°C.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1) The present invention first uses modified chitosan to flocculate and clarify the yellow pulp water to obtain pretreated yellow pulp water, effectively reducing the protein content of the yellow pulp water, and then uses a composite enzyme to enzymolyze the pretreated yellow pulp water to obtain an enzymolyzate, further reducing the protein content. The endoglucanase in the composite enzyme can also improve the water solubility of the yellow pulp water treated product. The enzymolyzate is then centrifuged and ultrafiltered to obtain an ultrafiltrate, and low molecular weight proteins are removed. Then, an ion exchange resin is used to desalt and purify the ultrafiltrate to obtain a clear and transparent desalted liquid. Finally, the desalted liquid is vacuum freeze-dried to obtain a yellow pulp water treated product. The yellow pulp water treated product has a high soybean oligosaccharide content and good water solubility, and can be applied to functional foods.

[0022] 2) Chitosan itself has a good flocculation effect on protein. Since yellow pulp water contains multiple components, chitosan is unstable in yellow pulp water, so it cannot fully exert its flocculation effect. Therefore, the present invention first acidifies 2.5G PAMAM to obtain acidified PAMAM with a carboxyl terminal, and then grafts the acidified PAMAM to the molecular chain of chitosan through the catalytic action of EDC·HCl and NHS to obtain modified chitosan. Compared with chitosan, the stability of modified chitosan in yellow pulp water is greatly improved, so it can fully exert its flocculation effect, thereby effectively reducing the protein in yellow pulp water and increasing the content of soybean oligosaccharides in the yellow pulp water treated product. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below in conjunction with specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0024] Example 1

[0025] Treat the yellow slurry water according to the following steps:

[0026] S1. Add 2.5G PAMAM to ultrapure water at a ratio of 3g:50mL, stir for 10 minutes, then dropwise add a hydrochloric acid aqueous solution with a mass concentration of 25%, adjust the pH value to 3, and stir and react at room temperature for 48 hours to obtain a reaction solution. Evaporate the ultrapure water from the reaction solution to obtain acidified PAMAM.

[0027] S2. Add EDC·HCl, NHS, chitosan, and the acidified PAMAM obtained in step S1 to ultrapure water at a ratio of 19mg:11mg:200mg:500mg:100mL, stir for 10 minutes, then adjust the pH value to 4 with a hydrochloric acid aqueous solution with a mass concentration of 10%, stir and react at 40°C for 48 hours to obtain a reaction solution. Filter the reaction solution to obtain a solid, wash the solid 3 times with distilled water, and then vacuum dry it at 40°C to constant weight to obtain modified chitosan.

[0028] S3. Add the modified chitosan obtained in step S2 to the yellow slurry water at a ratio of 1g:1L, adjust the pH value to 5 with a hydrochloric acid aqueous solution with a mass concentration of 10%, stir at room temperature for 20 minutes, let it stand for 1.2 hours, and then centrifuge at 5000 rpm for 20 minutes to obtain a supernatant, which is the pretreatment solution.

[0029] S4. Add the pretreatment solution obtained in step S3 to pure water, stir until evenly mixed, then adjust the pH value to 5 with a hydrochloric acid aqueous solution with a mass concentration of 10%, add a composite enzyme composed of papain and endoglucanase with the same enzyme activity, and enzymatically hydrolyze at 50°C for 6 hours. The ratio of the pretreatment solution obtained in step S3, pure water, and the composite enzyme is 1g:500mL:5000U. After the enzymatic hydrolysis is completed, raise the temperature to 100°C and keep it warm for 5 minutes to inactivate the enzyme to obtain an enzymatic hydrolysate.

[0030] S5. Centrifuge the enzymatic hydrolysate obtained in step S4 at 5000 rpm for 20 minutes to obtain a supernatant, and perform ultrafiltration treatment on the supernatant using an ultrafiltration membrane. The ultrafiltration membrane is a polyethersulfone membrane with a molecular weight cut-off of 5000Da. The conditions for ultrafiltration treatment are: temperature is 40°C, pressure is 50KPa, concentration multiple is 4 times, and pH value is 7.

[0031] S6. Perform desalting treatment on the ultrafiltrate obtained in step S6 using 001×7 strong acidic cation exchange resin and D301 weak basic anion resin connected in series at a volume ratio of 1:1 to obtain a desalted solution. The desalting temperature is 30°C and the flow rate is 12μs / cm. Vacuum freeze-dry the desalted solution at -35°C to constant weight to obtain the yellow slurry water treatment product.

[0032] Example 2

[0033] Treat the yellow slurry water according to the following steps:

[0034] S1. Add 2.5 G PAMAM to ultrapure water at a ratio of 3 g:50 mL. After stirring for 10 minutes, add an aqueous hydrochloric acid solution with a mass concentration of 25% dropwise, adjust the pH value to 3, and then stir and react at room temperature for 48 hours to obtain a reaction solution. Evaporate the ultrapure water from the reaction solution to obtain acidified PAMAM.

[0035] S2. Add EDC·HCl, NHS, chitosan, and the acidified PAMAM obtained in step S1 to ultrapure water at a ratio of 19 mg:11 mg:200 mg:500 mg:100 mL. After stirring for 10 minutes, adjust the pH value to 4 with an aqueous hydrochloric acid solution with a mass concentration of 10%. Stir and react at 40 °C for 48 hours to obtain a reaction solution. Filter the reaction solution to obtain a solid. Wash the solid 3 times with distilled water and then vacuum dry it at 40 °C to constant weight to obtain modified chitosan.

[0036] S3. Add the modified chitosan obtained in step S2 to yellow slurry water at a ratio of 1 g:1 L. Adjust the pH value to 5 with an aqueous hydrochloric acid solution with a mass concentration of 10%, stir at room temperature for 20 minutes, let it stand for 1 hour, and then centrifuge at 5000 rpm for 20 minutes to obtain a supernatant, which is the pretreatment solution.

[0037] S4. Add the pretreatment solution obtained in step S3 to pure water. After stirring until evenly mixed, adjust the pH value to 5 with an aqueous hydrochloric acid solution with a mass concentration of 10%. Add a composite enzyme composed of papain and endoglucanase with the same enzyme activity, and then carry out enzymatic hydrolysis at 60 °C for 5 hours. The ratio of the pretreatment solution obtained in step S3, pure water, and the composite enzyme is 1 g:500 mL:5000 U. After the enzymatic hydrolysis is completed, raise the temperature to 100 °C and keep it warm for 5 minutes to inactivate the enzyme to obtain an enzymatic hydrolysate.

[0038] S5. Centrifuge the enzymatic hydrolysate obtained in step S4 at 5000 rpm for 20 minutes to obtain a supernatant. Use an ultrafiltration membrane to perform ultrafiltration treatment on the supernatant. The ultrafiltration membrane is a polyethersulfone membrane with a molecular weight cut-off of 5000 Da. The conditions for ultrafiltration treatment are: temperature is 50 °C, pressure is 50 KPa, concentration multiple is 4 times, and pH value is 7.

[0039] S6. Use a 001×7 strongly acidic cation exchange resin and a D301 weakly basic anion resin connected in series at a volume ratio of 1:1 to perform desalting treatment on the ultrafiltrate obtained in step S6 to obtain a desalted solution. The desalting temperature is 40 °C and the flow rate is 12 μs / cm. Vacuum freeze-dry the desalted solution at -35 °C to constant weight to obtain a yellow slurry water treatment product.

[0040] Example 3

[0041] Treat yellow slurry water according to the following steps:

[0042] S1. Add 2.5 G PAMAM to ultrapure water at a ratio of 3 g:50 mL. After stirring for 10 minutes, dropwise add an aqueous hydrochloric acid solution with a mass concentration of 25%. Adjust the pH value to 3 and stir at room temperature for 48 hours to obtain a reaction solution. Evaporate the ultrapure water from the reaction solution to obtain acidified PAMAM;

[0043] S2. Add EDC·HCl, NHS, chitosan, and the acidified PAMAM obtained in step S1 to ultrapure water at a ratio of 19 mg:11 mg:200 mg:500 mg:100 mL. After stirring for 10 minutes, adjust the pH value to 4 with an aqueous hydrochloric acid solution with a mass concentration of 10%. Stir and react at 40 °C for 48 hours to obtain a reaction solution. Filter the reaction solution to obtain a solid. Wash the solid 3 times with distilled water and then vacuum dry at 40 °C to constant weight to obtain modified chitosan;

[0044] S3. Add the modified chitosan obtained in step S2 to yellow slurry water at a ratio of 1 g:1 L. Adjust the pH value to 5 with an aqueous hydrochloric acid solution with a mass concentration of 10% and then stir at room temperature for 20 minutes. Let it stand for 1.5 hours and then centrifuge at 5000 rpm for 20 minutes to obtain a supernatant, which is the pretreatment solution;

[0045] S4. Add the pretreatment solution obtained in step S3 to pure water. After stirring until evenly mixed, adjust the pH value to 5 with an aqueous hydrochloric acid solution with a mass concentration of 10%. Add a composite enzyme composed of papain and endoglucanase with the same enzyme activity and then enzymatically hydrolyze at 55 °C for 5.5 hours. The ratio of the pretreatment solution obtained in step S3, pure water, and the composite enzyme is 1 g:500 mL:5000 U. After the enzymatic hydrolysis is completed, raise the temperature to 100 °C and keep it warm for 5 minutes to inactivate the enzyme to obtain an enzymatic hydrolysate;

[0046] S5. Centrifuge the enzymatic hydrolysate obtained in step S4 at 5000 rpm for 20 minutes to obtain a supernatant. Use an ultrafiltration membrane to perform ultrafiltration treatment on the supernatant. The ultrafiltration membrane is a polyethersulfone membrane with a molecular weight cut-off of 5000 Da. The conditions for ultrafiltration treatment are: temperature is 45 °C, pressure is 50 KPa, concentration factor is 4 times, and pH value is 7;

[0047] S6. Use 001×7 strongly acidic cation exchange resin and D301 weakly basic anion resin connected in series at a volume ratio of 1:1 to perform desalting treatment on the ultrafiltrate obtained in step S6 to obtain a desalted solution. The desalting temperature is 35 °C and the flow rate is 12 μs / cm. Vacuum freeze-dry the desalted solution at -35 °C to constant weight to obtain a yellow slurry water treatment product.

[0048] Example 4

[0049] Treat yellow slurry water according to the following steps:

[0050] S1. Add 2.5 G PAMAM to ultrapure water at a ratio of 3 g:50 mL. After stirring for 10 minutes, add an aqueous hydrochloric acid solution with a mass concentration of 25% dropwise, adjust the pH value to 3, and then stir and react at room temperature for 48 hours to obtain a reaction solution. After evaporating the ultrapure water from the reaction solution, acidified PAMAM is obtained.

[0051] S2. Add EDC·HCl, NHS, chitosan, and the acidified PAMAM obtained in step S1 to ultrapure water at a ratio of 19 mg:11 mg:200 mg:500 mg:100 mL. After stirring for 10 minutes, adjust the pH value to 4 with an aqueous hydrochloric acid solution with a mass concentration of 10%. Stir and react at 40 °C for 48 hours to obtain a reaction solution. Filter the reaction solution to obtain a solid. Wash the solid 3 times with distilled water and then vacuum dry it at 40 °C to constant weight to obtain modified chitosan.

[0052] S3. Add the modified chitosan obtained in step S2 to yellow slurry water at a ratio of 1 g:1 L. Adjust the pH value to 5 with an aqueous hydrochloric acid solution with a mass concentration of 10%, stir at room temperature for 20 minutes, let it stand for 2 hours, and then centrifuge at 5000 rpm for 20 minutes to obtain a supernatant, which is the pretreatment solution.

[0053] S4. Add the pretreatment solution obtained in step S3 to pure water. After stirring until evenly mixed, adjust the pH value to 5 with an aqueous hydrochloric acid solution with a mass concentration of 10%. Add a composite enzyme composed of papain and endoglucanase with the same enzyme activity, and then carry out enzymatic hydrolysis at 57 °C for 5.2 hours. The ratio of the pretreatment solution obtained in step S3, pure water, and the composite enzyme is 1 g:500 mL:5000 U. After the enzymatic hydrolysis is completed, raise the temperature to 100 °C and keep it warm for 5 minutes to inactivate the enzyme, obtaining an enzymatic hydrolysate.

[0054] S5. Centrifuge the enzymatic hydrolysate obtained in step S4 at 5000 rpm for 20 minutes to obtain a supernatant. Use an ultrafiltration membrane to perform ultrafiltration treatment on the supernatant. The ultrafiltration membrane is a polyethersulfone membrane with a molecular weight cut-off of 5000 Da. The conditions for ultrafiltration treatment are: temperature is 48 °C, pressure is 50 KPa, concentration factor is 4 times, and pH value is 7.

[0055] S6. Use a 001×7 strong acidic cation exchange resin and a D301 weak basic anion resin connected in series at a volume ratio of 1:1 to perform desalting treatment on the ultrafiltrate obtained in step S6 to obtain a desalted solution. The desalting temperature is 36 °C and the flow rate is 12 μs / cm. Vacuum freeze-dry the desalted solution at -35 °C to constant weight to obtain a yellow slurry water treatment product.

[0056] Comparative Example 1

[0057] The difference from Example 3 is that steps S1 and S2 are missing, and the modified chitosan used in step S3 is replaced with unmodified chitosan.

[0058] Comparative Example 2

[0059] The difference from Example 3 is that the complex enzyme used in Step S4 is replaced with papain.

[0060] Experimental Example 1: Determination of Soybean Oligosaccharide Content

[0061] The sucrose content, stachyose content, and raffinose content in the yellow slurry water (the source of the yellow slurry water used in Examples 1-4 and Comparative Examples 1-2 is the same) and the yellow slurry water treatment products prepared in Examples 1-4 and Comparative Examples 1-2 were determined by high performance liquid chromatography. The chromatographic conditions were as follows: Kromasil-NH 2 chromatographic column, column temperature 30 °C; mobile phase composed of acetonitrile and water with a volume ratio of 7:3; flow rate 1 mL / min; maximum injection volume 20 μL; differential refractive index detector. The test results are shown in Table 1:

[0062] Table 1

[0063]

[0064]

[0065] As can be seen from Table 1, the sucrose content, stachyose content, and raffinose content in the yellow slurry water treatment products prepared in Examples 1-4 of the present invention are all higher than those in the untreated yellow slurry water, indicating that the present invention can effectively extract soybean oligosaccharides from yellow slurry water. Some steps of Comparative Examples 1 and 2 are different from those of Example 3. Compared with Example 3, the sucrose content, stachyose content, and raffinose content of Comparative Example 1 are all reduced, indicating that compared with unmodified chitosan, the modified chitosan prepared by the present invention can further increase the sucrose content, stachyose content, and raffinose content; compared with Example 3, the sucrose content, stachyose content, and raffinose content of Comparative Example 2 are also reduced, indicating that the endoglucanase used in the present invention can also play a role in increasing the sucrose content, stachyose content, and raffinose content.

[0066] Experimental Example 2: Water Solubility Test

[0067] 1 g of the yellow slurry water treatment products prepared in Examples 1-4 and Comparative Example 2 were weighed respectively, added to 100 g of deionized water, stirred until completely dissolved, and the transmittance was measured at 620 nm using a spectrophotometer. The higher the transmittance, the better the water solubility. The test results are shown in Table 2:

[0068] Table 2

[0069] Light transmittance (%) Example 1 90.8 Example 2 90.1 Example 3 91.0 Example 4 90.4 Comparative Example 2 82.6

[0070] As can be seen from Table 2, the light transmittance of Examples 1-4 of the present invention is relatively high, indicating that the yellow slurry treatment product prepared by the present invention has good water solubility. Some steps of Comparative Example 2 are different from those of Example 3. Compared with Example 3, the light transmittance of Comparative Example 2 is reduced, indicating that the endoglucanase used in the present invention can play a role in improving the water solubility of the yellow slurry treatment product.

[0071] The above embodiments are only illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A method for treating yellow pulp water based on modified chitosan, characterized in that: The following steps are involved: S1. 2.5G PAMAM was added to ultrapure water, stirred for 10 minutes, and then aqueous hydrochloric acid was added dropwise, and the pH value was adjusted to 3 and stirred at room temperature for 48 hours to obtain a reaction solution, and the reaction solution was evaporated to remove ultrapure water to obtain acidified PAMAM; S2. EDC·HCl, NHS, chitosan, and the acidified PAMAM obtained in step S1 were added to ultrapure water, stirred for 10 minutes, and then the pH value was adjusted to 4 with an aqueous hydrochloric acid solution, and the reaction was stirred at 40°C for 48 hours to obtain a reaction solution, and the reaction solution was filtered to obtain a solid, and the solid was washed with distilled water for 3 times and then vacuum dried at 40°C to constant weight to obtain modified chitosan; S3. The modified chitosan obtained in step S2 was added to the yellow slurry, the pH value was adjusted to 5 with aqueous hydrochloric acid, and then stirred at room temperature for 20 minutes, allowed to stand for 1-2 hours, and then centrifuged for 20 minutes to obtain a supernatant, which was the pre-treated solution; S4. The pretreatment solution obtained in step S3 is added to pure water, stirred until uniformly mixed, and then the pH value is adjusted to 5 with an aqueous hydrochloric acid solution. After adding the composite enzyme, enzymolysis is performed at 50-60°C for 5-6 hours. After the enzymolysis is completed, the temperature is raised to 100°C and the enzyme is incubated for 5 minutes to inactivate the enzyme to obtain an enzymatic solution. The composite enzyme is composed of papain and endoglucanase with the same enzyme activity. The ratio of the pretreatment solution obtained in step S3, pure water, and composite enzyme is 1g: 500mL: 5000U, and the mass concentration of the aqueous hydrochloric acid solution is 10%; S5. The enzymatic hydrolyzate obtained in step S4 is centrifuged for 20 minutes to obtain a supernatant, and the supernatant is ultrafiltered using an ultrafiltration membrane to obtain an ultrafiltrate; S6. Desalting the ultrafiltrate obtained in step S6 using 001×7 strongly acidic cation exchange resin and D301 weakly basic anion resin to obtain a desalted liquid, and freeze-drying the desalted liquid in vacuum to a constant weight to obtain a yellow slurry water treated product.

2. The method for treating yellow pulp water based on modified chitosan according to claim 1, characterized in that: In step S1, the ratio of 2.5G PAMAM to ultrapure water is 3g:50mL, and the mass concentration of the hydrochloric acid aqueous solution is 25%.

3. The method for treating yellow pulp water based on modified chitosan according to claim 1, characterized in that: In step S2, the ratio of EDC·HCl, NHS, chitosan, the acidified PAMAM obtained in step S1, and ultrapure water is 19 mg: 11 mg: 200 mg: 500 mg: 100 mL, and the mass concentration of the hydrochloric acid aqueous solution is 10%.

4. The method for treating yellow pulp water based on modified chitosan according to claim 1, characterized in that: In the step S3, the ratio of the modified chitosan obtained in step S2 to the yellow pulp water is 1 g: 1 L, the centrifugal speed is 5000 rpm, and the mass concentration of the hydrochloric acid aqueous solution is 10%.

5. The method for treating yellow pulp water based on modified chitosan according to claim 1, characterized in that: In step S5, the centrifugal speed is 5000 rpm, the ultrafiltration membrane is a polyethersulfone membrane with a molecular weight cutoff of 5000 Da, and the ultrafiltration treatment conditions are: temperature of 40-50° C., pressure of 50 KPa, concentration multiple of 4 times, and pH value of 7.

6. The method for treating yellow pulp water based on modified chitosan according to claim 1, characterized in that: In step S6, 001×7 strongly acidic cation exchange resin and D301 weakly basic anion resin are connected in series at a volume ratio of 1:1, and the conditions for desalination treatment are: temperature of 30-40° C. and flow rate of 12 μs / cm.

7. The method for treating yellow pulp water based on modified chitosan according to claim 1, characterized in that: In step S6, the vacuum freeze drying temperature is -35°C.

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