Method for preparing high deacetylation degree chitin with reaction waste liquid recycling

Abstract

The present application relates to the field of comprehensive utilization of biomass resources and preparation of polymer materials, and particularly relates to a green preparation method of high deacetylation degree chitin with reaction waste liquid recycling. The present application aims to solve the problems of serious environmental pollution, resource waste, low product purity and imperfect waste liquid recycling in chitin extraction. The present application uses chitinous biomass as raw material, and first carries out high pressure pulse electric field pretreatment to break the structure of the raw material and improve the reaction activity, and then carries out heating extraction in a choline chloride-lactic acid deep eutectic solvent system to realize efficient removal of protein and inorganic salt in the raw material. The present application uses microwave-assisted and phase transfer catalyst to realize rapid deacetylation reaction of chitin and alkali liquor. The present application uses a waste liquid grading recycling system: through electrodialysis, hydrochloric acid in the waste acid liquid generated by decalcification is recovered, through ultrafiltration-nanofiltration double membrane combination, alkali liquor is recovered, through reduced pressure distillation, deep eutectic solvent components are recovered, and recycling of acid, alkali and solvent is realized.

Description

Technical Field

[0001] This invention relates to the fields of comprehensive utilization of biomass resources and preparation of polymer materials, specifically to a green preparation method for chitin with high degree of deacetylation using recycled reaction waste liquid. Background Technology

[0002] Chitin and its deacetylated product, chitosan, are widely found in marine waste from crustaceans such as shrimp and crabs. As the second largest natural polymer resource after cellulose, chitin possesses excellent biocompatibility, biodegradability, and functional activity, making it valuable in food, medicine, agriculture, and environmental protection. Currently, the industrial production of chitin and chitosan typically employs the traditional acid-base method. This involves first demineralizing raw materials such as shrimp and crab shells with strong acid to remove calcium carbonate, then treating them at high temperatures with a strong alkali to remove proteins, obtaining crude chitin. Finally, the chitin is deacetylated under concentrated alkali conditions with prolonged heating to produce chitosan. This traditional process is lengthy, consumes large amounts of concentrated acid and alkali reagents, resulting in high processing costs. Furthermore, the generated acid and alkali wastewater, if discharged directly without proper treatment, can cause serious environmental pollution. Simultaneously, strong acids and alkalis can damage the structure of chitin, easily inducing polymer degradation, leading to a decrease in the molecular weight and viscosity of the resulting chitosan, and even affecting the degree of deacetylation and product performance. In addition, chitosan obtained by traditional methods is usually light yellow or beige due to the presence of residual pigments and impurities, and requires further decolorization and purification before it can be used in color-sensitive products.

[0003] In response to the aforementioned shortcomings of traditional processes, a series of improved technical solutions have been proposed in recent years. For example, to reduce the amount of acid and alkali used, some studies have utilized enzymatic hydrolysis or microbial fermentation to remove proteins and minerals from shrimp and crab shells. However, biological treatment methods have long processing cycles and high enzyme preparation costs, making industrial-scale application difficult. Processes relying solely on physical fields do not fundamentally change the use of acids and alkalis and still essentially generate a large amount of waste liquid; subsequent deacetylation steps often require alkali treatment, and the recycling and reuse of the organic solvent / eutectic system itself needs to be addressed.

[0004] In summary, current technologies lack a novel chitosan preparation process that organically combines multiple green and efficient methods with a wastewater recycling system. Therefore, it is necessary to provide a new technical solution to significantly reduce environmental pollution and resource consumption, improve product quality and production efficiency, and meet the needs of sustainable industrial development. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a green method for preparing high-degree-of-deacetylation chitosan by recycling reaction waste liquid. This invention uses crustacean biomass waste as raw material and comprehensively utilizes technologies such as high-voltage pulsed electric field, deep eutectic solvent extraction, microwave-assisted deacetylation, and phase transfer catalysis, supplemented by a recycling system for waste acid, waste alkali, and solvent, thereby achieving high-efficiency and low-pollution preparation of high-purity chitosan with a high degree of deacetylation.

[0006] The specific technical solution is as follows:

[0007] A green method for preparing high-degree-of-deacetylation chitin by recycling reaction waste liquid includes the following steps:

[0008] S1: Clean, dry and crush the crustacean biomass waste, add it to deionized water to form a suspension, perform high-pressure pulse, then add hydrochloric acid solution with half the volume of deionized water for decalcification, stirring while adding, the reaction temperature does not exceed 35℃, after no obvious bubbles are generated, let stand, filter to separate the solid, collect the decalcified filtrate, then wash and dry to prepare dry chitin crude material.

[0009] Furthermore, the high-voltage pulse is configured with the following parameters: peak pulse voltage of 20–25 kV / cm, duration of each pulse of 40–50 microseconds, pulse frequency of 100 Hz, and 10–15 pulses.

[0010] S2: Choline chloride and lactic acid are placed in a beaker in a certain proportion to form a deep eutectic solvent. The mixture is heated and stirred in a water bath at 70℃~80℃ and then transferred to a three-necked flask. The dried chitin crude material prepared in step S1 is added, and the mixture is stirred according to the solid-liquid mass ratio. The mixture is then heated in an oil bath, cooled and diluted, allowed to stand and filtered, and the waste deep eutectic solvent solution is collected. The filter residue is washed with ethanol aqueous solution and deionized water until neutral and then dried under vacuum to obtain pure chitin.

[0011] Furthermore, a certain proportion is used where the molar ratio of choline chloride to lactic acid is 1:1 to 1:3.

[0012] Furthermore, the solid-liquid mass ratio is 1:7 to 1:10.

[0013] Furthermore, oil bath heating is used, with specific parameters of temperature 100℃~120℃ and reaction time 1~2 hours.

[0014] S3: Mix the pure chitin prepared in step S2 with NaOH solution and add it to a three-necked glass reactor as the reaction liquid phase. Add toluene as the second phase, then add tetrabutylammonium bromide and stir evenly. Place the reactor in a microwave synthesis device for deacetylation reaction. Connect a reflux condenser to the top of the reactor. After the reaction is completed, cool and filter the mixture. Collect the alkaline solution after the deacetylation reaction. Wash the filter cake with deionized water until neutral. Collect the washing liquid. Dry the washed solid to constant weight to prepare crude chitosan.

[0015] Furthermore, the amount of tetrabutylammonium bromide used is 2.5% to 3.5% of the total amount of pure chitosan added.

[0016] Furthermore, the parameters of the microwave synthesis apparatus were set as follows: microwave power 600W, reaction temperature 90℃~120℃, time 8~12 minutes, and stirring speed 200rpm.

[0017] S4: Mix the crude chitosan prepared in step S3 with acetic acid solution, stir and let stand to form a 0.6% w / v solution. Then add activated carbon fiber, stir and filter. Then filter the solution through a ceramic membrane filter to obtain a purified chitosan solution. Under magnetic stirring, add NaOH solution to neutralize and precipitate. Finally filter and wash the solid with deionized water. Dry the solid to constant weight to prepare high degree of deacetylation chitin. The waste liquid generated in the preparation process is recycled.

[0018] Furthermore, the amount of activated carbon fiber used is 18% to 22% of the mass of the added crude chitosan.

[0019] To further neutralize the precipitation, the specific procedure is as follows: add NaOH solution dropwise at a rate of 5 mL / min while monitoring the acidity of the solution with a pH meter until the pH reaches 7.2, then stop adding and let stand for 30 minutes to allow for complete precipitation.

[0020] Furthermore, the waste liquid recycling and treatment specifically includes:

[0021] Decalcification waste acid recovery: The decalcification filtrate waste liquid collected in step S1 is placed in the feed chamber of the electrodialysis device. The cathode chamber and anode chamber are also configured with circulating liquids, initially water, respectively. A three-chamber electrodialysis device is selected, with a cation exchange membrane in the middle and an anion exchange membrane at the end.

[0022] Waste eutectic solvent recovery: Add the waste eutectic solvent collected in step S2 to a rotary evaporator, distill under reduced pressure, add activated carbon powder and stir, then filter;

[0023] Alkali solution and washing waste liquid recovery: The alkali solution and washing liquid after the deacetylation reaction in step S3 are separated by two-stage membrane separation: first, they are filtered with a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 3000 Da, and then concentrated with a nanofiltration membrane with a molecular weight cutoff of 200 Da, and then stored in a sealed container.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention adopts a combination of physical and chemical processes, and utilizes the synergistic effect of high voltage pulse electric field and deep eutectic solvent to significantly improve the purity and deacetylation degree of chitosan, thereby improving the extraction yield and purity of chitin.

[0026] (2) The present invention adopts a waste liquid graded recycling system, which greatly reduces the one-time consumption of strong acids and bases. It uses recyclable green solvents and physical field assistance in multiple steps to replace the traditional large-scale acid-base reaction. In particular, by recycling hydrochloric acid, sodium hydroxide and deep eutectic solvent, the main chemicals are recycled, reducing production costs and reducing the direct discharge of harmful waste acids and bases and environmental pollution.

[0027] (3) The present invention adopts process integration, which organically combines pretreatment, decalcification, deproteinization, deacetylation and purification, reducing intermediate waiting and transfer losses. Attached Figure Description

[0028] Figure 1 This is a flow chart of the process for preparing high-deacetylation chitin by recycling reaction waste liquid according to the present invention.

[0029] Figure 2 The images show ion chromatograms of the degree of deacetylation tested in Examples 1-4 and Comparative Examples 1-3.

[0030] Figure 3 The graph shows a comparison of the degree of deacetylation and wastewater discharge test data of the high-deacetylation chitin prepared in Examples 1-4 and Comparative Examples 1-3. Detailed Implementation

[0031] The following embodiments further explain and illustrate the technical solutions of the present invention. It is particularly noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention. (See attached...) Figure 1 The diagram shows the process flow for preparing high-deacetylation chitin using recycled reaction waste liquid. The detailed preparation steps are as follows:

[0032] 1. High-voltage pulsed electric field pretreatment of raw materials and decalcification

[0033] Cleaned and dried shell biomass was pulverized and placed in water to form a suspension slurry. The slurry was then briefly treated using a high-voltage pulsed electric field. Through the electroporation and perturbation effects of the high-voltage pulsed electric field, the hard shell structure was loosened and fractured, improving the efficiency of subsequent penetration and extraction of the effective components from the raw material by acids, alkalis, or solvents. After treatment with the high-voltage pulsed electric field, it was observed that some cell tissues in the shrimp shell suspension were destroyed, and the slurry viscosity decreased slightly, indicating that the electric field pretreatment played a preliminary role in breaking down and loosening the raw material structure.

[0034] The raw material suspension, after being treated with a high-voltage pulsed electric field, is decalcified using hydrochloric acid solution. Soaking and stirring at room temperature converts calcium carbonate into soluble calcium chloride, releasing carbon dioxide gas. This step removes most of the inorganic minerals from the raw material, yielding decalcified crude chitin solid. Excess hydrochloric acid and the generated calcium chloride waste liquid in the reaction solution are collected and centrally treated.

[0035] 2. Extraction of chitin using eutectic solvent

[0036] The decalcified crude chitin is extracted by heating in a choline chloride-lactic acid eutectic solvent. The eutectic solvent is a system composed of choline chloride and lactic acid in a molar ratio of 1:1 to 1:3. This system selectively dissolves proteins and other organic matter in the raw material upon heating and reacts with residual inorganic salts. The "eutectic solvent extraction" process effectively functions as a two-step process: firstly, lactic acid, as an organic acid, further reacts and removes residual minerals, generating calcium lactate dissolved in the liquid phase; secondly, the eutectic mixture formed by choline chloride and lactic acid has excellent solubility and separation capabilities for proteins, removing impurities such as proteins from the raw material without the need for a strong alkali. Specifically, the eutectic solvent and the decalcified raw material are mixed at a certain solid-liquid ratio, heated to approximately 100-120°C, and maintained for 1-2 hours, with intermittent microwave assistance to accelerate extraction. After treatment, the chitin, being poorly soluble in the eutectic solvent, remains in a solid state, while the proteins, most pigments, and residual inorganic salts in the raw material enter the eutectic solvent phase. By adding a small amount of water to dilute the system and cooling it, a small amount of dissolved chitin can be redissolved. High-purity chitin can then be obtained through solid-liquid separation, washing with water, and drying.

[0037] 3. Microwave-assisted deacetylation preparation of chitosan

[0038] The chitin obtained from the above extraction and purification was added to a concentrated alkaline solution and subjected to a deacetylation reaction in the presence of a phase transfer catalyst to generate chitosan. The alkaline solution was selected as sodium hydroxide; the phase transfer catalyst could be a quaternary ammonium salt. To improve the heterogeneous reaction rate, microwave heating was used in this step: chitin, alkaline solution, an appropriate amount of organic solvent, and the catalyst were placed together in a sealed microwave reactor and irradiated with microwaves at 90–120°C for several minutes to tens of minutes. The rapid heating by microwaves significantly shortened the reaction time, and the phase transfer catalyst allowed for more effective contact between the chitin and the solid phase. After the reaction was complete, the product was cooled, washed with water, and neutralized to obtain crude chitosan with a high degree of deacetylation.

[0039] 4. Chitosan purification and recovery

[0040] The crude chitosan was dissolved in dilute acetic acid to prepare a chitosan solution. An appropriate amount of activated carbon fiber was added and stirred for adsorption to remove pigments and residual organic impurities from the solution. The solution was then filtered using a ceramic microfiltration / ultrafiltration membrane to remove the activated carbon fiber and undissolved impurities, yielding a clear, acidic chitosan solution. Subsequently, the pH was adjusted to neutral by gradually adding dilute sodium hydroxide solution, causing the chitosan to precipitate again in solid form. The precipitate was collected by filtration, washed with pure water, neutralized and desalted, and then dried to obtain a high-purity chitosan product with a high degree of deacetylation. The waste liquid was then recovered.

[0041] To address the waste liquids and materials generated in each step of the process, this invention designs a corresponding recycling process to maximize resource recovery and reduce emissions: (a) Decalcification waste acid: generated in step 1, mainly composed of calcium chloride and unreacted hydrochloric acid. This invention recovers the hydrochloric acid through electrodialysis: selective migration is achieved using an electric field, regenerating high-concentration hydrochloric acid in the acid chamber; simultaneously, hydroxide ions are generated in the alkali chamber, precipitating as calcium hydroxide, which is then removed from the solution. Through this process, the recoverable hydrochloric acid can be used in the next batch of decalcification, and the by-product calcium hydroxide can be utilized as a by-product. (b) Deacetylation reaction waste alkali and chitosan precipitation mother liquor: both of these contain sodium hydroxide and the corresponding salts. This invention first uses ultrafiltration to remove any potentially suspended high molecular weight solids, and then uses nanofiltration to separate water and small molecule salts. The nanofiltration concentrate is enriched with sodium hydroxide and soluble salts, which, after adjustment, can be used as an alkali source for the alkali precipitation step or returned to the deacetylation reaction; the purified water permeated by nanofiltration can be recycled for raw material pretreatment, washing, and other processes. (c) Eutectic solvent solution: During the extraction process, a certain amount of water and some organic impurities will be mixed in. Water and low-boiling-point impurities in the eutectic solvent system are removed by vacuum distillation, and the distillate is collected and processed separately. The choline chloride and lactic acid remaining in the distillation vessel are replenished and adjusted according to the ratio and then mixed again as a new eutectic solvent, which can be reused in the next round of chitin extraction.

[0042] Example 1

[0043]

[0044] A green method for preparing chitin with high degree of deacetylation by recycling reaction waste liquid, the specific steps of which are as follows:

[0045] S1: Shrimp shells are rinsed in clean water to remove surface salt and soluble impurities, dried, and pulverized into powder. A 10% (w / v) suspension is prepared by adding deionized water and placing it in the processing chamber of a high-voltage pulsed electric field treatment device. The pulse voltage peak value is set to 23 kV / cm, each pulse duration is 45 microseconds, the pulse frequency is 100 Hz, and the pulses are repeated 12 times. Then, hydrochloric acid solution (half the volume of deionized water) is slowly added to the slurry for decalcification while stirring. The reaction temperature is controlled to not exceed 35℃. After no obvious bubbles are generated, the slurry is allowed to stand, filtered to separate the solid, and the decalcified filtrate is collected. The solid is rinsed with deionized water until neutral and then vacuum-dried at 60℃ for 2 hours to obtain dry chitin crude material.

[0046] S2: Choline chloride and lactic acid were placed in a beaker at a molar ratio of 1:2 and heated in a water bath at 75°C with stirring until a homogeneous and clear liquid was formed, which is the deep eutectic solvent. The solution was then transferred to a three-necked flask equipped with a reflux condenser and the dry chitin crude material prepared in step S1 was added. The solid-liquid mass ratio was 1:8. Stirring was started, and the system was heated in an oil bath to 110°C and maintained for 1.5 hours. Heating was stopped, and the slurry was cooled to 60°C. Deionized water was added for dilution and the mixture was stirred for 5 minutes. The mixture was then allowed to stand and cool to room temperature. It was then filtered using a Buchner funnel. The filtrate was the waste deep eutectic solvent solution, which was collected for subsequent vacuum distillation recovery. The filter residue was washed three times with an ethanol-water solution to remove the deep eutectic solvent residue. Finally, it was washed with deionized water until neutral and then dried in a vacuum freeze dryer at -40°C for 24 hours to obtain pure chitin.

[0047] S3: The pure chitin prepared in step S2 was mixed with NaOH solution at a solid-liquid ratio of 1:3 and added to a three-necked glass reactor as the reaction liquid phase. Toluene was added as the second phase, with a toluene-to-NaOH solution volume ratio of 3:2. Then, 3.0% (by weight of chitin) of the phase transfer catalyst tetrabutylammonium bromide was added and stirred until homogeneous. The reactor was then placed in a microwave synthesis apparatus to induce a deacetylation reaction. Apparatus settings: microwave power 600W, reaction temperature 105℃, time 10 minutes, stirring speed 200rpm. A reflux condenser was connected to the top of the reactor to prevent solvent evaporation. After the reaction, the reactor was cooled to below 60℃. The apparatus was opened, and the reaction mixture was filtered while hot using a Buchner funnel. The alkaline solution after the deacetylation reaction was collected. The filter cake was washed with 60℃ deionized water until neutral, and the washing liquid was collected. The washed solid was dried under vacuum to constant weight to obtain crude chitosan.

[0048] S4: Mix the crude chitosan prepared in step S3 with acetic acid solution, stir for 4 hours and let stand for 12 hours to allow the chitosan to fully dissolve, forming a 0.6% w / v solution. Then add 20% activated carbon fiber (by mass of crude chitosan) and stir magnetically at room temperature for 1 hour. Filter the activated carbon fiber using a 0.45µm microporous membrane, and then filter the solution through a ceramic membrane filtration device using a 0.2µm pore size ceramic membrane at 0.2MPa to obtain a purified chitosan solution. Under magnetic stirring, add NaOH solution for neutralization and precipitation. Specifically, add NaOH solution dropwise at a rate of 5mL / min, while monitoring the acidity of the solution with a pH meter. Stop adding when the pH reaches 7.2, let stand for 30 minutes to allow for complete precipitation, filter, and wash the solid three times with deionized water. Dry the solid under vacuum at 60℃ to constant weight to obtain high-deacetylated chitosan. Recycle the waste liquid generated during the preparation process.

[0049] Waste liquid recycling:

[0050] Decalcification waste acid recovery: The decalcification filtrate waste liquid collected in step S1 is placed in the feed chamber of the electrodialysis device. The cathode chamber and anode chamber are also configured with circulating liquids, initially water, respectively. A three-chamber electrodialysis device is selected, with a cation exchange membrane in the middle and an anion exchange membrane at the end. A voltage of 20V is applied, and electrodialysis is performed for 2 hours to obtain regenerated hydrochloric acid.

[0051] Waste eutectic solvent recovery: The waste eutectic solvent collected in step S2 is added to a rotary evaporator, the pressure is reduced to 0.05 MPa, and the mixture is distilled in a 60°C water bath. Activated carbon powder is added and stirred, then filtered to obtain regenerated eutectic solvent.

[0052] Alkali solution and washing waste liquid recovery: The alkali solution and washing liquid after the deacetylation reaction in step S3 are both alkaline waste liquids. Two-stage membrane separation is adopted: First, the hollow fiber ultrafiltration membrane with a molecular weight cutoff of 3000 Da is used to filter and remove the macromolecules and residual particles of the phase transfer catalyst; then, the solution is concentrated using a nanofiltration membrane with a molecular weight cutoff of 200 Da. The nanofiltration operation pressure is 3 MPa and the temperature is 25℃. The concentrated alkali solution is then stored in a sealed container to obtain regenerated alkali solution.

[0053] Example 2

[0054] The preparation method is the same as in Example 1, except that:

[0055] In step S1, the peak pulse voltage is set to 20kV / cm, the duration of each pulse is 50 microseconds, the pulse frequency is 100Hz, and the number of pulses is 15.

[0056] In step S2, the molar ratio of choline chloride to lactic acid is 1:1; the water bath heating temperature is 70°C; the solid-liquid mass ratio of the deep eutectic solvent to the dry chitin crude material is 1:7; the system is heated to 100°C in an oil bath and maintained for 2 hours.

[0057] In step S3, the phase transfer catalyst tetrabutylammonium bromide accounts for 2.5% of the mass of chitosan; in the microwave synthesis apparatus, the reaction temperature is set to 90°C and the time is 12 minutes.

[0058] In step S4, activated carbon fiber accounts for 18% of the mass of crude chitosan.

[0059] Example 3

[0060] The preparation method is the same as in Example 1, except that:

[0061] In step S1, the peak pulse voltage is set to 25kV / cm, the duration of each pulse is 40 microseconds, the pulse frequency is 100Hz, and the number of pulses is 10.

[0062] In step S2, the molar ratio of choline chloride to lactic acid is 1:3; the water bath heating temperature is 80℃; the solid-liquid mass ratio of the deep eutectic solvent to the dry chitin crude material is 1:10; the system is heated to 120℃ in an oil bath and the reaction is maintained for 1 hour.

[0063] In step S3, the phase transfer catalyst tetrabutylammonium bromide accounts for 3.5% of the mass of chitosan; in the microwave synthesis apparatus, the reaction temperature is set at 120°C and the time is 8 minutes.

[0064] In step S4, activated carbon fiber accounts for 22% of the mass of crude chitosan.

[0065] Example 4

[0066] The preparation method is the same as in Example 1, except that:

[0067] The acid solution used in step S1 is the regenerated hydrochloric acid recovered by electrodialysis in step S5 of Example 1;

[0068] The eutectic solvent used in step S2 is the eutectic solvent recovered in step S5 of Example 1;

[0069] The NaOH solution used in step S3 is the alkali solution recovered in step S5 of Example 1.

[0070] Comparative Example 1

[0071] The preparation method of Example 1 was followed, but without the high-voltage pulsed electric field; hydrochloric acid solution was directly added for decalcification. All other steps were the same.

[0072] Comparative Example 2

[0073] The preparation method of Example 1 was followed, but instead of using deep eutectic solvent extraction, a traditional alkaline method was used for protein removal. All other steps were the same.

[0074] Comparative Example 3

[0075] The preparation method of Example 1 was followed, but the reaction was not carried out in a microwave synthesis apparatus; instead, it was carried out directly under conventional heating conditions with mechanical stirring at 85°C. All other steps were the same.

[0076] The high-deacetylation chitosans prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to a deacetylation test: the degree of deacetylation was calculated by measuring the concentration of acetate ions produced after complete hydrolysis of chitosan. First, the chitosan sample was completely hydrolyzed in a strong acid to completely hydrolyze the acetylamino groups on its molecular chain, releasing acetic acid. Then, the acetate ions in the hydrolysate were separated and quantified by ion chromatography. An ion chromatography system equipped with a conductivity detector was selected, the column was IonPac AS11-HC, the eluent was potassium hydroxide aqueous solution, the flow rate was 1.0 mL / min, the column temperature was 30℃, and the retention times for each ion were: chloride ion retention time 4.2 min, formate ion retention time 6.1 min, acetic acid retention time 7.3 min, lactate ion retention time 8.8 min, phosphate ion retention time 11.5 min, and sulfate ion retention time 13.5 min.

[0077] The high-deacetylation chitosan prepared in Examples 1-4 and Comparative Examples 1-3 was subjected to a wastewater discharge test: the total volume of wastewater generated per kilogram of high-deacetylation chitosan (including decalcification wastewater, extraction wastewater, deacetylation wastewater, etc.) was measured, and the portion recycled was deducted. The wastewater volume was obtained by collecting and measuring the unrecovered wastewater from each step.

[0078] The specific test comparison results are shown in Table 2. Figure 2 , Figure 3 As shown:

[0079]

[0080] As can be seen from the above comparison results, in Example 4, the solution recovered in step S5 of Example 1 was used. Its degree of deacetylation and waste liquid discharge were not significantly different from those of Examples 1-3, achieving a closed-loop cycle of the main chemicals. In Comparative Example 1, the degree of deacetylation without a high-voltage pulse electric field was lower, indicating that insufficient pretreatment may affect the subsequent deacetylation efficiency, resulting in the need for more hydrochloric acid in the decalcification step and the generation of more waste acid liquid. In Comparative Example 2, the degree of deacetylation of protein deacetylation by the traditional alkaline method was the lowest, because the traditional alkaline method may destroy the chitin structure, resulting in insufficient deacetylation reaction, the use of a large amount of strong alkali, the generation of a large amount of waste alkaline liquid, and the difficulty of recovery. In Comparative Example 3, the degree of deacetylation of conventional heating deacetylation was lower than that of the examples, indicating that microwave assistance can promote the deacetylation reaction more efficiently. Conventional heating reaction time is longer, which may increase the amount of waste liquid generated, and the recovery efficiency may be slightly lower.

[0081] In summary, this invention significantly reduces wastewater discharge while increasing the degree of deacetylation through process integration and wastewater recycling, demonstrating the advantages of green environmental protection and comprehensive resource utilization.

Claims

1. A green method for preparing chitin with a high degree of deacetylation by recycling reaction waste liquid, characterized in that, The preparation method uses crustacean biomass as raw material, which undergoes pretreatment under a high-voltage pulsed electric field. Through the electroporation and perturbation effects of the high-voltage pulsed electric field, the crustacean structure is disrupted, achieving preliminary fragmentation and loosening of the raw material structure. Extraction is then performed by heating in a choline chloride-lactic acid eutectic solvent system. Microwave-assisted, synergistic phase-transfer catalyst action is employed to rapidly deacetylate chitin with alkaline solution. A waste liquid classification and recycling system is used: hydrochloric acid in the waste acid solution generated during decalcification is recovered via electrodialysis; alkaline solution is recovered using an ultrafiltration-nanofiltration dual-membrane combination; and eutectic solvent components are recovered via vacuum distillation. The preparation method includes the following specific steps: S1: The crustacean biomass waste is washed, dried and crushed, then added to deionized water to form a suspension slurry. High-voltage pulse is applied, with the peak pulse voltage set at 20-25 kV / cm, each pulse duration at 40-50 microseconds, pulse frequency at 100 Hz, and pulses repeated 10-15 times. Then, hydrochloric acid solution with half the volume of deionized water is added for decalcification. The reaction temperature does not exceed 35℃. After no obvious bubbles are generated, the mixture is allowed to stand, filtered to separate the solid, and the decalcified filtrate is collected. Then, it is washed and dried to prepare dry chitin crude material. S2: Choline chloride and lactic acid are placed in a beaker at a molar ratio of 1:1 to 1:

3. The mixture is heated and stirred in a water bath at 70℃ to 80℃. The mixture is then transferred to a three-necked flask, and the dry chitin crude material prepared in step S1 is added. The mixture is stirred and stirred at a solid-liquid mass ratio of 1:7 to 1:

10. The mixture is then heated in an oil bath at 100℃ to 120℃ for 1 to 2 hours. After cooling and dilution, the mixture is allowed to stand and filtered. The waste eutectic solvent solution is collected. The filter residue is washed with an aqueous ethanol solution and deionized water until neutral. The residue is then dried under vacuum to obtain pure chitin. S3: Mix the pure chitin prepared in step S2 with NaOH solution and add it to a container as the reaction liquid phase. Add toluene as the second phase, and then add 2.5% to 3.5% tetrabutylammonium bromide, which accounts for 2.5% to 3.5% of the pure chitin mass. Stir evenly and place it in a microwave synthesis device for deacetylation reaction. Set the microwave power to 600W, the reaction temperature to 90℃ to 120℃, the time to 8 to 12 minutes, and the stirring speed to 200rpm. Connect a reflux condenser to the top of the reactor. After the reaction is completed, cool and filter, collect the alkaline solution after the deacetylation reaction, wash the filter cake with deionized water until neutral, collect the washing liquid, and dry the washed solid to constant weight to prepare crude chitosan. S4: Mix the crude chitosan prepared in step S3 with acetic acid solution, stir and let stand to form a 0.6% w / v solution. Then add activated carbon fiber, stir and filter. Then filter the solution through a ceramic membrane filter to obtain a purified chitosan solution. Under magnetic stirring, add NaOH solution to neutralize and precipitate. Finally filter and wash the solid with deionized water. Dry the solid to constant weight to prepare high degree of deacetylation chitin. The waste liquid generated in the preparation process is recycled.

2. The green preparation method for high-deacetylation chitin by recycling reaction waste liquid as described in claim 1, characterized in that, The activated carbon fiber mentioned in step S4 is used in an amount that accounts for 18% to 22% of the mass of the added crude chitosan.

3. The green preparation method for high-degree-of-deacetylation chitin by recycling reaction waste liquid as described in claim 1, characterized in that, The neutralization precipitation described in step S4 is specifically performed as follows: NaOH solution is added at a rate of 5 mL / min, while the acidity of the solution is monitored with a pH meter. When the pH reaches 7.2, the addition is stopped, and the solution is allowed to stand for 30 minutes.

4. The green preparation method for high-deacetylation chitin by recycling reaction waste liquid as described in claim 1, characterized in that, The waste liquid recycling and treatment described in step S4 specifically includes: recycling decalcification waste acid, placing the decalcification filtrate waste collected in step S1 into the feed chamber of an electrodialysis device, and using a three-chamber electrodialysis device for electrolysis; recycling waste eutectic solvent, adding the waste eutectic solvent collected in step S2 to a rotary evaporator, distilling under reduced pressure, adding activated carbon powder and stirring, and filtering; and recycling alkali and washing waste liquid, separating the alkali and washing liquid after the deacetylation reaction in step S3 using a two-stage membrane separation process, successively using a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 3000 Da and a nanofiltration membrane with a molecular weight cutoff of 200 Da for concentration.

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