A carboxymethyl starch hydrogel adsorbent with high methylene blue adsorption efficiency and a preparation method thereof

Abstract

The application discloses a kind of carboxymethyl starch hydrogel adsorbents of high-efficiency methylene blue and preparation method thereof.Firstly, cassava starch is used as raw material, and after low-temperature gelatinization in alkaline urea solution, crosslinking is carried out using epichlorohydrin and freeze-thaw treatment is carried out, to build a stable three-dimensional network hydrogel skeleton;Subsequently, in ethanol aqueous solution system, the gel skeleton is subjected to alkalization and etherification reaction, and carboxymethyl groups are introduced in situ.The method adopts the innovative strategy of "gelation before carboxymethylation", and successfully solves the technical problem of high-substitution carboxymethyl starch easy to dissolve by the space confinement effect of crosslinked network, and the prepared adsorbent has a substitution degree of 1.43 and is not soluble in water, and has excellent anti-swelling and mechanical strength.The adsorbent has high methylene blue adsorption capacity, fast adsorption rate, and stable performance in the pH range of 4-11.The process is simple, low in cost and easy to scale up, and has wide application prospect in water treatment field.

Description

Technical Field

[0001] This invention relates to the field of bio-based material preparation technology, specifically to a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue and its preparation method. Background Technology

[0002] With the rapid development of the global dyeing and printing industry, the problem of wastewater discharge containing cationic dyes is becoming increasingly serious. Cationic dyes (such as methylene blue and crystal violet) pose a serious threat to aquatic ecosystems and human health due to their high color intensity, biotoxicity, and recalcitrant nature. Adsorption technology, due to its ease of operation, controllable cost, and high efficiency and selectivity, has become a research hotspot in the field of wastewater treatment. In recent years, the research and development of adsorption materials has gradually shifted towards high efficiency and renewability, with continuous deepening of related process optimization and mechanism exploration. Natural polymer materials, with their renewable, biodegradable, and environmentally friendly characteristics, have become a new direction for adsorption material research. Among them, starch-based adsorbents have attracted much attention due to their wide availability and easy modification of molecular structures, while cassava starch, due to its unique physicochemical properties, has become an ideal substrate for modification research.

[0003] Cassava starch is a natural polysaccharide extracted from cassava tubers. Its molecular chain consists of amylose (20%–30%) and amylopectin (70%–80%). Compared to corn and potato starch, cassava starch has a lower amylose content and more branched molecular chains, resulting in higher gelatinization viscosity and better potential for chemical modification. In recent years, research on starch modification has gradually shifted from single chemical modification to multi-step synergistic modification. This involves enhancing reactivity through physical pretreatment, improving stability through chemical cross-linking, and optimizing the distribution of functional groups through enzymatic hydrolysis, forming a multi-dimensional performance regulation strategy. The numerous hydroxyl groups (-OH) on its molecular chain can introduce functional groups through reactions such as etherification, esterification, and cross-linking, thereby regulating the adsorption performance of the material. However, natural cassava starch is difficult to use directly as an adsorbent due to its high water solubility, low mechanical strength, and tendency to swell and disintegrate. Therefore, researchers often improve its stability through chemical modification or composite cross-linking. Currently, the green transformation of modification processes has become an important trend, such as developing solvent-free reaction systems, low-temperature energy-saving processes, and bio-based crosslinking agents to reduce environmental impact and production costs. Among these, carboxymethylation modification has become an important direction for the functionalization of cassava starch due to its simple operation, mild reaction conditions, and ability to significantly enhance the charge properties of materials.

[0004] Carboxymethyl starch (CMS) is an etherified derivative prepared by a nucleophilic substitution reaction of starch and chloroacetic acid under alkaline conditions. During the reaction, the hydroxyl groups on the starch molecular chain are replaced by carboxymethyl groups (-CH₂COO⁻), imparting a strong negative charge to the material, enabling it to efficiently adsorb cationic dyes via electrostatic interactions. However, highly substituted CMS is too water-soluble, making solid-liquid separation after adsorption difficult through simple filtration or centrifugation, requiring additional cross-linking or compounding with other polymers to improve its stability. This process not only increases the complexity of the material but may also lead to low cross-linking efficiency, reduced environmental friendliness, and decreased adsorption performance. Furthermore, the strongly negatively charged carboxymethyl groups on CMS reduce its reactivity, making further modification of highly substituted CMS to enhance its adsorption performance difficult.

[0005] Therefore, to address the aforementioned problems, this invention provides a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue and its preparation method. Through an innovative synthetic route of "first constructing a cross-linked network, then performing carboxymethylation," the invention aims to simultaneously achieve high substitution degree, water insolubility, and three-dimensional network stability in the material. This strategy utilizes the spatial confinement effect of the hydrogel network, restricting the carboxymethylation reaction within a fixed grid. This effectively introduces negatively charged groups while fundamentally avoiding dissolution problems caused by excessive hydrophilicity, ultimately yielding an adsorbent material with both high adsorption capacity and excellent recyclability. Summary of the Invention

[0006] The purpose of this invention is to provide a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue and its preparation method, aiming to simultaneously achieve high substitution degree, water insolubility and three-dimensional network stability of the material.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue includes the following steps:

[0009] S1. Prepare an aqueous solution containing sodium hydroxide and urea, mix it with cassava starch to form cassava starch milk, place the cassava starch milk in an ice water bath and stir at low temperature to form a gelatinized solution, and obtain cassava starch hydrogel.

[0010] S2. Continuously stir the cassava starch hydrogel, add epichlorohydrin to it to carry out cross-linking reaction, and then freeze-thaw the reaction system to obtain cassava starch hydrogel. Then wash the gel with distilled water, break the gel into granules, and freeze-dry it.

[0011] S3. Add the freeze-dried cassava starch hydrogel to an ethanol aqueous solution and add sodium hydroxide to carry out an alkalization reaction.

[0012] S4. Add an ethanol-water solution of dissolved chloroacetic acid and sodium hydroxide to the alkalized reaction system to carry out an etherification reaction. After the reaction is completed, the carboxymethyl starch hydrogel is obtained through post-treatment.

[0013] Preferably, in step S1, the concentration of the cassava starch milk is 5 wt%; the concentration of sodium hydroxide in the gelatinization solution is 4 wt% and the concentration of urea is 14 wt%; the temperature of the gelatinization solution is 0°C, the stirring speed is 200 rpm, and the stirring time is 5-10 min.

[0014] Preferably, in step S2, the epichlorohydrin is slowly and uniformly added to the cassava starch hydrogel over 30 minutes; the freeze-thaw treatment conditions are as follows: the gel freezing temperature is -30°C, and the gel freezing time is 24 hours; the gel melting temperature is 25°C, and the gel melting time is 1 to 1.5 hours; the stirring speed of the cassava starch hydrogel is 200 rpm, and the stirring time is 30 to 40 minutes; the stirring speed for the crosslinking reaction is 200 rpm, and the crosslinking reaction time is 1 to 1.5 hours.

[0015] Preferably, in step S2, the number of washing cycles is 5 to 8; the particle size of the gel particles that are broken into granules is 0.4 to 0.8 mm; the freeze-drying temperature is -95°C; and the freeze-drying time is 72 h.

[0016] Preferably, in step S3, the amount of freeze-dried cassava starch hydrogel added is 3-7 wt%; the concentration of the ethanol aqueous solution is 95%; the heating temperature for the alkalization reaction is 40°C; the stirring speed for the alkalization reaction is 200 rpm; and the alkalization reaction time is 2-12 h.

[0017] Preferably, in step S4, the concentration of the ethanol aqueous solution for dissolving chloroacetic acid is 95%; the heating temperature for the etherification reaction is 40°C, the stirring speed for the etherification reaction is 200 rpm, and the etherification reaction time is 2~16 h.

[0018] Preferably, the molar amounts of sodium hydroxide added in steps S3 and S4 are the same, and the ratio of the total molar amount of chloroacetic acid to the total molar amount of sodium hydroxide added in steps S3 and S4 is 1:1.5~2.5.

[0019] Preferably, in step S4, the molar ratio of sugar units in the starch hydrogel during the etherification reaction to chloroacetic acid is 1:14~24.

[0020] Preferably, in step S4, the post-processing includes: adding glacial acetic acid to the product after the etherification reaction to adjust the pH value to 6.5-7, washing and filtering with 90% ethanol aqueous solution 5-8 times, and then freeze-drying at a temperature of -95°C for 72 hours.

[0021] This application also claims a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue prepared by the above-mentioned method.

[0022] The working mechanism of this invention lies in the synthetic strategy of "gelation first, followed by carboxymethylation". This strategy first constructs a stable three-dimensional network framework through epichlorohydrin crosslinking. This framework acts as a solid-phase microreactor, and through its spatial confinement effect, it effectively restricts the excessive movement and free extension of starch molecular chains during the subsequent carboxymethylation process, fundamentally avoiding the dissolution problem caused by high degree of substitution and hydrophilicity. At the same time, this pre-designed rigid network allows the starch chains to fully extend, ensuring that the etherification reaction can proceed uniformly and efficiently, thereby achieving a high degree of substitution. Finally, the crosslinked network remains intact throughout the reaction, and the large number of carboxyl groups introduced synergistically endow the hydrogel product with excellent three-dimensional stability, anti-swelling properties, and efficient electrostatic adsorption capacity for cationic dyes.

[0023] Due to the application of the above technical solution, the present invention has the following beneficial effects compared with the prior art:

[0024] 1. This invention adopts a synthesis strategy of "gelation followed by carboxymethylation", which successfully solves the technical paradox that high-substituted carboxymethyl starch must dissolve. The prepared hydrogel adsorbent can maintain a complete three-dimensional network structure while having a substitution degree as high as 1.43. It is insoluble in water and has excellent anti-swelling and mechanical strength. This characteristic ensures that the adsorbent is structurally stable during use and is easy to separate and recover from the treated wastewater, solving the industry pain point of "easy to use but difficult to recover" of high-performance adsorbent materials.

[0025] 2. The entire preparation process of this invention involves only simple unit operations such as constant temperature stirring, washing and freeze drying, without the need for complex or expensive special equipment. The reaction conditions are mild and the requirements for production facilities are low. This simplified process is not only easy to implement and scale up, but also significantly reduces the consumption of raw materials and energy, keeping the preparation cost of the final product at a low level, laying a solid foundation for large-scale industrial applications.

[0026] 3. The product of this invention exhibits excellent comprehensive adsorption performance. The hydrogel adsorbent prepared by this method shows outstanding characteristics such as fast adsorption rate (adsorption equilibrium can be reached in 60 minutes) and huge adsorption capacity (maximum adsorption capacity up to 1359 mg / g) when facing the cationic dye methylene blue. Moreover, it can maintain high adsorption efficiency in a wide pH range (pH=4-11), has strong environmental adaptability, and does not require precise adjustment of the influent pH value, which greatly simplifies the operation of actual water treatment process and has broad application prospects in water treatment and other fields.

[0027] 4. The preparation method of the present invention is not only applicable to cassava starch, but its core "first cross-linking and then etherification" strategy can also be extended to other types of starch such as corn and potato, demonstrating strong raw material adaptability and technology transfer capability. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, some of the drawings in the following description are some embodiments of the present invention. For those skilled in the art, other drawings can be made based on these drawings without creative effort.

[0029] Figure 1 These are scanning electron microscope (SEM) images of the materials prepared in Example 5 of the present invention and the original cassava starch; wherein, Figure (a) is a scanning electron microscope image of the original cassava starch magnified 200X; Figure (b) is a scanning electron microscope image of the cassava starch hydrogel magnified 200X; Figure (c) is a scanning electron microscope image of the carboxymethyl starch hydrogel magnified 200X.

[0030] Figure 2 These are the infrared spectra and X-ray diffraction patterns of the materials prepared in Example 5 of the present invention and the original cassava starch; wherein, Figure (a) is the infrared spectrum of the original cassava starch and the materials prepared in Example 5; Figure (b) is the X-ray diffraction pattern of the original cassava starch and the materials prepared in Example 5;

[0031] Figure 3 Figure 5 shows the adsorption capacity of the material prepared in Example 5 of this invention in methylene blue solutions at different pH values, as well as the pseudo-first-order kinetic adsorption fitting curves and pseudo-second-order kinetic adsorption fitting curves in methylene blue solutions of different concentrations. Figure (a) shows the adsorption capacity of the material prepared in Example 5 in methylene blue solutions (concentration of 500 mg / L) at different pH values. Figure (b) shows the pseudo-first-order kinetic adsorption fitting curves and pseudo-second-order kinetic adsorption fitting curves of the material prepared in Example 5 in a methylene blue solution with a concentration of 500 mg / L. Detailed Implementation

[0032] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific implementation schemes are now described in detail.

[0033] The present invention will be further described below with reference to embodiments, but the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0034] Example 1

[0035] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0036] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of cassava starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform cassava starch hydrogel.

[0037] S2. Continuously stir the cassava starch hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min, then continue stirring for 60 min. After stirring, place the beaker containing the cassava starch hydrogel in a freezer at -30℃ for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. After the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95℃ for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4~0.8 mm.

[0038] S3. Add 5g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 17.27g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 8h.

[0039] S4. Dissolve 40.81g of chloroacetic acid in 45mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 17.27g of NaOH and continue stirring at 200rpm in a water bath at 40℃ for 12h. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 0.85.

[0040] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 896.4 mg / g.

[0041] Example 2

[0042] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0043] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0044] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min, then continue stirring for 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. After the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0045] S3. Add 5g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 27.14g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 8h.

[0046] S4. Dissolve 64.13g of chloroacetic acid in 45mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 27.14g of NaOH and continue stirring at 200rpm in a water bath at 40℃ for 12h. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.02.

[0047] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1182 mg / g.

[0048] Example 3

[0049] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0050] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0051] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Continue stirring for another 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. Once the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0052] S3. Add 5g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 27.14g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 12h.

[0053] S4. Dissolve 64.13g of chloroacetic acid in 45mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 27.14g of NaOH and continue stirring at 200rpm in a water bath at 40℃ for 12h. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.38.

[0054] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1188 mg / g.

[0055] Example 4

[0056] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0057] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0058] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Continue stirring for another 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. Once the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0059] S3. Add 5g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 27.14g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 6h.

[0060] S4. Dissolve 64.13g of chloroacetic acid in 45mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 27.14g of NaOH and continue stirring at 200rpm in a water bath at 40℃ for 12h. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.39.

[0061] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1220 mg / g.

[0062] Example 5

[0063] See appendix Figure 1 ~Appendix Figure 3 This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0064] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0065] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Continue stirring for another 60 min. After stirring, place the beaker containing the hydrogel in a -30°C freezer for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. Once the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0066] S3. Add 7g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 37.96g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 6h.

[0067] S4. Dissolve 89.78 g of chloroacetic acid in 45 mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 37.96 g of NaOH and continue stirring at 200 rpm for 12 h in a water bath at 40 °C. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95 °C to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.43.

[0068] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1359 mg / g.

[0069] Figure 1 These are scanning electron microscope (SEM) images of the original starch of the present invention and the material prepared in Example 5;

[0070] Figure (a) is a scanning electron microscope image of raw cassava starch magnified 200X; Figure (b) is a scanning electron microscope image of cassava starch hydrogel magnified 200X; Figure (c) is a scanning electron microscope image of carboxymethyl starch hydrogel magnified 200X.

[0071] As can be seen from the scanning electron microscope images, the original cassava starch consists of dense and smooth spherical or hemispherical particles. After gelation, the starch hydrogel exhibits a sheet-like structure under the electron microscope. The carboxymethylation process does not significantly change the microstructure of the hydrogel, and the carboxymethyl starch hydrogel also exhibits a sheet-like structure similar to that of the starch hydrogel.

[0072] Figure 2 The infrared spectrum and X-ray diffraction pattern of the original starch of the present invention and the material prepared in Example 5 are shown below.

[0073] Figure (a) shows the infrared spectrum of the original cassava starch and the material prepared in Example 5;

[0074] The infrared spectra of native starch and starch hydrogel can be seen from the figure. Figure 1 In conclusion, the chemical structure of starch was not altered during the gelation process, while the infrared spectrum of carboxymethyl starch hydrogel showed no change at 1600 cm⁻¹. -1 An absorption peak at 1425 cm⁻¹, attributed to the antisymmetric stretching vibration of the carboxyl group, was observed. -1 An absorption peak at 1317 cm⁻¹, attributed to the stretching vibration of the methylene group, was observed. -1 The presence of a symmetric stretching vibration absorption peak of the carboxyl group indicates that the carboxyl group was successfully grafted onto the starch chain during the carboxymethylation process.

[0075] Figure (b) shows the X-ray diffraction patterns of the original cassava starch and the material prepared in Example 5;

[0076] The XRD pattern shows that the gelation process completely destroys the crystalline regions in the starch granules, and the starch hydrogel is composed entirely of amorphous regions without any crystalline regions. Similarly, the carboxymethyl starch hydrogel is also composed entirely of amorphous regions without any crystalline regions.

[0077] like Figure 3 The figure shows the adsorption capacity of the material prepared in Example 5 of this invention in methylene blue solutions at different pH values, as well as the pseudo-first-order kinetic adsorption fitting curves and pseudo-second-order kinetic adsorption fitting curves in a 500 mg / L methylene blue solution.

[0078] Figure (a) shows the change in adsorption capacity of the material prepared in Example 5 in methylene blue solutions (concentration of 500 mg / L) at different pH values;

[0079] As can be seen from the figure, carboxymethyl starch hydrogels exhibit good adsorption effects at pH values ​​of 4 to 11, with adsorption capacities exceeding 950 mg / g and removal rates exceeding 95%, reaching a maximum of 97.6%.

[0080] Figure (b) shows the pseudo-first-order kinetic adsorption fitting curves and pseudo-second-order kinetic adsorption fitting curves of the material prepared in Example 5 in a 500 mg / L methylene blue solution.

[0081] The correlation coefficient R of the pseudo-first-order kinetic adsorption fitting curve 2 =0.9743, correlation coefficient R of the pseudo-second-order kinetic adsorption fitting curve 2 =0.9966;

[0082] Example 6

[0083] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0084] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0085] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Continue stirring for another 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. Once the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0086] S3. Add 5g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 27.14g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 6h.

[0087] S4. Dissolve 64.13g of chloroacetic acid in 45mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 27.14g of NaOH and continue stirring at 200rpm for 2h in a water bath at 40℃. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.41.

[0088] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1196 mg / g.

[0089] Example 7

[0090] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0091] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0092] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Continue stirring for another 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. Once the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0093] S3. Add 5g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 27.14g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 6h.

[0094] S4. Dissolve 64.13g of chloroacetic acid in 45mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 27.14g of NaOH and continue stirring at 200rpm in a water bath at 40℃ for 16h. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.22.

[0095] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1301 mg / g.

[0096] Example 8

[0097] This embodiment provides a method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, comprising the following steps:

[0098] S1. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of tapioca starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0099] S2. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Continue stirring for another 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and allow it to thaw at room temperature. Once the gel is completely thawed, soak it in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, pulverize the gel and freeze-dry it at -95°C for 72 h. Finally, sieve it using a 20-mesh standard sieve to obtain cassava starch hydrogel particles with a particle size of 0.4-0.8 mm.

[0100] S3. Add 3g of tapioca starch hydrogel particles to a three-necked flask, add 100mL of 95% ethanol aqueous solution and 16.27g of NaOH, place the three-necked flask in a 40℃ water bath, and stir with a stirrer at 200rpm for 6h.

[0101] S4. Dissolve 38.48 g of chloroacetic acid in 45 mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 16.27 g of NaOH and continue stirring at 200 rpm for 12 h in a water bath at 40 °C. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95 °C to finally obtain carboxymethyl starch hydrogel particles with a degree of substitution of 1.10.

[0102] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 1127 mg / g.

[0103] Comparative Example 1

[0104] This comparative example uses the traditional "carboxymethylation followed by gelation" synthesis method, which includes the following steps:

[0105] S1. Add 30g of tapioca starch, 100mL of 95% ethanol aqueous solution and 15g of NaOH to a three-necked flask. Place the three-necked flask in a 40℃ water bath and stir with a stirrer at 200rpm for 1h.

[0106] S2. Dissolve 35g of chloroacetic acid in 25mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 15g of NaOH and continue stirring at 200rpm for 1.5h in a water bath at 40℃. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch with a degree of substitution of 0.52.

[0107] S3. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of carboxymethyl starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0108] S4. The hydrogel was continuously stirred in an ice-water bath, and 15 mL of epichlorohydrin was slowly and uniformly added dropwise over 30 min. Stirring was continued for another 60 min. After stirring, the beaker containing the hydrogel was placed in a freezer at -30°C for 24 h. After freezing, the beaker was removed and allowed to thaw at room temperature. After the gel was completely thawed, it was soaked in deionized water to remove urea, sodium hydroxide, and unreacted epichlorohydrin. Then, it was freeze-dried at -95°C for 72 h to obtain carboxymethyl starch hydrogel.

[0109] In the adsorption performance test, a 1500 mg / L methylene blue solution was used. 50 mg of carboxymethyl starch hydrogel particles were weighed and added to 100 mL of 1500 mg / L methylene blue solution. The mixture was shaken and stirred at a shaking rate of 150 rpm and a temperature of 15 °C. After shaking for 240 min, the adsorption capacity of the carboxymethyl starch hydrogel particles was 608.78 mg / g.

[0110] Comparative Example 2

[0111] This comparative example uses the traditional "carboxymethylation followed by gelation" synthesis method, which includes the following steps:

[0112] S1. Add 30g of tapioca starch, 100mL of 95% ethanol aqueous solution and 15g of NaOH to a three-necked flask. Place the three-necked flask in a 40℃ water bath and stir with a stirrer at 200rpm for 10h.

[0113] S2. Dissolve 35g of chloroacetic acid in 25mL of 95% ethanol aqueous solution, and add the solution to a three-necked flask. Then add 15g of NaOH and continue stirring at 200rpm in a water bath at 40℃ for 14h. After that, add glacial acetic acid dropwise to the reaction system to make the pH reach 6.5 to end the reaction. Then wash the hydrogel particles several times with 90% ethanol aqueous solution. Then freeze-dry them in a freeze dryer at -95℃ to finally obtain carboxymethyl starch with a degree of substitution of 1.03.

[0114] S3. Add 56g of urea and 16g of sodium hydroxide to 328g of deionized water, stir to dissolve and prepare a solution. Add 20g of carboxymethyl starch to the solution and stir continuously for 5 minutes until the starch is fully gelatinized in the solution to form a uniform hydrogel.

[0115] S4. Continuously stir the hydrogel in an ice-water bath, and slowly and uniformly add 15 mL of epichlorohydrin over 30 min. Then continue stirring for 60 min. After stirring, place the beaker containing the hydrogel in a freezer at -30°C for 24 h. After freezing, remove the beaker and let it thaw at room temperature. If the gel does not form after complete thawing, carboxymethyl starch hydrogel cannot be obtained.

[0116] When the degree of substitution of carboxymethyl starch is less than or equal to 0.52, carboxymethyl starch can be successfully gelled. However, as the degree of substitution gradually increases, the reactivity of carboxymethyl starch gradually decreases, and it cannot successfully form a hydrogel. Therefore, the maximum degree of substitution of carboxymethyl starch hydrogel synthesized using the traditional "carboxymethylation first, then gelation" synthesis strategy is 0.52, and the maximum methylene blue adsorption capacity is 608.78 mg / g, which is significantly lower than the high degree of substitution of 1.43 and adsorption capacity of 1359 mg / g in Example 5.

[0117] The adsorption kinetic parameters of methylene blue dye in Example 5 above are shown in Table 1.

[0118] Table 1

[0119]

[0120] As can be seen from the table above, the pseudo-second-order kinetic equation has a higher fitting degree, so the adsorption of carboxymethyl starch hydrogel can be described by pseudo-second-order kinetics.

[0121] In summary, this invention employs an innovative strategy of "gelation followed by carboxymethylation," successfully overcoming the technical challenge of achieving both high adsorption capacity and structural stability in highly substituted carboxymethyl starch. The resulting hydrogel adsorbent remains insoluble in water even at a substitution degree as high as 1.43, exhibiting excellent anti-swelling properties and mechanical strength, thus solving the industry pain point of "easy to use but difficult to recover" in high-performance adsorbent materials. This preparation process involves only simple operations such as constant-temperature stirring, washing, and freeze-drying, with mild conditions, low equipment requirements, and economical costs, making it highly potential for large-scale application. The resulting product exhibits excellent adsorption performance for methylene blue, with not only a fast adsorption rate (reaching equilibrium in 60 minutes) and high capacity (1359 mg / g), but also maintaining high efficiency over a wide pH range of 4-11, demonstrating strong environmental adaptability. Furthermore, this technology has good versatility; its core process can be extended to various starches such as corn and potato, providing a new technical path for the high-value development of natural polymer materials and the field of water treatment.

[0122] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, characterized in that, Includes the following steps: S1. Prepare an aqueous solution containing sodium hydroxide and urea, mix it with cassava starch to form cassava starch milk, place the cassava starch milk in an ice water bath and stir at low temperature to form a gelatinized solution, and obtain cassava starch hydrogel. S2. Continuously stir the cassava starch hydrogel, add epichlorohydrin to it to carry out cross-linking reaction, and then freeze-thaw the reaction system to obtain cassava starch hydrogel. Then wash the gel with distilled water, break the gel into granules, and freeze-dry it. S3. Add the freeze-dried cassava starch hydrogel to an ethanol aqueous solution and add sodium hydroxide to carry out an alkalization reaction. S4. Add an ethanol-water solution of dissolved chloroacetic acid and sodium hydroxide to the alkalized reaction system to carry out an etherification reaction. After the reaction is completed, the carboxymethyl starch hydrogel is obtained through post-treatment.

2. The method for preparing the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S1, the concentration of the cassava starch milk is 5 wt%; the concentration of sodium hydroxide in the gelatinization solution is 4 wt% and the concentration of urea is 14 wt%; the temperature of the gelatinization solution is 0°C, the stirring speed is 200 rpm, and the stirring time is 5-10 min.

3. The preparation method of the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S2, epichlorohydrin is slowly and uniformly added to the cassava starch hydrogel over 30 minutes; the freeze-thaw treatment conditions are as follows: the gel freezing temperature is -30℃, the gel freezing time is 24h; the gel melting temperature is 25℃, the gel melting time is 1~1.5h; the stirring speed of the cassava starch hydrogel is 200rpm, and the stirring time is 30~40min; the stirring speed of the crosslinking reaction is 200rpm, and the crosslinking reaction time is 1~1.5h.

4. The preparation method of the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S2, the number of washing cycles is 5 to 8; the particle size of the gel particles broken into granules is 0.4 to 0.8 mm; the freeze-drying temperature is -95°C; and the freeze-drying time is 72 hours.

5. The preparation method of the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S3, the amount of freeze-dried cassava starch hydrogel added is 3~7wt%; the concentration of the ethanol aqueous solution is 95%; the heating temperature for the alkalization reaction is 40℃, the stirring speed for the alkalization reaction is 200rpm, and the alkalization reaction time is 2~12h.

6. The preparation method of the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S4, the concentration of the ethanol aqueous solution for dissolving chloroacetic acid is 95%; the heating temperature for the etherification reaction is 40°C, the stirring speed for the etherification reaction is 200 rpm, and the etherification reaction time is 2~16 h.

7. The preparation method of the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, The molar amounts of sodium hydroxide added in steps S3 and S4 are the same, and the ratio of the total molar amount of chloroacetic acid to the total molar amount of sodium hydroxide added in steps S3 and S4 is 1:1.5~2.

5.

8. The method for preparing the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S4, the molar ratio of sugar units in the starch hydrogel during the etherification reaction to chloroacetic acid is 1:14~24.

9. The preparation method of the highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue according to claim 1, characterized in that, In step S4, the post-processing includes: adding glacial acetic acid to the product after the etherification reaction to adjust the pH value to 6.5-7, washing and filtering with 90% ethanol aqueous solution 5-8 times, and then freeze-drying at a temperature of -95°C for 72 hours.

10. A highly efficient carboxymethyl starch hydrogel adsorbent for adsorbing methylene blue, prepared by the method described in any one of claims 1 to 9.

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