A cross-linked carboxymethyl corn starch adsorbent and a preparation method and application thereof

Abstract

This invention discloses a cross-linked carboxymethyl corn starch adsorbent, its preparation method, and its application. The method involves first mixing corn starch with 85%-95% ethanol (by mass) and stirring until homogeneous; adjusting the pH of the starch emulsion to 10-12, preheating at 50-60°C, adding 0.1%-0.15% epichlorohydrin (by dry starch weight), and continuously stirring for 1-2 hours; after cooling to room temperature, adding solid sodium hydroxide for alkalization, adding chloroacetic acid to the alkalized system, and reacting at 50-60°C for 2-4 hours; after the reaction, adjusting the pH of the reaction system to 6-7, filtering, washing, drying, and sieving to obtain the cross-linked carboxymethyl corn starch adsorbent. This invention overcomes the defects of poor structural stability and low adsorption capacity of unmodified starch. The resulting adsorbent is environmentally friendly, has excellent adsorption performance, and the preparation process is simple, controllable, and low-cost.

Description

Technical Field

[0001] This invention relates to the field of starch modification and adsorption materials technology, and in particular to a cross-linked carboxymethyl corn starch adsorbent, its preparation method and application. Background Technology

[0002] In recent years, the rapid advancement of industrialization in my country has led to increasingly severe heavy metal wastewater pollution, which has become one of the key bottlenecks restricting the sustainable development of the ecological environment. Heavy metal ions such as copper, lead, cadmium, and mercury contained in this type of wastewater are highly toxic, difficult to degrade, and prone to bioaccumulation. Once they seep into water bodies or soil, they not only disrupt soil fertility and the ecological balance of aquatic bodies, but also accumulate through the food chain, damaging vital organs such as the liver and kidneys, posing a serious and lasting threat to public health and safety. However, current mainstream heavy metal adsorption treatment methods have many limitations. Traditional adsorption materials, such as synthetic cation exchange resins and activated carbon, while possessing certain adsorption efficiencies, suffer from non-biodegradability and high cost. Long-term use can easily lead to secondary environmental burdens and resource waste. Therefore, the development of environmentally friendly heavy metal adsorption materials has become an urgent need that combines environmental and economic benefits.

[0003] Chinese invention patent CN105642244B discloses a method for preparing and applying a cross-linked-enzymatic hydrolysis composite ultra-micro modified starch adsorbent. The method involves ultra-micro pulverizing corn, sweet potato, and potato starches separately, then mixing them in a mass ratio of 1:4:3, adding water and stirring to obtain a starch emulsion. Sodium tripolyphosphate is then added for cross-linking, followed by washing with deionized water and freeze-drying. The cross-linked starch is then mixed with water again to form a starch emulsion, and α-amylase and isoamylase are added. After centrifugation, washing, drying, and sieving, the cross-linked-enzymatic hydrolysis composite ultra-micro modified starch is obtained. While this technology significantly increases the specific surface area and mechanical strength of starch and improves its adsorption capacity for heavy metals, the preparation of the material through multiple steps such as ultra-micro pulverization, mixing of various starches, cross-linking treatment, freeze-drying, and enzymatic hydrolysis is cumbersome, costly, and energy-intensive. Furthermore, relying solely on physical pore formation for adsorption results in uncontrollable pore structure. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for preparing cross-linked carboxymethyl corn starch adsorbent that is inexpensive, environmentally friendly, simple in process, and has good adsorption performance.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for preparing a cross-linked carboxymethyl corn starch adsorbent includes the following steps:

[0007] 1) Mix corn starch with 85%-95% ethanol by mass and stir until homogeneous to obtain a starch emulsion;

[0008] 2) Adjust the pH of the starch emulsion to 10-12, preheat it at 50-60 ℃, add 0.1%-0.15% epichlorohydrin by weight of dry starch, and stir continuously for 1-2 h; the epichlorohydrin is added after being dispersed in 85%-95% ethanol by weight.

[0009] 3) After the obtained system is cooled to room temperature, sodium hydroxide solid is added for alkalization. Chloroacetic acid is added to the alkalized system and reacted at 50-60 °C for 2-4 h. After the reaction is completed, the pH value of the reaction system is adjusted to 6-7, filtered, washed, dried and sieved to obtain cross-linked carboxymethyl corn starch adsorbent. The molar ratio of chloroacetic acid to native starch is controlled at (0.1-0.3):1. The chloroacetic acid is added after being dispersed in 85%-95% ethanol by mass.

[0010] To further achieve the purpose of this invention, preferably, the starch milk in step 1) has a mass content of 10%-30%; the corn starch is high amylose corn starch, ordinary corn starch or waxy corn starch.

[0011] Preferably, the adjustment of the pH value of the starch emulsion to 10-12 in step 2) is done using sodium hydroxide solution; the preheating at 50-60 ℃ is achieved by placing the starch emulsion in a constant temperature water bath at 50-60 ℃.

[0012] Preferably, the sodium hydroxide solution has a mass fraction of 3%-7%.

[0013] Preferably, the molar ratio of sodium hydroxide solid to original starch in step 3) is 3:2-5:2.

[0014] Preferably, in step 3), the pH of the reaction system is adjusted to 6-7 after the reaction is completed by adjusting with a 1-3 M glacial acetic acid-ethanol solution.

[0015] Preferably, the washing in step 3) is performed using an ethanol solution with a mass fraction of 70%-90%.

[0016] Preferably, the drying in step 3) is performed in an oven at 50-60 ℃.

[0017] A cross-linked carboxymethyl corn starch adsorbent is prepared by the above-described preparation method.

[0018] The cross-linked carboxymethyl corn starch adsorbent described above is used in the treatment of industrial wastewater containing copper heavy metals.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1) This invention uses corn starch as the core raw material. Compared with the artificially synthesized cation exchange resins, activated carbon and other materials relied upon in the prior art, corn starch is widely available, inexpensive and has the characteristics of being natural, non-toxic and completely biodegradable. It avoids the problems of traditional adsorption materials being difficult to degrade after use, causing secondary pollution and wasting resources from the source, and has both environmental and economic benefits.

[0021] 2) Compared to unmodified corn starch and single-modified starch, this invention achieves a significant improvement in adsorption performance through a cross-linked carboxymethyl composite modification process. On the one hand, the carboxymethylation reaction introduces a large number of carboxyl anionic active groups into the starch molecular chain, enhancing the adsorption capacity for heavy metal ions such as copper, lead, cadmium, and mercury through ion exchange and complexation, which can meet the actual efficiency requirements of heavy metal wastewater treatment and can replace traditional synthetic adsorption materials. On the other hand, the cross-linking reaction utilizes epichlorohydrin to construct a three-dimensional network covalent structure between starch molecules, greatly improving the mechanical strength and structural stability of the adsorbent, ensuring that it is not easily collapsed or dissolved in the wastewater treatment system, and can stably exert its adsorption effect. Moreover, the cross-linking produces a steric hindrance effect, increasing the specific surface area and the number of adsorption sites, assisting the carboxymethyl group to capture ions more efficiently.

[0022] 3) The preparation method of this invention is highly controllable and easier to industrialize; the raw materials used in the preparation method of this invention are all conventional chemical raw materials, which are easy to obtain and the amount can be controlled, and there is no need for expensive special reagents; the entire preparation process is simple, the conditions are mild, the subsequent washing, drying and other steps are convenient to operate, the energy consumption is low, and the overall preparation cost is significantly lower than the traditional synthetic adsorbent materials and complex modified starch preparation processes, which has obvious cost advantages. Attached Figure Description

[0023] Figure 1 Fourier transform infrared spectra of Examples 1-3, Comparative Example 1, and blank starch are shown.

[0024] Figure 2 The images are scanning electron microscope (SEM) images of Examples 1-3, Comparative Example 1, and blank starch. Detailed Implementation

[0025] To better understand the present invention, it will be further described below with reference to the accompanying drawings and specific embodiments. However, the implementation of the present invention is not limited thereto. The described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] To address the problems of cation exchange resins used in the treatment of copper-containing heavy metal wastewater, such as non-biodegradability, high cost, and the potential for secondary environmental burden and resource waste; and to address the issues of existing starch adsorbent preparation processes requiring specific starch blends, cross-linking, freeze-drying, enzymatic hydrolysis, and even ultrasonic, electric field, or ultrafine grinding treatments, resulting in high costs, numerous steps, and difficulties in industrialization, the present invention aims to provide a cross-linked carboxymethyl corn starch adsorbent and its preparation method that uses ordinary starch as raw material, requires no specific enzyme treatment, and requires no special processes.

[0027] The raw material for the preparation method of this invention is corn starch. Among many resources, natural corn starch has outstanding advantages such as low price, non-toxicity, and excellent biodegradability. However, unmodified corn starch has defects such as poor structural stability, low adsorption capacity, and strong water solubility, and is easily dissolved and lost during adsorption, making it difficult to meet the actual needs of wastewater treatment. The corn starch of this invention is preferably high amylose corn starch, ordinary corn starch, or waxy corn starch; these three types of corn starch are classified according to their amylose content; wherein the amylose content of high amylose corn starch is 55%-65%, the amylose content of ordinary corn starch is 30%-40%, and the amylose content of waxy corn starch is 15%-25%.

[0028] The preparation method of this invention embodies the cross-linking carboxymethyl composite modification of corn starch. The cross-linking method using epichlorohydrin assists the carboxymethylation modification using chloroacetic acid. The cross-linking reaction utilizes epichlorohydrin to construct a three-dimensional network covalent structure between starch molecules, which significantly improves the mechanical strength and structural stability of the adsorbent, ensuring that it is not easily collapsed or dissolved in the wastewater treatment system and can stably exert its adsorption effect. Moreover, the cross-linking produces a steric hindrance effect, increasing the specific surface area and the number of adsorption sites, especially assisting carboxymethyl groups to capture metal ions more efficiently. The carboxymethylation reaction introduces a large number of carboxyl anionic active groups into the starch molecular chain, enhancing the adsorption capacity for heavy metal ions, represented by ions, through ion exchange and complexation, thus meeting the actual efficiency requirements of heavy metal wastewater treatment.

[0029] In the preparation method of this invention, the starch emulsion preparation, cross-linking reaction, and carboxymethylation reaction are all carried out in an 85%-95% ethanol aqueous solution. This specific concentration of ethanol aqueous solution can prevent starch from gelatinizing during the reaction, maintain the particle morphology, and ensure that cross-linking and carboxymethylation proceed uniformly. However, this invention found that an ethanol concentration below 85% easily leads to starch gelatinization, agglomeration, and uncontrolled reaction; while a concentration above 95% results in decreased reagent dispersibility and incomplete reaction.

[0030] Therefore, the present invention provides a method for preparing a cross-linked carboxymethyl corn starch adsorbent, comprising the following steps:

[0031] 1) Mix corn starch with 85%-95% ethanol by mass and stir until homogeneous to obtain a starch emulsion;

[0032] 2) Adjust the pH of the starch emulsion to 10-12, preheat it at 50-60 ℃, add 0.1%-0.15% epichlorohydrin by weight of dry starch, and stir continuously for 1-2 h; the epichlorohydrin is added after being dispersed in 85%-95% ethanol by weight.

[0033] 3) After the obtained system is cooled to room temperature, sodium hydroxide solid is added for alkalization. Chloroacetic acid is added to the alkalized system and reacted at 50-60 °C for 2-4 h. After the reaction is completed, the pH value of the reaction system is adjusted to 6-7, filtered, washed, dried and sieved to obtain cross-linked carboxymethyl corn starch adsorbent. The molar ratio of chloroacetic acid to native starch is controlled at (0.1-0.3):1. The chloroacetic acid is added after being dispersed in 85%-95% ethanol by mass.

[0034] Example 1

[0035] A method for preparing a cross-linked carboxymethyl corn starch adsorbent includes the following steps:

[0036] 1) Select high amylose corn starch and mix it with 85% ethanol by mass, and stir evenly to obtain starch emulsion;

[0037] 2) Adjust the pH of the starch emulsion to 10 with sodium hydroxide solution, preheat it at 60 ℃, add 0.1% epichlorohydrin by weight of dry starch, wherein the epichlorohydrin is dispersed in 85% ethanol by weight before being added, and stir continuously for 1.5 h.

[0038] 3) After the obtained system was cooled to room temperature, sodium hydroxide solid was added for alkalization (the molar ratio of added sodium hydroxide solid to native starch was 3:2). Chloroacetic acid was added to the alkalized system, with a molar ratio of chloroacetic acid to native starch of 0.1. The chloroacetic acid was dispersed in 85% ethanol and reacted at 60 °C for 2 h. After the reaction was completed, the pH of the reaction system was adjusted to 6 with 3 M glacial acetic acid-ethanol solution. The system was filtered, washed with 70% ethanol solution, dried in an oven at 55 °C, and sieved to obtain cross-linked carboxymethyl corn starch. The product test results are shown in Table 1.

[0039] Example 2

[0040] A method for preparing a cross-linked carboxymethyl corn starch adsorbent includes the following steps:

[0041] 1) Select ordinary corn starch and mix it with 90% ethanol by mass, and stir evenly to obtain starch emulsion;

[0042] 2) Adjust the pH of the starch emulsion to 11 with sodium hydroxide solution, preheat at 55 ℃, add 0.13% epichlorohydrin by weight of dry starch, the epichlorohydrin is added after being dispersed in 90% ethanol by weight, and stir continuously for 1 h.

[0043] 3) After the obtained system was cooled to room temperature, sodium hydroxide solid was added for alkalization (the molar ratio of added sodium hydroxide solid to native starch was 4:2). Chloroacetic acid was added to the alkalized system, with a molar ratio of chloroacetic acid to native starch of 0.3. The chloroacetic acid was dispersed in 90% ethanol and reacted at 55 °C for 3 h. After the reaction was completed, the pH of the reaction system was adjusted to 7 with 2 M glacial acetic acid-ethanol solution. The system was filtered, washed with 80% ethanol solution, dried in an oven at 60 °C, and sieved to obtain cross-linked carboxymethyl corn starch. The product test results are shown in Table 1.

[0044] Example 3

[0045] A method for preparing a cross-linked carboxymethyl corn starch adsorbent includes the following steps:

[0046] 1) Select waxy corn starch and mix it with 95% ethanol by mass, and stir evenly to obtain starch emulsion;

[0047] 2) Adjust the pH of the starch emulsion to 12 with sodium hydroxide solution, preheat it at 50°C, add 0.15% epichlorohydrin by weight of dry starch, the epichlorohydrin is added after being dispersed in 95% ethanol by weight, and stir continuously for 2 hours.

[0048] 3) After the obtained system was cooled to room temperature, sodium hydroxide solid was added for alkalization (the molar ratio of added sodium hydroxide solid to native starch was 5:2). Chloroacetic acid was added to the alkalized system, with a molar ratio of chloroacetic acid to native starch of 0.2. The chloroacetic acid was dispersed in 95% ethanol and reacted at 50 °C for 4 h. After the reaction was completed, the pH of the reaction system was adjusted to 6.5 with 1 M glacial acetic acid-ethanol solution. The system was filtered, washed with 70% ethanol solution, dried in an oven at 50 °C, and sieved to obtain cross-linked carboxymethyl corn starch. The product test results are shown in Table 1.

[0049] Comparative Example 1

[0050] 1) Select ordinary corn starch and mix it with 90% ethanol by mass, and stir evenly to obtain starch emulsion;

[0051] 2) Add sodium hydroxide solid to the emulsion for alkalization. Add chloroacetic acid to the alkalized system. The molar ratio of chloroacetic acid to native starch is 0.1. Disperse the chloroacetic acid with 90% ethanol before adding it. React at 60 °C for 2 h. After the reaction is complete, adjust the pH of the reaction system to 6 with 2 M glacial acetic acid-ethanol solution. Filter, wash with 80% ethanol solution, dry in an oven at 60 °C, and sieve to obtain cross-linked carboxymethyl corn starch.

[0052] Comparative Example 1 was prepared without the treatment in step 2 of the technical solution of the present invention, and a single carboxymethyl modified ordinary corn starch was obtained.

[0053] Fourier transform infrared spectroscopy observation

[0054] Take an appropriate amount of starch sample (3-5 mg) and mix it with potassium bromide powder, then grind and compress it into tablets. Place the tablets into a Fourier transform infrared spectrometer for infrared spectroscopy testing. The wavelength range for sample testing is 4000-400 cm⁻¹. -1 The resolution is 4cm. -1 Each sample was scanned 64 times.

[0055] Depend on Figure 1 It can be observed that, compared to the unmodified blank starch, the characteristic peak width of the hydroxyl stretching vibration at 3446 cm⁻¹ is narrower and the intensity is weaker in the single carboxymethyl modified starch of Comparative Example 1 and the cross-linked carboxymethyl composite modified starch of Examples 1-3. This indicates that the hydroxyl groups of the starch molecular chains undergo a substitution reaction during carboxymethylation, resulting in a decrease in hydroxyl content. Meanwhile, both Comparative Example 1 and Examples 1-3 show a peak width of 1611 cm⁻¹. -1 and 1430 cm -1 The peaks represent the asymmetric and symmetric stretching vibrations of -COO-, respectively. These phenomena effectively prove that the carboxyl group has been successfully introduced into the starch molecular chain of corn starch. In addition, compared with Comparative Example 1, the characteristic peak of the hydroxyl stretching vibration at 3446 cm⁻¹ in Examples 1-3 is narrower, which indicates that the cross-linking reaction also occurred effectively in Comparative Examples 1-3, that is, the cross-linked carboxymethyl composite modified starch was successfully prepared.

[0056] Scanning electron microscopy observation

[0057] First, attach the conductive adhesive to the copper plate, then carefully sprinkle starch onto the conductive adhesive and blow away any excess starch with a bulb syringe. Spray gold under vacuum for 30 seconds, fix the prepared sample in the sample chamber of a scanning electron microscope, magnify it appropriately, and observe and photograph the microscopic morphology of the starch particles.

[0058] Scanning electron microscopy observations of Examples 1-3, Comparative Example 1, and blank ordinary corn starch are as follows: Figure 2As shown, compared to the single carboxymethyl modified starch and blank starch in Comparative Example 1, the starch granules in Examples 1-3 all have small protrusions and slight cracks on their surfaces. Furthermore, the starch granules are more tightly bonded, the interfaces gradually become blurred, and the surface of the granules shows obvious pits and depressions, exhibiting a certain network structure (arrows indicate this), resulting in an increased specific surface area. This unique loose structure facilitates the adsorption of small molecules and ions into its interior, indicating that the cross-linking reaction connects molecules in a bridging manner to form a certain network structure, enabling the cross-linked carboxymethyl composite modified starch to effectively remove heavy metal ions.

[0059] Solubility and swelling

[0060] Accurately weigh 0.6000 g (dry basis) of starch sample, add 30 ml of distilled water to prepare a 2% (w / w) starch slurry, and pour it into a pre-weighed centrifuge tube. Vortex thoroughly to mix evenly. Heat the starch slurry at 50°C, 70°C, and 90°C respectively, stirring for 30 min. After stirring, rapidly cool to room temperature, then centrifuge at 3500 r·min⁻¹ for 20 min. Transfer the supernatant to an aluminum box and dry at 105°C to constant weight. Record the mass of starch in the aluminum box, i.e., the mass of water-soluble starch (B); and the mass of the precipitate in the centrifuge tube, i.e., the mass of expanded starch (C). The formulas for calculating the solubility and expansion of starch paste are as follows:

[0061] solubility

[0062] Expansion degree P(g / g) = C / (m(1-S))

[0063] In the formula: B - mass of dissolved starch, g; C - mass of precipitate, g; m - dry starch mass, g. The same sample was measured three times, and the average value was taken.

[0064] Table 1 Solubility Test Results

[0065] "—": indicates that the starch paste cannot be separated by centrifugation.

[0066] Table 2. Expansion Test Results

[0067] "—": indicates that the starch paste cannot be separated by centrifugation.

[0068] Solubility and swelling directly reflect the structural stability and particle integrity of the adsorbent in aqueous solution, and are key indicators determining whether heavy metal adsorption can proceed stably and efficiently. As shown in Tables 1 and 2, the solubility and swelling of the cross-linked carboxymethyl composite modified starch (Example 2) of this invention are significantly lower than those of single carboxymethyl starch (Comparative Example 1). This indicates that the epichlorohydrin cross-linking forms a three-dimensional network covalent structure, effectively limiting the water absorption, swelling, dissolution, and disintegration of starch particles, allowing the adsorbent to maintain its intact particle shape in the wastewater system, avoiding dissolution and loss during adsorption that leads to decreased adsorption efficiency and secondary pollution. Simultaneously, moderate swelling characteristics can maintain open internal pores within the particles, which is beneficial for Cu²⁺. + It enters the interior of the particles and binds to carboxyl groups. Test results show that epichlorohydrin and chloroacetic acid have a synergistic effect: the introduction of epichlorohydrin enables starch to first construct a stable three-dimensional framework, improving structural strength, controlling solubility and swelling, and increasing specific surface area; while chloroacetic acid subsequently introduces carboxyl active sites, and the two work together to achieve a dual improvement in structural stability and adsorption efficiency.

[0069] Determination of the degree of substitution of carboxymethyl etherified groups

[0070] Acidification: Weigh 1 g of starch sample and place it in a 100 mL beaker. Add 40 mL of 2 mol / L hydrochloric acid solution (prepared with 70% ethanol), seal with plastic wrap, and stir at room temperature for 1 h. Filter, then wash with 80% ethanol solution (v / v) until the solution is free of chloride ions (tested with AgNO3 solution), and then dry in a 40 ℃ oven.

[0071] Determination: Accurately weigh 0.5000 g of acidified and dried starch sample into a 250 ml Erlenmeyer flask. Dissolve the starch sample thoroughly in 40 ml of 0.1 mol / L NaOH standard solution. Place the flask in a 35℃ constant temperature water bath and stir until the solution becomes transparent. Add 2 drops of phenolphthalein indicator; the solution will then turn red. Immediately back-titrate with 0.1 mol / L hydrochloric acid standard solution until the red color disappears. Calculate the carboxyl content and degree of substitution of the modified starch using the following formula.

[0072]

[0073]

[0074] in: M is the mass of the sample taken, in grams. V NaOH The volume of 0.1 mol / L NaOH solution added is in mL. V HCl This represents the volume, in mL, of the 0.1 mol / L HCl solution consumed during the titration. A represents the number of millimoles of NaOH consumed in neutralizing carboxymethyl starch, in mmol.

[0075] Copper ion adsorption determination

[0076] Choose Cu 2+ As the adsorbent target, a Cu(NO3)2 solution with a concentration of 50 mg / L was used as the initial adsorbent solution. 50 mL of the heavy metal ion solution was placed in a 250 mL stoppered conical flask, and 0.3000 g of adsorbent was accurately weighed and added to the flask. The flask was placed in a constant-temperature shaker and shaken at room temperature for 3 hours. After the shake, the solution was centrifuged, and the Cu concentration in the supernatant was determined using flame atomic absorption spectrometry. 2+ The concentration of metal ions. The adsorption capacity Q of the adsorbent for metal ions is calculated using the following formula:

[0077] Adsorption capacity calculation formula:

[0078]

[0079] in:

[0080] Q represents the amount of metal ions adsorbed by the adsorbent, expressed in mg / g.

[0081] C0 and C1 are the concentrations of metal ions in the solution before and after adsorption, in mg / L;

[0082] V is the volume of the solution containing the adsorbed metal ions, in L;

[0083] W represents the mass of the adsorbent, in grams.

[0084] The results of testing the degree of carboxyl substitution and the amount of copper ion adsorption in the examples and comparative examples using the two methods described above are shown in Table 1 below:

[0085] Table 1. Results of Substitution Degree and Adsorption Capacity Tests

[0086] Table 1 shows that the blank starch has a carboxyl substitution degree of 0 and mainly relies on physical adsorption. Therefore, its adsorption capacity for copper ions is only 2.547 mg / g. Comparative Example 1 improves its adsorption capacity to some extent by introducing carboxyl groups and adding chemical adsorption of anionic groups. However, single carboxymethyl starch does not have a spatial network structure, has a small specific surface area and few adsorption sites, so the improvement in adsorption capacity is not significant. The synergistic introduction of carboxyl groups and cross-linking bonds can significantly improve the spatial structure and ion adsorption of starch. Therefore, the adsorption capacity of cross-linked carboxymethyl corn starch in Examples 1-3 for copper ions is 3-6 times higher than that of blank starch and 2-4 times higher than that of single carboxymethyl starch. Compared with existing adsorption materials, the adsorption capacity of the adsorbent of this invention can reach the level of conventional cation exchange resins. However, this invention uses natural starch as raw material, which has the advantages of being biodegradable, having no secondary pollution, and having low raw material cost, making it more suitable for the green treatment of industrial wastewater.

[0087] Compared to the cross-linking-enzymatic hydrolysis composite ultrafine modified starch adsorbent in Chinese invention patent CN105642244B, this invention provides a one-step method for preparing cross-linked carboxymethyl corn starch. The cross-linking and carboxymethylation reactions are carried out sequentially in the same reaction system, resulting in a simpler process, milder conditions, and eliminating the need for freeze-drying and enzymatic hydrolysis. This significantly reduces costs, simplifies the process, and consumes less energy. This invention utilizes the synergistic effect of epichlorohydrin and chloroacetic acid: epichlorohydrin first constructs a stable three-dimensional framework, enhancing structural strength and increasing specific surface area; chloroacetic acid then introduces carboxyl ion adsorption groups. The combination of these two substances achieves a dual improvement in both the physical and chemical aspects of structural stability and ion adsorption, making it more suitable for the green treatment of industrial wastewater.

[0088] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a cross-linked carboxymethyl corn starch adsorbent, characterized in that... Includes the following steps: 1) Mix corn starch with 85%-95% ethanol by mass and stir until homogeneous to obtain a starch emulsion; 2) Adjust the pH of the starch emulsion to 10-12, preheat it at 50-60 ℃, add 0.1%-0.15% epichlorohydrin by weight of dry starch, and stir continuously for 1-2 h; the epichlorohydrin is added after being dispersed in 85%-95% ethanol by weight. 3) After the obtained system is cooled to room temperature, sodium hydroxide solid is added for alkalization. Chloroacetic acid is added to the alkalized system and reacted at 50-60 °C for 2-4 h. After the reaction is completed, the pH value of the reaction system is adjusted to 6-7, filtered, washed, dried and sieved to obtain cross-linked carboxymethyl corn starch adsorbent. The molar ratio of chloroacetic acid to native starch is controlled at (0.1-0.3):

1. The chloroacetic acid is added after being dispersed in 85%-95% ethanol by mass.

2. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 1, characterized in that, The starch milk in step 1) has a mass content of 10%-30%; the corn starch is high amylose corn starch, ordinary corn starch or waxy corn starch.

3. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 1, characterized in that, In step 2), the pH value of the starch emulsion is adjusted to 10-12 using sodium hydroxide solution; the preheating at 50-60 ℃ is achieved by placing the starch emulsion in a constant temperature water bath at 50-60 ℃.

4. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 3, characterized in that, The sodium hydroxide solution has a mass fraction of 3%-7%.

5. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 1, characterized in that, The molar ratio of sodium hydroxide solid to original starch in step 3) is 3:2-5:

2.

6. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 1, characterized in that, In step 3), after the reaction is completed, the pH of the reaction system is adjusted to 6-7 by using a 1-3 M glacial acetic acid-ethanol solution.

7. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 1, characterized in that, The washing described in step 3) is done with an ethanol solution of 70%-90% by mass.

8. The method for preparing the cross-linked carboxymethyl corn starch adsorbent according to claim 1, characterized in that, The drying described in step 3) is performed in an oven at 50-60 ℃.

9. A cross-linked carboxymethyl corn starch adsorbent, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. The cross-linked carboxymethyl corn starch adsorbent according to claim 8 is used in the treatment of industrial wastewater containing copper heavy metals.

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