Chitosan-based adsorbent as well as preparation method and application thereof

Abstract

The invention relates to a chitosan-based adsorbent as well as a preparation method and application thereof, and belongs to the technical field of environmental functional materials and water pollution treatment. Chitosan is used as a matrix and is cross-linked with polyethyleneimine and glutaraldehyde to form an intermediate with a stable three-dimensional network structure, and under the catalytic action of triethylamine, the intermediate and dimethyl chlorophosphate are subjected to a functional grafting reaction to introduce a phosphate group into an amino site of CS-P to prepare the chitosan-based adsorbent, which is recorded as CS-PN, and the structural formula of the chitosan-based adsorbent is shown in the specification. The structure of the chitosan-based adsorbent CS-PN can be kept under the acidic condition, and the chitosan-based adsorbent CS-PN shows high adsorption capacity of 311.72 mg / g to hexavalent chromium in water, and has excellent selectivity and good recoverability and regeneration performance.

Description

Technical Field

[0001] This invention relates to a chitosan-based adsorbent, its preparation method, and its application, belonging to the fields of environmental functional materials and water pollution control technology. Background Technology

[0002] Chromium (Cr) is a typical example of heavy metal pollutants in industrial wastewater with outstanding toxicity, especially when it exists in the form of Cr(VI), due to its high water solubility, high mobility, and significant carcinogenic and mutagenic risks.

[0003] Currently, common Cr(VI) removal technologies include ion exchange, membrane separation, chemical reduction precipitation, biological treatment, and adsorption. Among these, adsorption shows promising application prospects in practical water treatment due to its simple operation, low cost, and ease of implementation. However, traditional adsorption materials such as activated carbon suffer from limited adsorption capacity, difficult regeneration, and generally poor selectivity; biomass-based materials (such as straw), while widely available, often exhibit defects such as poor structural stability and insufficient active sites; and nanomaterials such as carbon nanotubes are costly and may pose environmental release risks, thus limiting their large-scale application.

[0004] Chitosan, a natural cationic polysaccharide, has molecular chains rich in active groups such as amino and hydroxyl groups. It exhibits excellent coordination and electrostatic adsorption capabilities for heavy metal ions such as hexavalent chromium, and possesses advantages such as wide availability, biodegradability, and good environmental compatibility. It is considered an ideal adsorption matrix. However, unmodified chitosan easily dissolves and loses its properties in strongly acidic aquatic environments, exhibits poor mechanical strength, and has limited selectivity for target ions. This severely limits its direct application in treating wastewater containing acidic chromium. Furthermore, common single modification methods (such as simple cross-linking or grafting) often fail to simultaneously achieve the dual functions of material structural stability, high adsorption capacity, excellent selectivity, and reduced toxicity. Therefore, developing a novel chitosan-based adsorbent material that is stable under acidic conditions and possesses both highly efficient adsorption and chemical reduction functions has significant scientific and practical application value. Summary of the Invention

[0005] To address the shortcomings of existing chitosan-based adsorbents, such as structural instability under acidic conditions, insufficient selectivity for Cr(VI) adsorption, and the difficulty of achieving multiple performance characteristics with a single modification method, this invention proposes a method for preparing a chitosan-based adsorbent. Using chitosan as a matrix, a stable three-dimensional network structure intermediate is formed by crosslinking with polyethyleneimine and glutaraldehyde. Under the catalysis of triethylamine, the intermediate undergoes a functionalization grafting reaction with dimethyl chlorophosphate to introduce phosphate groups into the amino sites of CS-PN, thus obtaining the chitosan-based adsorbent, denoted as CS-PN. The chitosan-based adsorbent exhibits structural stability in acidic media, high adsorption capacity and selectivity for Cr(VI), and can reduce adsorbed Cr(VI) to low-toxicity Cr(III) in situ, demonstrating chemical detoxification capabilities and achieving synergistic treatment of adsorption and detoxification.

[0006] A chitosan-based adsorbent, denoted as CS-PN, is prepared by cross-linking chitosan with polyethyleneimine and glutaraldehyde to form an intermediate, followed by a functionalization grafting reaction of the intermediate with dimethyl chlorophosphate under the catalysis of triethylamine. Its structural formula is as follows: .

[0007] The preparation method of the chitosan-based adsorbent includes the following specific steps: (1) Chitosan was dissolved in acetic acid solution to obtain chitosan solution. Polyethyleneimine and glutaraldehyde were added to chitosan solution and mixed evenly. The mixture was reacted at 50~60℃ for 1.5~2.5h and then freeze-dried to obtain intermediate product CS-P. The reaction formula is as follows: ; (2) The intermediate product CS-P was dissolved in dichloromethane to obtain a CS-P solution. Dimethyl chlorophosphate and triethylamine were added to the chitosan solution and mixed evenly. The mixture was reacted at 50-60℃ for 10-14 h. After washing, the mixture was freeze-dried to obtain the chitosan-based adsorbent CS-PN. The reaction formula is as follows: ; Preferably, the molar ratio of chitosan, polyethyleneimine and glutaraldehyde in step (1) is 1:1~1.5:1~1.3.

[0008] Preferably, the mass ratio of the intermediate product CS-P in step (2) to dimethyl chlorophosphate is 2~2.5:1, and the amount of triethylamine added is 36.3 wt. of dimethyl chlorophosphate.

[0009] Application of the chitosan-based adsorbent in the selective adsorption of Cr(VI) in chromium-containing wastewater.

[0010] The principle of selective adsorption and chemical detoxification of Cr(VI) by chitosan-based adsorbent CS-PN: (1) Material structure design principle: composite modification process to construct functional skeleton Crosslinking enhances stability: Chitosan (CS) is crosslinked with amino-rich polyethyleneimine (PEI) using glutaraldehyde (GLA) as a crosslinking agent to form a three-dimensional network structure, which solves the problems of easy dissolution and poor mechanical strength of the original chitosan under acidic conditions, while introducing a large number of amino (-NH2) active sites.

[0011] Grafting enhances selectivity: Phosphate-containing groups (-PO3) are introduced through grafting modification. 2- A multi-site synergistic adsorption system of "ammonia-phosphoryl group" was constructed using dimethyl chlorophosphate (NP2). According to the hard-soft acid-base theory (HSAB), the phosphate group (hard base) preferentially binds to Cr(VI) (hard acid), significantly improving the selectivity for Cr(VI).

[0012] Mesoporous structure optimizes mass transfer efficiency: The modified CS-PN exhibits a mesoporous structure (specific surface area 90.186 m²). 2 / g, with an average pore size of 3.826nm), and good thermal stability (stable below 333K), providing ample space for Cr(VI) diffusion and adsorption.

[0013] (2) Selective adsorption mechanism: synergistic effect of multiple effects to achieve preferential adsorption of Cr(VI) Electrostatic attraction dominates: Under acidic conditions (optimal pH=3), amino groups on the CS-PN surface are protonated to form -NH3. + The main form of Cr(VI) (HCrO4) - Cr2O7 2- This generates a strong electrostatic attraction. Zeta potential analysis shows that the CS-PN surface remains positively charged within the pH range of 1-7, and the zeta potential reaches its peak at pH 3, indicating the strongest electrostatic adsorption.

[0014] Selective coordination enhancement: phosphate group (-PO3) 2- The N / O functional group and the amino group (-NH2) act as cooperating active sites, forming a weak chemical coordination bond with the empty orbital of Cr(VI) through lone pair electrons. DFT calculations confirm that the binding energy between the N / O functional group and Cr(VI) is -13 to -14.13 eV, which is thermodynamically stable, and this coordination is specific to Cr(VI) (hard acid-hard base matching).

[0015] Differences in physicochemical properties drive the development of Cr(VI) ions: Cr(VI) has the smallest ionic radius (0.62 Å) and the highest charge density compared to the coexisting Cu. 2+ Zn 2+ Mg 2+Plasma is more likely to bind to the protonation sites on the CS-PN surface; at the same time, Cr(VI) has a small atomic weight (51.996) and a faster diffusion rate, which allows it to preferentially occupy adsorption sites.

[0016] Differences in electrochemical properties: Cyclic voltammetry analysis showed that Cr(VI) had a characteristic reduction peak at +0.97V, which did not overlap with other coexisting ions (reduction peak range -0.48~-0.08V), ensuring that it could interact with the adsorbent through a dedicated electron transfer pathway during adsorption without interference from other ions.

[0017] (3) Chemical detoxification pathway: the intrinsic mechanism of Cr(VI) reduction to less toxic Cr(III). CS-PN achieves chemical detoxification while adsorbing Cr(VI). The core is to reduce the highly toxic and highly mobile Cr(VI) to the less toxic and easily precipitated Cr(III). The specific pathway is as follows: Reduction active sites: The amino groups (-NH2) in CS-PN and the amino groups introduced by PEI have reducing properties and can act as electron donors to undergo redox reactions with Cr(VI).

[0018] Detoxification mechanism verification: XPS characterization showed that after adsorption, the CS-PN surface simultaneously exhibited characteristic peaks of Cr(VI) (Cr2p3 / 2=575.79eV, Cr2p1 / 2=585.38eV) and Cr(Ⅲ) (Cr2p3 / 2=577.08eV, Cr2p1 / 2=586.94eV); and there was no significant difference in the Cr2p spectrum under dark conditions and under ultraviolet irradiation, proving that the reduction effect originated from the adsorbent itself, rather than external photocatalysis.

[0019] Synergistic stabilizing effect: The Cr(III) generated by reduction can be fixed on the surface of the adsorbent by forming more stable coordination bonds with the amino and phosphate groups on the surface of CS-PN, avoiding secondary pollution and realizing integrated detoxification of "adsorption-reduction-fixation".

[0020] The beneficial effects of this invention are: (1) This invention improves the acid stability of chitosan-based adsorbent CS-PN material through a two-step process of "crosslinking-grafting" and introduces phosphate groups with dual functions of coordination and reduction, thereby achieving synergistic optimization of structural stability and adsorption and detoxification performance. (2) The chitosan-based adsorbent CS-PN of the present invention has an extremely high adsorption capacity (311.71 mg / g) for Cr(VI) and excellent selective adsorption ability, with a high partition coefficient, and is suitable for complex aquatic environments. (3) The chitosan-based adsorbent CS-PN of the present invention has excellent chemical and thermal stability. It can be efficiently regenerated through simple desorption treatment. Its performance decays little after multiple cycles of use, which reduces the cost of use. (4) The adsorption process of the chitosan-based adsorbent CS-PN of the present invention not only physically adsorbs Cr(VI) through electrostatic interaction, but also reduces some Cr(VI) to trivalent chromium Cr(III) with lower toxicity by the reducing functional groups on the surface of CS-PN, thereby achieving a synergistic effect of chromium adsorption and in-situ chemical detoxification, and reducing environmental risks. Attached Figure Description

[0021] Figure 1 Here is a SEM image of the chitosan-based adsorbent CS-PN from Example 1; Figure 2 EDS diagram of chitosan-based adsorbent CS-PN in Example 1; Figure 3 The image shows the FT-IR spectrum of chitosan-based adsorbent CS-PN from Example 1. Figure 4 The XPS spectra of chitosan-based adsorbent CS-PN before and after adsorption of Cr(VI) in Example 1 are shown below. Figure 5 This is a SEM image of Cr(VI) adsorbed by the chitosan-based adsorbent CS-PN in Example 1. Figure 6 The image shows the EDS diagram of Cr(VI) after adsorption by the chitosan-based adsorbent CS-PN in Example 1. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0023] Example 1: A chitosan-based adsorbent, denoted as CS-PN, is prepared by cross-linking chitosan with polyethyleneimine and glutaraldehyde to form an intermediate, and then functionalizing the intermediate with dimethyl chlorophosphate under the catalysis of triethylamine. The preparation method of the chitosan-based adsorbent includes the following specific steps: (1) Chitosan was dissolved in acetic acid solution to obtain a chitosan solution with a concentration of 0.05 g / mL. Polyethyleneimine and glutaraldehyde were added to the chitosan solution and mixed evenly. The mixture was reacted at 50 °C for 2 h and then freeze-dried to obtain the intermediate product CS-P. The molar ratio of chitosan, polyethyleneimine and glutaraldehyde was 1:1.32:1.2. The structural formula of the intermediate product CS-P is as follows: ; (2) The intermediate product CS-P was dissolved in dichloromethane to obtain a CS-P solution. Dimethyl chlorophosphate and triethylamine were added to the chitosan solution and mixed evenly. The mixture was reacted at 50°C for 12 h, washed, and freeze-dried to obtain the chitosan-based adsorbent CS-PN. The mass ratio of the intermediate product CS-P to dimethyl chlorophosphate was 2.25:1, and the amount of triethylamine added was 36.3 wt.% of dimethyl chlorophosphate. The structural formula of the chitosan-based adsorbent CS-PN is: ; The SEM image of the chitosan-based adsorbent CS-PN in this embodiment is shown below. Figure 1 The EDS diagram of the chitosan-based adsorbent CS-PN is shown in the figure. Figure 2 The FT-IR spectrum of the chitosan-based adsorbent CS-PN is shown in the figure. Figure 3 The chitosan-based adsorbent CS-PN is mainly composed of elements C, N, O, and P, with weight percentages of 53.93%, 15.78%, 25.69%, and 4.6%, respectively. In the FT-IR spectra, CS-P and CS-PN show significant differences in chemical bonds and functional groups. After two-step modification, the absorption peaks of hydroxyl (-OH) and amino (-NH2) groups in CS decreased from 3433.15 cm⁻¹. -1 Moved to 3429.30cm -1 Meanwhile, the CO stretching vibration peak decreased from 1077.05 cm⁻¹. -1 It became 1058.73cm -1 This confirms the success of its structural modification; the CS-P spectrum at 1631.48 cm⁻¹... -1 The absorption peak at [value missing] is attributed to the C=N bond formed by the Schiff base reaction during the crosslinking of glutaraldehyde and polyethyleneimine with CS. This peak further shifted to 1638.23 cm⁻¹ after secondary modification. -1 This confirms successful crosslinking; 1399.59 cm⁻¹ in CS-P -1 The NH2 bending vibration peak at 1206.74 cm⁻¹ disappears after the formation of CS-PN, indicating that the introduction of NP2 replaces the original amino group, thus confirming the successful incorporation of NP2; in addition, the 1206.74 cm⁻¹ peak in the CS-PN spectrum... -1 The new absorption peak appearing at 825.26 cm⁻¹ corresponds to the stretching vibration of the P=O bond in NP₂. -1 The peak at [value] is attributed to the PNP bonds formed by the NP2 chlorination reaction; XPS analysis of the chitosan-based adsorbent CS-PN (see [reference]). Figure 4 The peak values ​​in the spectrum of the chitosan-based adsorbent CS-PN mainly come from C1s, N1s, O1s and P2p; Adsorption performance test of chitosan-based adsorbent CS-PN for Cr(VI): K₂Cr₂O₇ solutions with different initial concentrations (150-850 mg / L) were prepared, and the pH was adjusted to 3 with HCl or NaOH. 10 mL of Cr(VI) solution was added to several 15 mL centrifuge tubes, followed by 10 mg of the chitosan-based adsorbent CS-PN prepared in this example. The tubes were shaken at 200 rpm in isothermal shakers at 298 K, 308 K, and 318 K until adsorption equilibrium was reached (24 hours). Furthermore, a set of kinetic studies was conducted at 298 K according to the above steps at different time points (10 min-480 min), and samples were filtered to determine the residual Cr(VI) concentration in the filtrate and calculate the adsorption capacity. The results showed that the maximum adsorption capacity reached 311.72 mg / g, and the adsorption behavior conformed to the RP isothermal model and the pseudo-second-order kinetic model, indicating that the chitosan-based adsorbent CS-PN mainly exhibits monolayer chemisorption. Selective adsorption performance in the presence of competing ions: At 298 K, chitosan-based adsorbent CS-PN (40 mg) was added to simulated adsorption solutions (Cr(VI) 43.85 mg / L, Mg(II) 102.57 g / L, Zn(II) 100.51 mg / L, Mn(II) 102.82 mg / L, Cu(II) 100.11 mg / L, Ni(I) 106.31 mg / L) and actual adsorption solutions (Cr(VI) 40.2 mg / L, Mg(II) 184.83 g / L, Zn(II) 122.72 mg / L, Mn(II) 116.17 mg / L, Cu(II) 117.01 mg / L, Ni(I) 227.93 mg / L). The solutions were shaken at 200 rpm for 24 hours, and then centrifuged to separate the adsorbent. The supernatant was obtained, and the residual concentration of the remaining metal ions in the supernatant was determined by ICP-OES. In the simulated adsorption solution, the removal rates of Cr(VI) were 53.41%, Mg(II) 0.10%, Zn(II) 0.21%, Mn(II) 0.70%, Cu(II) 14.4%, and Ni(I) 0.48%. In the actual adsorption solution, the removal rates of Cr(VI) were 79.83%, Mg(II) 0.93%, Zn(II) 2.45%, Mn(II) 1.27%, Cu(II) 12.05%, and Ni(I) 0.13%. Therefore, the chitosan-based adsorbent CS-PN has extremely strong selectivity for Cr(VI). Adsorbent desorption and cycling performance: At room temperature, 10 mg of chitosan-based adsorbent CS-PN and Cr(VI) solutions (pH=3, 10 mL, 350 mg / L) were added to a 15 mL centrifuge tube. The tube was shaken at 200 rpm for 20 h. The chitosan-based adsorbent was then separated by centrifugation, and the supernatant was obtained. The chitosan-based adsorbent CS-PN after Cr(VI) adsorption was analyzed by XPS, SEM, and EDS (see [link to analysis]). Figures 4-6 ),from Figure 4 Characteristic peaks of Cr were found in the XPS image. Figure 5 The SEM image shows that Cr is uniformly distributed on the chitosan-based adsorbent. Figure 6 EDS analysis showed that the Cr content reached 26%; ICP-OES analysis showed that the residual Cr(VI) concentration in the supernatant was 141.7771 mg / L, and the adsorption capacity of chitosan-based adsorbent CS-PN for Cr(VI) was 202.5075 mg / g, with an adsorption efficiency of 58.82%; after adsorbing Cr(VI), the chitosan-based adsorbent CS-PN was eluted with sodium thiosulfate desorption solution (40 mL) for 24 h; after centrifugation, the chitosan-based adsorbent was washed with distilled water until the solution was neutral to complete the regeneration of chitosan-based adsorbent CS-PN; after 5 repeat experiments, the adsorption capacity of Cr(VI) decreased by only 14.41 mg / g in the 5th adsorption; XPS characterization showed that the CS-PN surface after adsorption simultaneously exhibited characteristic peaks of Cr(VI) (Cr2p3 / 2=575.79eV, Cr2p1 / 2=585.38eV) and Cr(Ⅲ) (Cr2p3 / 2=577.08eV, Cr2p1 / 2=586.94eV); and there was no significant difference in the Cr2p spectrum under dark conditions and under ultraviolet irradiation, proving that the reduction effect originated from the adsorbent itself, rather than external photocatalysis.

[0024] Example 2: A chitosan-based adsorbent, denoted as CS-PN, is prepared by cross-linking chitosan with polyethyleneimine and glutaraldehyde to form an intermediate, and then functionalizing the intermediate with dimethyl chlorophosphate under the catalysis of triethylamine. The preparation method of the chitosan-based adsorbent includes the following specific steps: (1) Chitosan was dissolved in acetic acid solution to obtain a chitosan solution with a concentration of 0.04 g / mL. Polyethyleneimine and glutaraldehyde were added to the chitosan solution and mixed evenly. The mixture was reacted at 55 °C for 1.5 h and then freeze-dried to obtain the intermediate product CS-P. The molar ratio of chitosan, polyethyleneimine and glutaraldehyde was 1:1.1:1.1. The structural formula of the intermediate product CS-P is as follows: ; (2) The intermediate product CS-P was dissolved in dichloromethane to obtain a CS-P solution. Dimethyl chlorophosphate and triethylamine were added to the chitosan solution and mixed evenly. The mixture was reacted at 55°C for 10 h, washed, and freeze-dried to obtain the chitosan-based adsorbent CS-PN. The mass ratio of the intermediate product CS-P to dimethyl chlorophosphate was 2.0:1, and the amount of triethylamine added was 36.3 wt.% of dimethyl chlorophosphate. The structural formula of the chitosan-based adsorbent CS-PN is: ; Adsorption performance test of chitosan-based adsorbent CS-PN for Cr(VI): K₂Cr₂O₇ solutions with different initial concentrations (150-850 mg / L) were prepared, and the pH was adjusted to 3 with HCl or NaOH. 10 mL of Cr(VI) solution was added to several 15 mL centrifuge tubes, followed by 10 mg of the chitosan-based adsorbent CS-PN prepared in this example. The tubes were shaken at 200 rpm in isothermal shakers at 298 K, 308 K, and 318 K until adsorption equilibrium was reached (24 hours). Furthermore, a set of kinetic studies was conducted at 298 K according to the above steps at different time points (10 min-480 min), and samples were filtered to determine the residual Cr(VI) concentration in the filtrate and calculate the adsorption capacity. The results showed that the maximum adsorption capacity reached 296.13 mg / g, and the adsorption behavior conformed to the RP isothermal model and the pseudo-second-order kinetic model, indicating that the chitosan-based adsorbent CS-PN mainly exhibits monolayer chemisorption. Selective adsorption performance in the presence of competing ions: At 298 K, chitosan-based adsorbent CS-PN (40 mg) was added to simulated adsorption solutions (Cr(VI) 43.85 mg / L, Mg(II) 102.57 g / L, Zn(II) 100.51 mg / L, Mn(II) 102.82 mg / L, Cu(II) 100.11 mg / L, Ni(I) 106.31 mg / L) and actual adsorption solutions (Cr(VI) 40.2 mg / L, Mg(II) 184.83 g / L, Zn(II) 122.72 mg / L, Mn(II) 116.17 mg / L, Cu(II) 117.01 mg / L, Ni(I) 227.93 mg / L). The solutions were shaken at 200 rpm for 24 hours, and then centrifuged to separate the adsorbent. The supernatant was obtained, and the residual concentration of the remaining metal ions in the supernatant was determined by ICP-OES. In the simulated adsorption solution, the removal rates of Cr(VI) were 50.41%, Mg(II) 0.10%, Zn(II) 0.20%, Mn(II) 0.50%, Cu(II) 14.1%, and Ni(I) 0.38%. In the actual adsorption solution, the removal rates of Cr(VI) were 75.83%, Mg(II) 0.73%, Zn(II) 2.05%, Mn(II) 1.17%, Cu(II) 11.05%, and Ni(I) 0.13%. Therefore, the chitosan-based adsorbent CS-PN has extremely strong selectivity for Cr(VI). Adsorbent desorption and cycling performance: At room temperature, 10 mg of chitosan-based adsorbent CS-PN and Cr(VI) solutions (pH=3, 10 mL, 350 mg / L) were added to a 15 mL centrifuge tube and shaken at 200 rpm for 20 h. The chitosan-based adsorbent was then separated by centrifugation, and the supernatant was obtained. XPS, SEM, and EDS analyses were performed on the chitosan-based adsorbent CS-PN after Cr(VI) adsorption. The XPS image showed a characteristic peak for Cr, the SEM image showed that Cr was uniformly distributed on the chitosan-based adsorbent, and the EDS analysis showed that the Cr content reached a certain level. ICP-OES determination was also performed. The remaining Cr(VI) concentration in the supernatant was 163.464 mg / L, and the adsorption capacity of chitosan-based adsorbent CS-PN for Cr(VI) was 180.8207 mg / g, with an adsorption efficiency of 52.52%. After adsorbing Cr(VI), the chitosan-based adsorbent CS-PN was eluted with sodium thiosulfate desorption solution (40 mL) for 24 h. After centrifugation, the chitosan-based adsorbent was washed with distilled water until the solution was neutral to complete the regeneration of chitosan-based adsorbent CS-PN. After 5 repeat experiments, the adsorption capacity of Cr(VI) decreased by only 18.53 mg / g in the 5th adsorption.

[0025] Example 3: A chitosan-based adsorbent, denoted as CS-PN, is prepared by cross-linking chitosan with polyethyleneimine and glutaraldehyde to form an intermediate, and then functionalizing the intermediate with dimethyl chlorophosphate under the catalysis of triethylamine. The preparation method of the chitosan-based adsorbent includes the following specific steps: (1) Chitosan was dissolved in acetic acid solution to obtain a chitosan solution with a concentration of 0.06 g / mL. Polyethyleneimine and glutaraldehyde were added to the chitosan solution and mixed evenly. The mixture was reacted at 60 °C for 2.5 h and then freeze-dried to obtain the intermediate product CS-P. The molar ratio of chitosan, polyethyleneimine and glutaraldehyde was 1:1.5:1.3. The structural formula of the intermediate product CS-P is as follows: ; (2) The intermediate product CS-P was dissolved in dichloromethane to obtain a CS-P solution. Dimethyl chlorophosphate and triethylamine were added to the chitosan solution and mixed evenly. The mixture was reacted at 60°C for 14 hours, washed, and freeze-dried to obtain the chitosan-based adsorbent CS-PN. The mass ratio of the intermediate product CS-P to dimethyl chlorophosphate was 2.5:1, and the amount of triethylamine added was 36.3 wt.% of dimethyl chlorophosphate. The structural formula of the chitosan-based adsorbent CS-PN is: ; Adsorption performance test of chitosan-based adsorbent CS-PN for Cr(VI): K₂Cr₂O₇ solutions with different initial concentrations (150-850 mg / L) were prepared, and the pH was adjusted to 3 with HCl or NaOH. 10 mL of Cr(VI) solution was added to several 15 mL centrifuge tubes, followed by 10 mg of the chitosan-based adsorbent CS-PN prepared in this example. The tubes were shaken at 200 rpm in isothermal shakers at 298 K, 308 K, and 318 K until adsorption equilibrium was reached (24 h). Furthermore, a set of kinetic studies was conducted at 298 K according to the above steps at different time points (10 min-480 min), and samples were filtered to determine the residual Cr(VI) concentration in the filtrate and calculate the adsorption capacity. The results showed that the maximum adsorption capacity reached 280.55 mg / g, and the adsorption behavior conformed to the RP isothermal model and the pseudo-second-order kinetic model, indicating that the chitosan-based adsorbent CS-PN mainly exhibits monolayer chemisorption. Selective adsorption performance in the presence of competing ions: At 298 K, 40 mg of chitosan-based adsorbent CS-PN was added to simulated adsorption solutions (Cr(VI) 43.85 mg / L, Mg(II) 102.57 g / L, Zn(II) 100.51 mg / L, Mn(II) 102.82 mg / L, Cu(II) 100.11 mg / L, Ni(I) 106.31 mg / L) and actual adsorption solutions (Cr(VI) 40.2 mg / L, Mg(II) 184.83 g / L, Zn(II) 122.72 mg / L, Mn(II) 116.17 mg / L, Cu(II) 117.01 mg / L, Ni(I) 227.93 mg / L). The adsorbent was separated by centrifugation after shaking at 200 rpm for 24 h. The supernatant was obtained, and the residual concentration of the remaining metal ions in the supernatant was determined by ICP-OES. The removal rates of Cr(VI) in the simulated adsorption solution were 51.41%, Mg(II) 0.10%, Zn(II) 0.19%, Mn(II) 0.60%, Cu(II) 9.4%, and Ni(I) 0.38%. The removal rates of Cr(VI) in the actual adsorption solution were 77.68%, Mg(II) 0.73%, Zn(II) 2.43%, Mn(II) 1.25%, Cu(II) 10.05%, and Ni(I) 0.21%. It can be seen that the chitosan-based adsorbent CS-PN has extremely strong selectivity for Cr(VI). Adsorbent desorption and cycling performance: At room temperature, 10 mg of chitosan-based adsorbent CS-PN and Cr(VI) solutions (pH=3, 10 mL, 350 mg / L) were added to a 15 mL centrifuge tube and shaken at 200 rpm for 20 h. The chitosan-based adsorbent was then separated by centrifugation, and the supernatant was obtained. The chitosan-based adsorbent CS-PN after Cr(VI) adsorption was analyzed by XPS, SEM, and EDS. The XPS image showed a characteristic peak of Cr, the SEM image showed that Cr was uniformly distributed on the chitosan-based adsorbent, and the EDS analysis could determine the Cr content. The supernatant was analyzed by ICP-OES. The remaining Cr(VI) concentration in the solution was 160.9388 mg / L, and the adsorption capacity of chitosan-based adsorbent CS-PN for Cr(VI) was 183.4195 mg / g, with an adsorption efficiency of 46.73%. After adsorbing Cr(VI), the chitosan-based adsorbent CS-PN was eluted with sodium thiosulfate desorption solution (40 mL) for 24 h. After centrifugation, the chitosan-based adsorbent was washed with distilled water until the solution was neutral to complete the regeneration of chitosan-based adsorbent CS-PN. After 5 repeat experiments, the adsorption capacity of Cr(VI) decreased by only 16.23 mg / g in the 5th adsorption. The specific embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A chitosan-based adsorbent, characterized in that, Using chitosan as a matrix, an intermediate is formed by crosslinking with polyethyleneimine and glutaraldehyde. This intermediate is then functionalized with dimethyl chlorophosphate under triethylamine catalysis. The resulting product is denoted as CS-PN, and its structural formula is as follows: 。 2. The method for preparing the chitosan-based adsorbent according to claim 1, characterized in that, The specific steps are as follows: (1) Chitosan was dissolved in acetic acid solution to obtain chitosan solution. Polyethyleneimine and glutaraldehyde were added to chitosan solution and mixed evenly. The mixture was reacted at 50~60℃ for 1.5~2.5h and then freeze-dried to obtain intermediate product CS-P. (2) The intermediate product CS-P was dissolved in dichloromethane to obtain a CS-P solution. Dimethyl chlorophosphate and triethylamine were added to the chitosan solution and mixed evenly. The mixture was reacted at 50~60℃ for 10~14h, washed, and freeze-dried to obtain the chitosan-based adsorbent CS-PN.

3. The method for preparing the chitosan-based adsorbent according to claim 2, characterized in that: Step (1) The molar ratio of chitosan, polyethyleneimine and glutaraldehyde is 1:1~1.5:1~1.

3.

4. The method for preparing the chitosan-based adsorbent according to claim 2, characterized in that: In step (2), the mass ratio of intermediate product CS-P to dimethyl chlorophosphate is 2~2.5:1, and the amount of triethylamine added is 36.3 wt. of dimethyl chlorophosphate.

5. The application of the chitosan-based adsorbent according to claim 1 in the selective adsorption of Cr(VI) in chromium-containing wastewater.

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