Application of Aerogel Beads in Treatment of Uranium-Containing Wastewater

Abstract

The present invention provides an application of aerogel beads in the treatment of uranium-containing wastewater. The aerogel beads are polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads, namely PVA-SA / CS-SH aerogel beads. The PVA-SA / CS-SH aerogel beads provided by the present invention for uranium removal have a strong adsorption capacity, and the removal amount of uranyl ions is as high as 350.33 mg g-1, showing excellent adsorption performance. Moreover, the PVA-SA / CS-SH provided by the present invention has good reusability and adsorption selectivity for uranyl ions, and its cost is low and economical, and it is expected to become a new type of efficient uranium removal adsorbent.

Description

Technical Field

[0001] The present invention belongs to the field of chemistry, and particularly relates to the application of aerogel beads in the treatment of uranium-containing wastewater. Background Art

[0002] In recent years, nuclear energy, as a safe, stable and efficient clean energy, has been applied all over the world. In the large-scale use of nuclear energy, the mining of uranium ore and the discharge of nuclear fuel have caused a large amount of uranium-containing wastewater to enter nature. Uranium in nuclear wastewater generally exists in the form of hexavalent, which can cause radiation in the human body and damage various organs of the human body, and even cause cancer. Uranium existing in nature, due to its radioactivity and heavy metal toxicity, will cause very serious harm to the environment and humans. Therefore, it is very necessary to develop a safe and efficient method to remove uranium from wastewater.

[0003] At present, the methods for removing uranium from aqueous solutions mainly include the following several, such as photocatalysis, solvent extraction, membrane separation, bioremediation, electrochemistry and adsorption methods, etc. Among these methods, the adsorption method has become a commonly used and effective method because of its simple equipment, convenient operation, low cost and large adsorption capacity. Commonly used adsorbents include polymer adsorbents, biosorbents and porous materials, etc. Among them, porous materials are widely used in the adsorption field because of their excellent pore structures and active sites conducive to adsorption. For example, layered double metal hydroxides, organic covalent materials and aerogels. Aerogel is a new type of three-dimensional macro-porous material, with characteristics such as high surface area, high porosity, good expansibility and fast mass transfer rate, and is suitable for removing pollutants and impurities in aqueous solutions, so it can be used as an effective and efficient adsorbent. Natural polymers represented by polysaccharides are widely used in the preparation of aerogels because their long chains carry rich functional groups and cross-linkable properties. Among them, chitosan (CS) and sodium alginate (SA) are both natural polysaccharides, with rich active functional groups, biodegradability and good biocompatibility, and are widely used in the adsorption field. However, the mechanical strength of pure chitosan aerogel and pure sodium alginate aerogel is low, the acid resistance is weak, and the stability is poor, which greatly limits their application in adsorbing heavy metal ions in wastewater.

[0004] Y.-R. He et al. published the paper "Aerogel based on melamine-formaldehyde and alginate: Simply removing of uranium from aqueous solutions" in the Journal of Molecular Liquids in 2019, which reported the preparation of a melamine-formaldehyde / alginate (MF-A) aerogel, obtaining a bulk structure with a porous network and good mechanical stability, and used it for adsorbing U(VI) from aqueous solutions. Therefore, chitosan can be used as the polycationic component and sodium alginate as the polyanionic component, and the two are crosslinked to form gel beads, which not only obtain a large number of active groups but also a porous internal space structure. Other functional groups can also be grafted onto the surfaces of chitosan and sodium alginate for surface modification to enrich the active sites. Moreover, the gel beads prepared by crosslinking are also more easily separated from the solution after adsorption and can be recycled after adsorption-desorption. However, the preparation cost of this aerogel is relatively high, and the adsorption performance of this aerogel for uranyl ions is not high enough.

[0005] Therefore, there is a need in the art for a new type of aerogel beads to be applied in the treatment of uranium-containing wastewater. Summary of the Invention

[0006] First, using chitosan and sodium alginate as raw materials, (3-mercaptopropyl)trimethoxysilane is grafted to introduce mercapto functional groups, polyvinyl alcohol is used, and the gel-vacuum freeze-drying method is adopted to prepare polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads (PVA-SA / CS-SH).

[0007] The present invention provides an application of aerogel beads in the treatment of uranium-containing wastewater, and the aerogel beads are polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads, that is, PVA-SA / CS-SH aerogel beads.

[0008] In a specific embodiment, when the aerogel beads are used for the treatment of uranium-containing wastewater, the pH value of the wastewater is 5-7, and the wastewater treatment temperature is 35-50°C, preferably 40-50°C.

[0009] In a specific embodiment, when the aerogel beads are used for the treatment of uranium-containing wastewater, the adsorption treatment time is more than 5 h, preferably 5-10 h.

[0010] In a specific embodiment, when the aerogel beads are used for the treatment of uranium-containing wastewater, the initial concentration C 0 of uranyl ions in the wastewater is 30-70 mg / L.

[0011] In a specific embodiment, when the aerogel beads are used for treating uranium-containing wastewater, the solid-liquid ratio of PVA-SA / CS-SH aerogel beads to the wastewater is 0.1-0.15 g / L.

[0012] In a specific embodiment, when the aerogel beads are used for treating uranium-containing wastewater, Cu in the wastewater is removed first. 2+ , Fe 3+ , CO 3 2- and SO 4 2- , and then the aerogel beads are brought into contact with the uranium-containing wastewater for uranium adsorption.

[0013] In a specific embodiment, when the aerogel beads are used for treating uranium-containing wastewater, the PVA-SA / CS-SH aerogel beads are added to the wastewater, and reciprocating oscillation adsorption is adopted. Preferably, the oscillation adsorption speed is 100-150 rpm.

[0014] In a specific embodiment, the PVA-SA / CS-SH aerogel beads are prepared by the following method and steps:

[0015] Step A: First, prepare benzaldehyde-chitosan, i.e., BCS, using benzaldehyde and chitosan.

[0016] Step B: Then, prepare benzaldehyde-thiol-functionalized chitosan, i.e., BCS-SH.

[0017] Step C: Then, prepare thiol-functionalized chitosan, i.e., CS-SH.

[0018] Step D: Use polyvinyl alcohol, sodium alginate, and CS-SH prepared in Step C to prepare polyvinyl alcohol-sodium alginate / thiol-functionalized chitosan aerogel beads, i.e., PVA-SA / CS-SH.

[0019] In a specific embodiment, in Step A, chitosan is dissolved in an acetic acid solution, and methanol is added. After the chitosan is completely dissolved, a mixed solution of benzaldehyde and methanol is added; the reaction is carried out fully at room temperature to obtain a pale yellow gel; the gel is washed several times with methanol and absolute ethanol respectively to wash away the unreacted benzaldehyde, and then it is dried to obtain amino-protected chitosan, i.e., BCS.

[0020] In Step B, the BCS prepared in Step A is dispersed in toluene, (3-mercaptopropyl)trimethoxysilane is added to obtain a mixture, the mixture is ultrasonically dispersed, and then a hydrothermal reaction is carried out; after the reaction is complete, it is naturally cooled to room temperature, and the obtained solid reactant is washed several times with toluene and absolute ethanol respectively, and dried to obtain BCS-SH.

[0021] In step C, the BCS-SH prepared in step B is hydrolyzed with hydrochloric acid to remove the benzaldehyde used to protect the amino group and the chitosan that has not been successfully grafted with the mercapto group. After washing several times with absolute ethanol and deionized water, it is dried to obtain CS-SH;

[0022] In step D, first dissolve polyvinyl alcohol and sodium alginate in deionized water, heat and stir to form a homogeneous mixture; then transfer it to a low-temperature water bath, add the CS-SH prepared in step C and stir. After complete dissolution, it is naturally cooled to room temperature to obtain a mixture; the mixture is dropped into a calcium chloride solution to crosslink and form gel beads; the gel beads are soaked in the calcium chloride solution, and then washed several times with deionized water to remove the uncrosslinked calcium chloride; then the gel beads are frozen and vacuum freeze-dried to obtain the PVA-SA / CS-SH.

[0023] In a specific embodiment, in step A, the concentration of the acetic acid solution is 1-3%, the reaction time to obtain a light yellow gel at room temperature is more than 24 h, the drying temperature of the gel is 60-70 °C, and the drying time is more than 10 h;

[0024] In step B, the ultrasonic dispersion time of the mixture is more than 10 min. The hydrothermal reaction includes loading the mixture into a hydrothermal reaction kettle with a polytetrafluoroethylene inner liner and reacting at 105-120 °C for more than 12 h. The drying is carried out at 60-70 °C, and the drying time is more than 6 h;

[0025] In step C, the concentration of hydrochloric acid is 0.2-1 mol / L, the temperature for hydrolysis with hydrochloric acid is 50-70 °C, the hydrolysis time is more than 6 h, the drying is carried out at 60-70 °C, and the drying time is more than 6 h;

[0026] In step D, water bath heating is used during heating and stirring, and the water bath heating temperature is 85-95 °C for more than 1 h; the temperature of the low-temperature water bath is 75-82 °C, and the time for adding CS-SH and stirring is more than 3 h. It is preferred to use a syringe to drop the mixture into the calcium chloride solution. The concentration of the calcium chloride solution is 2-5 wt%, and the soaking time of the gel beads in the calcium chloride solution is more than 5 h. Preferably, the freezing temperature is -35 °C to -25 °C, the freezing time is more than 2 h, and the vacuum freeze-drying temperature is -55 °C to -45 °C for more than 6 h.

[0027] The PVA-SA / CS-SH aerogel beads for uranium removal provided by the present invention have strong adsorption capacity, and the removal amount of uranyl ions is as high as 350.33 mg g -1, showing excellent adsorption performance. Moreover, the PVA-SA / CS-SH provided by the present invention has good reusability and adsorption selectivity for uranyl ions, with low cost and economic benefits, and is expected to become a new type of efficient uranium removal adsorbent. Description of the Drawings

[0028] Figure 1 Shows the macroscopic and microscopic structures of the PVA-SA / CS-SH gel beads prepared in Example 1, where Figure 1 a is the gel bead before vacuum freeze-drying, Figure 1 b is the gel bead after vacuum freeze-drying, Figure 1 c is the SEM image of the gel bead, Figure 1 d is the SEM image characterizing the internal structure of the gel bead.

[0029] Figure 2 Is the EDX analysis result diagram of PVA-SA / CS-SH and PVA-SA / CS-SH+U, where Figure 2 a is the gel bead before uranium adsorption, Figure 2 b is the gel bead after uranium adsorption.

[0030] Figure 3 Is the thermogravimetric curve (TGA) of the PVA-SA / CS-SH gel bead.

[0031] Figure 4 Shows the isotherm and pore size distribution of the PVA-SA / CS-SH gel bead, where Figure 4 a is the adsorption-desorption isotherm of the gel bead, Figure 4 b is the pore size distribution of the gel bead.

[0032] Figure 5 Is the XRD spectrum of CS, BCS, BCS-SH, CH-SH and PVA-SA / CS-SH.

[0033] Figure 6 Is the FTIR spectrum of CS, BCS, BCS-SH, CH-SH and PVA-SA / CS-SH.

[0034] Figure 7 Is the adsorption effect of the gel bead on uranium under different pH conditions and its Zeta potential diagram. Among them, Figure 7 a is the adsorption effect of the PVA-SA / CS-SH gel bead and the sodium alginate / chitosan gel bead (SA / CS) on uranium under different pH conditions. Figure 7 b is the Zeta potential diagram of the PVA-SA / CS-SH gel bead under different pH conditions.

[0035] Figure 8Schematic diagram of PVA-SA / CS-SH adsorbing U(VI) at different contact times. Figure 8 a shows the effect of contact time on the adsorption rate change of U(VI) by PVA-SA / CS-SH. Figure 8 b is the linear fitting of the intraparticle diffusion curve.

[0036] Figure 9 Effect of coexisting ions during the uranium adsorption experiment with PVA-SA / CS-SH aerogel beads.

[0037] Figure 10 Structural diagram of the five-cycle adsorption-desorption experiment on PVA-SA / CS-SH.

[0038] Figure 11 EDS mapping and EDS characterization diagrams of PVA-SA / CS-SH aerogel beads after uranium adsorption, where Figure 11 a is the EDS mapping diagram, Figure 11 b is the EDS characterization diagram.

[0039] Figure 12 FTIR spectra and full XPS spectra of PVA-SA / CS-SH before and after uranium adsorption. Figure 12 a is the FTIR spectrum, Figure 12 b is the XPS spectrum. Detailed implementation mode

[0040] Example 1

[0041] This example is for the preparation of PVA-SA / CS-SH aerogel beads.

[0042] Materials: Chitosan (degree of deacetylation: 95%), acetic acid, hydrochloric acid, and benzaldehyde were all purchased from Beijing Chemical Reagent Co., Ltd., Sinopharm Group. Polyvinyl alcohol (degree of alcoholysis: 98.0 - 99.0 mol%, viscosity: 3.2 - 3.8 mPa·s), sodium alginate (90%), (3-mercaptopropyl)trimethoxysilane, methanol, and absolute ethanol were purchased from Shanghai Macklin Biochemical Co., Ltd. Toluene was purchased from Tianjin Tianli Technology Company. Uranyl nitrate hexahydrate (UO 2 (NO 3 ) 2 ·6H 2 O) was purchased from Hubei Chushengwei Chemical Co., Ltd. All reagents were of analytical grade. Deionized water was prepared in the laboratory. The preparation method of the PVA-SA / CS-SH aerogel beads includes the following steps.

[0043] 1. Preparation of benzaldehyde-chitosan (BCS)

[0044] Dissolve 5.0g chitosan (CS) in 160mL 2% acetic acid solution, and add 100mL methanol. After the chitosan is completely dissolved, add a mixture of 40mL benzaldehyde and 100mL methanol. Fully react at room temperature for 24h to obtain a light yellow gel. Wash the gel several times with methanol and anhydrous ethanol to wash away the unreacted benzaldehyde, and then dry it at 65°C for 12h to obtain amino-protected chitosan - benzaldehyde-chitosan (BCS). This can prevent the amino groups in chitosan from being consumed in large quantities during the reaction of grafting thiol groups.

[0045] 2. Preparation of benzaldehyde-thiol functionalized chitosan (BCS-SH)

[0046] 1.5 g of benzaldehyde-chitosan (BCS) was dispersed in 30 mL of toluene, and 3 mL of (3-mercaptopropyl) trimethoxysilane was added. The mixture was ultrasonically dispersed for 15 min, and then placed in a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 110 ° C for 24 h. After cooling naturally to room temperature, the reactants were washed several times with toluene and anhydrous ethanol, respectively, and dried at 65 ° C for 12 h to obtain benzaldehyde-thiol functionalized chitosan (BCS-SH).

[0047] 3. Preparation of thiol-functionalized chitosan (CS-SH)

[0048] 1.0g of thiolated benzaldehyde-chitosan (BCS-SH) was hydrolyzed with 50mL 0.5mol / L hydrochloric acid at 60℃. After 12h of hydrolysis, the benzaldehyde used to protect the amino groups and the chitosan that was not successfully grafted with thiol groups were removed. After washing with anhydrous ethanol and deionized water several times, it was dried at 65℃ for 12h to obtain thiol-functionalized chitosan (CS-SH).

[0049] 4. Preparation of polyvinyl alcohol-sodium alginate / thiol-functionalized chitosan aerogel beads (PVA-SA / CS-SH)

[0050] 1) Dissolve 2.0 g polyvinyl alcohol (PVA) and 0.5 g sodium alginate (SA) in 60 mL deionized water and heat in a water bath at 90°C for 2 h to form a uniform mixture. Then transfer to a water bath at 80°C, add 0.5 g thiol-functionalized chitosan (CS-SH), and stir magnetically for 6 h. After complete dissolution, cool naturally to room temperature.

[0051] 2) The mixture was dropped into 100 mL of 3 w% calcium chloride solution with a syringe to form gel beads by crosslinking. The gel beads were soaked in 3 w% calcium chloride solution overnight, and then washed several times with deionized water to remove the uncrosslinked calcium chloride. The gel beads were frozen at -30 °C for 4 h and finally freeze-dried under vacuum at -51 °C for 12 h to obtain polyvinyl alcohol-sodium alginate / thiol-functionalized chitosan aerogel beads (PVA-SA / CS-SH).

[0052] Example 2

[0053] This example characterized the PVA-SA / CS-SH aerogel beads prepared in Example 1 above.

[0054] 1. Characterization methods and equipment:

[0055] A field emission scanning electron microscope (SEM, Apreo 2) was used to observe the morphology and microstructure of the aerogel beads. An X-Max EDS instrument (Oxford, UK) was used for elemental analysis of the aerogel beads. The Fourier transform infrared spectrum of the aerogel beads was measured on a Nicolet-iS10 FTIR spectrometer (Thermo Fisher Scientific, USA) in the range of 4000 - 400 cm -1 -1. The XRD analysis of the aerogel beads was carried out using a smartlab9 X-ray diffractometer (Kα radiation, copper target, Rigaku Corporation, Japan). The specific surface area of the aerogel beads was measured using Brunauer-Emmett-Teller (BET, Micro for TriStar IIPlus 3030). The surface potential of the aerogel beads was detected using ZS-90 (Malvern, UK). Thermogravimetric analysis of the aerogel beads was performed using a NETZSCH STA 449F3 instrument (NETZSCH, Germany). The XPS spectrum of the aerogel beads was measured by a Thermo Fisher Scientific Escalab 250Xi spectrometer in the USA. The concentration of uranium in the solution was determined using a double-beam ultraviolet-visible spectrophotometer (Shanghai Youke Instrument Co., Ltd.).

[0056] Figure 1 shows the macroscopic and microscopic structures of the PVA-SA / CS-SH gel beads prepared in Example 1, where Figure 1 a is the gel bead before vacuum freeze-drying, Figure 1 b is the gel bead after vacuum freeze-drying, Figure 1 c is the SEM image of the gel bead, Figure 1 d is the SEM image characterizing the internal structure of the gel bead.

[0057] From Figure 1As can be seen, PVA-SA / CS-SH is a white colloidal sphere with uniform size and a diameter of about 2 mm( Figure 1 a). After vacuum freeze-drying, the shape of PVA-SA / CS-SH is basically unaffected, the volume is slightly smaller, and the surface roughness increases( Figure 1 b). The SEM image of PVA-SA / CS-SH is as Figure 1 shown in c. The surface of the gel beads is rough and uneven. Figure 1 d is the electron micrograph characterizing the internal structure of the gel beads. It can be observed that there are a large number of groove structures inside the gel beads, resulting in wrinkled and curved surfaces inside, which is beneficial to the multi-channel diffusion of uranium solution. This structure provides sufficient contact positions, increases the adsorption surface area, improves the adsorption capacity, and helps the PVA-SA / CS-SH gel beads to rapidly adsorb U(VI).

[0058] Figure 2 Figure shows the EDX analysis results of PVA-SA / CS-SH and PVA-SA / CS-SH+U, where Figure 2 a is the gel beads before uranium adsorption, Figure 2 b is the gel beads after uranium adsorption. Elemental analysis of the samples was carried out, and the EDX analysis results of PVA-SA / CS-SH and PVA-SA / CS-SH+U are as Figure 2 shown. Figure 2 In the analysis diagram of PVA-SA / CS-SH in a, the elements Si and S appear, indicating that the grafting of (3-mercaptopropyl)trimethoxysilane is successful and the mercapto functional group is introduced into the material. In addition, the element Ca appears, indicating that Ca 2+ exchanges ions with Na + and the material crosslinks successfully to form gel beads. Figure 2 b is the EDX characterization result after PVA-SA / CS-SH adsorbs U(VI). It can be seen that the element U is detected in the characterization result, indicating that PVA-SA / CS-SH successfully adsorbs U(VI) during the adsorption process.

[0059] Figure 3 Figure shows the thermogravimetric curve (TGA) of PVA-SA / CS-SH gel beads. Figure 3The abscissa represents temperature, the left ordinate represents weight, corresponding to the solid line curve, and the right ordinate represents DTG, corresponding to the dashed line curve. This solid line curve is mainly divided into three stages. The first stage is the decomposition of crystal water and the evaporation of adsorbed water, which occurs at 30 - 170 °C. The second stage occurs at 214.45 - 308.22 °C, with a sharp mass loss, mainly attributed to the breaking of chemical bonds of organic reagents and the loss of organic functional groups. In the third stage, the mass continues to decrease (308.22 - 501.06 °C), which may be due to the chain-breaking decomposition of the SA and CS structures and the large-scale thermal decomposition of the polymer backbone. The remaining residue is mainly the inorganic salt of sodium alginate.

[0060] Figure 4 shows the isotherm and pore size distribution of PVA - SA / CS - SH gel beads, where Figure 4 a is the adsorption - desorption isotherm of the gel beads, Figure 4 b is the pore size distribution of the gel beads. Figure 4 In a, the abscissa is the relative pressure and the ordinate is the adsorption volume. Figure 4 In b, the abscissa is the pore size in nm, and the ordinate is the pore volume in cm 3 / g. The BET surface area of PVA - SA / CS - SH gel beads is calculated from the nitrogen adsorption / desorption isotherm. As can be seen from Figure 4 a, PVA - SA / CS - SH gel beads belong to type Ⅳ isotherm, showing an H3 - type hysteresis loop. A monolayer adsorption layer is formed in the low - pressure region, and then the monolayer gradually reaches saturation and multi - layer adsorption begins. The H3 - type hysteresis loop indicates that the pore structure of the material is formed by the stacking of layered structures. The specific surface area of PVA - SA / CS - SH gel beads is 5.0680 m 2 / g, the total pore volume calculated by the single - point method is 0.0159 cm 3 / g, and the pore size distribution is between 2 - 10 nm, with an average pore size of 7.4747 nm. This indicates that PVA - SA / CS - SH gel beads are mesoporous - dominated adsorption materials, providing a good mass transfer environment for the adsorption of U(VI).

[0061] Figure 5 are the XRD spectra of CS, BCS, BCS - SH, CH - SH, and PVA - SA / CS - SH. Figure 5The horizontal coordinate is the degree of 2θ, and the vertical coordinate is the intensity. The five curves from top to bottom in the figure are the XRD spectra of CS, BCS, BCS-SH, CH-SH, and PVA-SA / CS-SH, respectively. Among them, the characteristic peak of CS appears at 20.3°, and there is no obvious characteristic peak near 10°. The reason is that the chitosan used in this experiment is of high deacetylation degree (95%). This corresponds to the Form II crystal form of chitosan, indicating strong crystallinity. BCS is chitosan grafted with benzaldehyde. The characteristic peak near 20° becomes wider and the intensity decreases. This is because the reaction of -NH 2 of chitosan with benzaldehyde causes a decrease in the hydrogen bond content and a decrease in the crystallinity of the material. Compared with the XRD patterns of CS, BCS, BCS-SH, and CH-SH, the XRD pattern of PVA-SA / CS-SH changes significantly. Small characteristic peaks appear at 2θ = 32.04° and 45.82°. Other miscellaneous peaks disappear, and the main characteristic peak shifts from 20° to 22°. This can be attributed to the characteristic peak of sodium alginate. Moreover, the characteristic peak becomes wider and the intensity decreases. It may be that part of sodium alginate cross-links with Ca 2+ and the other part cross-links with CS. The XRD pattern of PVA-SA / CS-SH shows a new configuration, and interactions occur among polyvinyl alcohol, sodium alginate, and mercaptochitosan to form a new structure.

[0062] Figure 6 are the FTIR spectra of CS, BCS, BCS-SH, CH-SH, and PVA-SA / CS-SH. Figure 6 In it, the horizontal coordinate is the wave number, and the vertical coordinate is the transmittance. The curves from bottom to top in the figure are CS, BCS (or BACS), BCS-SH (or BACS-SH), CH-SH, and PVA-SA / CS-SH, respectively. The functional groups of CS, BCS, BCS-SH, CH-SH, and PVA-SA / CS-SH gel beads are further analyzed by FTIR spectra. The broad peak at 3411 cm -1 in CS corresponds to the superimposed peak of -OH stretching vibration and -NH 2 stretching vibration. The absorption peaks at 2923 and 2876 cm -1 are due to C-H symmetric and asymmetric stretching vibrations. The characteristic peaks at 1653 and 1596 cm -1 correspond to amide-I (C=O stretching vibration) and amide-II (N-H bending vibration) groups. In BCS, the characteristic peaks corresponding to the benzene ring appear at 758 and 690 cm -1 , and at the same time, the characteristic peak appearing at 1598 cm -1 corresponds to C=N, indicating that the C 2 -NH 2It undergoes a Schiff base reaction with benzaldehyde. The characteristic peaks of the above-mentioned benzaldehyde also appear in the spectrum of BCS-SH, and new characteristic peaks appear at 484 cm -1 for BCS-SH, CH-SH, and PVA-SA / CS-SH, corresponding to the bending vibration of Si-O-Si. Among them, the characteristic peaks at 2349 cm -1 for CH-SH and PVA-SA / CS-SH correspond to the stretching vibration peak of -SH. These results indicate that -SH has been successfully grafted onto the material. In the spectra of CH-SH and PVA-SA / CS-SH, the characteristic peaks at 758, 690, and 1598 cm -1 have disappeared, indicating that the benzaldehyde used to protect the amino group has been successfully removed. The above analysis shows that the PVA-SA / CS-SH gel beads have been successfully prepared.

[0063] Example 3

[0064] In this example, the uranium adsorption experiment was carried out on the PVA-SA / CS-SH aerogel beads prepared in Example 1 above, and the influence of pH was specifically investigated.

[0065] Dissolve 2.1091 g of uranyl nitrate hexahydrate in deionized water, add 10 mL of 0.1 mol / L HNO 3 , and make up the volume to 1 L with deionized water to obtain a 1 g / L uranium stock solution. In subsequent experiments, the uranium stock solution was diluted to 50 mg / L, and 0.1 M HNO 3 or NaOH was used to adjust the pH (2 - 10) of the solution. Add a certain amount of PVA-SA / CS-SH to a conical flask containing the U(VI) solution. All adsorption experiments were carried out in a reciprocating water bath shaker at 130 rpm. After reaching the adsorption equilibrium, the solid and liquid phases were separated with a 0.22 μm microfiltration membrane. Using arsenazo (III) as the chromogenic agent, the absorbance of the supernatant was measured at 652 nm with a UV-visible spectrophotometer, and thus the U(VI) concentration was calculated. The removal amount q e (mg / g) and removal rate R of the adsorbent for U(VI) were calculated by the following two formulas:

[0066]

[0067]

[0068] where C 0 and C e are the initial concentration and equilibrium concentration of U(VI) ions (mg L -1 ), respectively. V is the volume of the U(VI) solution (L), and m is the amount of the adsorbent (g).

[0069] Figure 7Adsorption effect of gel beads on uranium under different pH conditions and its Zeta potential diagram. Among them, Figure 7 a shows the adsorption effect of PVA-SA / CS-SH gel beads and sodium alginate / chitosan gel beads (SA / CS) on uranium under different pH conditions. Among them, the upper curve is the curve of PVA-SA / CS-SH gel beads, and the lower curve is the curve of SA / CS gel beads. The pH value of the initial solution is the most important factor in the adsorption process of U(VI) on PVA-SA / CS-SH gel beads. Therefore, in this invention, the adsorption capacity of PVA-SA / CS-SH for U(VI) was explored in the range of pH = 2 - 10 of the initial solution, and compared with sodium alginate / chitosan gel beads (SA / CS). V from Figure 7 As can be seen from a, the adsorption amount of gel beads crosslinked by SA and CS without group modification and material modification for U(VI) is not high. In contrast, PVA-SA / CS-SH has excellent adsorption effect on U(VI). It can be seen that PVA-SA / CS-SH has the strongest adsorption capacity for U(VI) at pH = 6, and the adsorption amount reaches 340.51 mg / g. In the high acid region, the adsorption amount of PVA-SA / CS-SH is relatively low. As the pH increases, the adsorption amount also increases continuously and reaches the maximum value at pH = 6.

[0070] Figure 7 b is the Zeta potential diagram of PVA-SA / CS-SH gel beads under different pH conditions, and its ordinate is the Zeta potential value. From Figure 7 b, it can be known that the Zeta potential of PVA-SA / CS-SH is positive in the range of pH = 2 - 7, which indicates that the surface of PVA-SA / CS-SH gel beads is positively charged. This phenomenon can be attributed to the existence of a large number of H + in a strongly acidic environment, resulting in the protonation of the -NH 2 at the surface active center of PVA-SA / CS-SH In this pH range, uranium mainly exists in the form of UO 2 2+ , so there is electrostatic repulsion between PVA-SA / CS-SH and UO 2 2+ , affecting its adsorption capacity. As the pH increases, -NH 3 + undergoes deprotonation and changes back to -NH 2 . At this time, the content of OH - in the solution increases, forming coordination compounds with uranyl ions, such as UO 2 (OH) + , (UO 2 ) 2 (OH) 22+ and (UO 2 ) 3 (OH) 5 + , the adsorption capacity increases. When the pH value continues to increase, the uranyl ion complex changes again and becomes a negatively charged group, such as UO 2 (OH) 3 - and (UO 2 ) 3 (OH) 7 - and so on. And the Zeta of PVA-SA / CS-SH gradually becomes negative potential, the surface of the gel beads begins to carry negative charges, and there is electrostatic repulsion with uranyl ions, resulting in a low adsorption amount in the high-alkali region.

[0071] Example 4

[0072] In this example, a uranium adsorption experiment was carried out on the PVA-SA / CS-SH aerogel beads prepared in Example 1 above, and the influence of contact time was specifically investigated.

[0073] Figure 8 is a schematic diagram of the adsorption of U(VI) by PVA-SA / CS-SH at different contact times. Figure 8 a shows the influence of contact time on the change rate of U(VI) adsorption by PVA-SA / CS-SH. Figure 8 b is the linear fitting of the intraparticle diffusion curve. Figure 8 Multiple dots in a represent experimental data. At the inflection point of the curve around 80 min, the curves from top to bottom represent the pseudo-first-order kinetic model, the pseudo-second-order kinetic model, the Elovich model, and the intraparticle diffusion kinetic model. The reaction conditions among them are: C 0 = 50 mg / L, m / V = 0.125 g / L, pH = 6.0, T = 313 K.

[0074] Contact time is a key factor affecting the adsorption rate of the adsorbent for U(VI). By establishing multiple kinetic models to analyze the adsorption process and explore the rate-limiting step of mass transfer in the adsorption process. From Figure 8It can be seen that within 0 - 2 h, the adsorption capacity of PVA - SA / CS - SH for U(VI) increases rapidly. Because in the initial stage, the concentration of U(VI) in the solution is relatively high, which can diffuse rapidly on the surface of the adsorbent, and the surface of the adsorbent can also provide sufficient adsorption active centers. Therefore, the adsorption capacity shows an obvious upward trend. As the contact time increases, U(VI) is continuously captured by the adsorbent, resulting in a decrease in the U(VI) concentration, which reduces the diffusion rate. The adsorption active centers of the adsorbent gradually decrease and cannot provide enough sites. The gel beads stay in the solution for a long time, and the steric hindrance increases, further increasing the mass transfer resistance, leading to a slowdown in the adsorption rate and showing a downward trend. The reaction almost reaches equilibrium at about 6 h. To explain the influence of contact time on the adsorption process of PVA - SA / CS - SH, pseudo - first - order (Eq.3), pseudo - second - order (Eq.4), intraparticle diffusion kinetic model (Eq.5) and Elovich model (Eq.6) are established to simulate the adsorption kinetic data:

[0075]

[0076]

[0077]

[0078]

[0079] where q t (mg / g) is the adsorption capacity at the adsorption time t (min), and q e is the adsorption capacity at adsorption equilibrium. k 1 (min -1 ) and k 2 (g mg -1 min -1 ) are the rate constants of the pseudo - first - order and pseudo - second - order kinetic equations respectively. k ip is the intraparticle diffusion rate constant (mg g –1 min –0.5 ), C i is the boundary layer constant. α (mg g -1 -min) is the initial adsorption rate, and β (g mg -1 ) is related to the surface coverage and the activation energy of chemisorption.

[0080] The fitting results are as Figure 8 shown. Figure 8In a, through the comparison of the pseudo-first-order, pseudo-second-order, intraparticle diffusion kinetic models and Elovich simulation, it can be seen that the fitting coefficient of the pseudo-second-order kinetic model is 0.996, which is the highest among the four models. The theoretical equilibrium adsorption capacity simulated by the pseudo-second-order kinetic model is 358.76 mg / g, which is closer to the actual equilibrium adsorption capacity (350.33 mg / g). This indicates that the pseudo-second-order kinetic model is more suitable for describing the adsorption process of PVA-SA / CS-SH for U(VI). The results show that chemical adsorption is the rate-limiting step of this adsorption process, and the functional groups on PVA-SA / CS-SH complex with U(VI). Figure 8 b is the linear fitting of the intraparticle diffusion curve, which can intuitively observe the steps of intraparticle diffusion. The fitting results show three linear components, indicating that there are three stages in the adsorption process. The three straight lines from left to right in the figure represent the first stage, the second stage, and the third stage respectively. In the first stage, rapid diffusion of U(VI) occurs on the surface of the adsorbent, and the adsorption rate is the fastest. In the second stage, intraparticle diffusion occurs. As the adsorption proceeds, the active sites of the adsorbent are gradually occupied, and the diffusion rate of this process is lower than that of the first stage. The third stage represents the dynamic adsorption equilibrium of adsorption and desorption between U(VI) and PVA-SA / CS-SH gel beads, and the adsorption reaction almost reaches equilibrium. The fitting curve is not a straight line passing through the origin of the coordinate axis, indicating that the adsorption process is not controlled by a single factor of intraparticle diffusion, and other processes are also involved in the rate control of the adsorption process.

[0081] Example 5

[0082] In this example, the uranium adsorption experiment was carried out on the PVA-SA / CS-SH aerogel beads prepared in Example 1 above, and the adsorption isotherm and the maximum adsorption amount of uranium by the aerogel beads were specifically investigated.

[0083] Using the isotherm model can facilitate the exploration of the binding mode between the adsorbent and the adsorbate, as well as the adsorption behavior and mechanism of the adsorbent in the solid-liquid two phases. Three models, the Langmuir, Freundlich, and Dubinin-Raduschkevich (D-R) isotherm models, were established to explore the adsorption behavior and mechanism of PVA-SA / CS-SH in the range of initial uranium concentration from 10 to 100 mg / L.

[0084] Among the three isotherm models, the average fitting coefficient of the Langmuir model is the highest, which is 0.996, far greater than the average fitting coefficients of the Freundlich model (0.975) and the D-R model (0.763). The theoretical maximum adsorption capacity simulated by the Langmuir model is also more consistent with the actual adsorption results. Therefore, it is most suitable for describing the adsorption process of PVA-SA / CS-SH for U(VI).

[0085] Combined with the fitting data of the Langmuir, Freundlich, and D-R isotherm models, it shows that the adsorption process of PVA-SA / CS-SH for U(VI) belongs to homogeneous monolayer chemisorption. At 313 K, the maximum adsorption capacity of PVA-SA / CS-SH for U(VI) simulated by the Langmuir isotherm model is 664.56 mg / g, indicating that PVA-SA / CS-SH is a potential and excellent adsorbent material.

[0086] Example 6

[0087] In this example, a uranium adsorption experiment was carried out on the PVA-SA / CS-SH aerogel beads prepared in Example 1 above, and the adsorption temperature and adsorption thermodynamics were specifically investigated.

[0088] In order to further explore the adsorption mechanism, a thermodynamic analysis of PVA-SA / CS-SH for U(VI) was carried out in the range of 303 - 323 K to explore the change of its adsorption capacity. The experimental results show that increasing the temperature based on 313 K does not significantly improve the adsorption amount. Considering energy conservation and environmental protection, 313 K was selected as the optimal reaction temperature.

[0089] Example 7

[0090] In this example, a uranium adsorption experiment was carried out on the PVA-SA / CS-SH aerogel beads prepared in Example 1 above, and the influence of coexisting ions and the reusability of PVA-SA / CS-SH were specifically investigated.

[0091] Figure 9 The influence of coexisting ions during the uranium adsorption experiment for PVA-SA / CS-SH aerogel beads. The adsorption conditions in the figure are: m / V = 0.125 g / L, pH = 6.0, T = 313 K, t = 300 min. In order to explore the influence of the material's adsorption of U(VI) in the presence of other ions, coexisting ion experiments with different concentrations were carried out. Uranium wastewater is generally in an acidic environment and contains a large amount of Cl - and SO 4 2- , and there are also other cations and anions in addition. Therefore, the following ions were selected for exploration experiments. The results are as Figure 9 shown. Among the anions, Cl - and NO 3 - have almost no influence on the adsorption process. While the presence of CO 3 2- and SO 4 2- reduces the adsorption effect of the material. This is because SO 4 2-will complex with uranyl ions to form coordination compounds, such as (UO 2 ) 4 (OH) 7 (SO 4 ) 4 7- , and will also form insoluble complexes, such as UO 2 SO 4 (aq), which cannot be adsorbed by the adsorbent, resulting in a decrease in the adsorption capacity. Among the cations, K + and Na + have little effect on the adsorption of U(VI). This is because the ionic radii of the above ions are quite different from that of the uranyl ion and cannot form competitive adsorption with the uranyl ion. Cu 2+ and Fe 3+ have the greatest impact on the adsorption effect. This is mainly because the ionic radius of Cu 2+ is similar to that of the uranyl ion, and the two form competitive adsorption, interfering with the adsorption process. For Fe 3+ , the amino and mercapto groups on PVA-SA / CS-SH are prone to coordinate complexation with Fe 3+ , and the active centers of the adsorbent are occupied, inhibiting the adsorption of U(VI). Generally speaking, PVA-SA / CS-SH has good selective adsorption for U(VI).

[0092] Figure 10 is the structural diagram of five cyclic adsorption-desorption experiments on PVA-SA / CS-SH. The adsorption conditions in the figure are: C 0 = 50 mg / L, m / V = 0.125 g / L, pH = 6.0, T = 313 K, t = 300 min. The abscissa in the figure is the number of cycles, the left ordinate is the removal amount of uranium by the adsorbent, corresponding to the bar chart, and the right ordinate is the desorption rate, corresponding to the dot line.

[0093] To explore the reusability of the material, as a consideration of green environmental protection and economic benefits. A 1.0 M HNO 3 solution was used as the desorbent for PVA-SA / CS-SH to test its repeatability. The results are as Figure 10 shown. As the number of adsorption-desorption cycles increases, the adsorption capacity of the material gradually decreases. In the fifth cyclic adsorption-desorption experiment, the removal rate of U(VI) can still reach 81.19%. In multiple cyclic adsorption-desorption experiments, a part of the material itself may undergo acidolysis, thus affecting the overall adsorption effect. The uranyl ion will also combine with the adsorbent, consuming a part of the active sites, resulting in a decrease in the adsorption capacity. The results show that PVA-SA / CS-SH has excellent reusability and good recyclability.

[0094] Example 8

[0095] This example is for the characterization and adsorption mechanism analysis of the PVA-SA / CS-SH aerogel beads prepared in Example 1 above before and after uranium adsorption.

[0096] Figure 11 They are the EDS mapping and EDS characterization diagrams of the PVA-SA / CS-SH aerogel beads after uranium adsorption, where Figure 11 a is the EDS mapping diagram, Figure 11 b is the EDS characterization diagram. The two-dimensional elemental analysis method (EDS-Mapping) was used to characterize and analyze the sample after adsorbing U(VI). In Figure 11 a, it can be clearly seen that uranium elements appear on the material, Figure 11 and the uranium ray peak can also be observed in b, indicating that U(VI) has been successfully adsorbed by the material. In addition, the presence of elements such as sulfur and calcium can be observed in both the EDS mapping and EDS characterization, further indicating the successful preparation of the modified material.

[0097] Figure 12 They are the FTIR spectra and full XPS spectra of PVA-SA / CS-SH before and after adsorption. Figure 12 a is the FTIR spectrum, Figure 12 b is the XPS spectrum. Figure 12 In a, the upper curve is the aerogel beads before uranium adsorption, and the lower curve is the aerogel beads after uranium adsorption. The abscissa in the figure is the wave number, and the ordinate is the transmittance. Figure 12 In b, the upper curve is the aerogel beads after uranium adsorption, and the lower curve is the aerogel beads before uranium adsorption. The abscissa in the figure is the binding energy, and the ordinate is the intensity. In order to explore the adsorption mechanism of PVA-SA / CS-SH for U(VI), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize and analyze PVA-SA / CS-SH before and after adsorbing U(VI). Figure 12 In a, a new characteristic peak appears at 896 cm -1 for the antisymmetric stretching vibration of O=U=O, indicating that U(VI) has been successfully adsorbed by PVA-SA / CS-SH. The characteristic absorption peak corresponding to -SH at 2349 cm -1 shifts to a lower wave number after adsorption, and the characteristic absorption peak changes from sharp to smooth and broad. This indicates that -SH participates in the adsorption reaction during the adsorption of U(VI) and plays an important role. Figure 12In Figure b, characteristic peaks corresponding to U 4f appeared in the XPS spectrum after adsorption, further confirming the successful adsorption of U(VI) by PVA-SA / CS-SH. In addition to U 4f, S 2p, N 1s, C 1s, and O 1s also appeared in the full XPS spectrum of PVA-SA / CS-SH, further indicating the successful preparation of the material.

[0098] In addition, PVA-SA / CS-SH was compared with other efficient U(VI) adsorbents, such as MWCA, aMSP / SA, ZnNiAl-LDHs, PCG, CS / AL-AP, etc. reported in the literature. The removal amount of U(VI) by PVA-SA / CS-SH was as high as 350.33 mg g -1 , showing excellent adsorption performance.

[0099] In summary, in the present invention, sodium alginate and chitosan were used as raw materials, and mercapto functional groups were successfully introduced by grafting (3-mercaptopropyl) trimethoxysilane. Calcium chloride was used as a crosslinking agent, and polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads (PVA-SA / CS-SH) were prepared by the gel-vacuum freeze-drying method. Specifically, in the present invention, Schiff base reaction was first carried out between benzaldehyde and the amino group in chitosan to synthesize benzaldehyde-chitosan (B-CS) that protected the amino group, and then reacted with (3-mercaptopropyl) trimethoxysilane to introduce mercapto functional groups, and then benzaldehyde was removed to obtain mercapto-chitosan (CS-SH). The mercapto-chitosan was crosslinked with the mixed solution of polyvinyl alcohol and sodium alginate by calcium chloride solution, and polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads (PVA-SA / CS-SH) were prepared by gel-vacuum freeze-drying. The present invention also provides the application of the aerogel beads in the treatment of uranium-containing wastewater.

[0100] The characterization results showed that there were a large number of groove structures and stacked cavities inside the PVA-SA / CS-SH gel beads, providing excellent mass transfer conditions for the adsorption of U(VI). At pH = 6, T = 313 K, m / V = 0.125 g / L (solid-liquid ratio), C 0 = 50 mg L -1Under the experimental conditions, the adsorption reaction reached equilibrium at 300 min, and the maximum adsorption capacity of PVA-SA / CS-SH for U(VI) was 350.33 mg / g. The adsorption process was more in line with the Langmuir isothermal adsorption model and the pseudo-second-order kinetic model. The adsorption isotherm, adsorption kinetics and adsorption thermodynamics parameters indicated that the adsorption was a monolayer homogeneous, mainly chemisorption, spontaneous endothermic process. The intraparticle diffusion model showed that the adsorption process proceeded in three steps. In the first stage, rapid surface diffusion occurred; in the second stage, intraparticle diffusion occurred; in the third stage, the adsorption rate slowed down and entered the dynamic adsorption equilibrium state. The adsorption of U(VI) by PVA-SA / CS-SH was mainly due to the coordination / chelating effects of amino, hydroxyl and mercapto groups.

[0101] PVA-SA / CS-SH had good reusability and adsorption selectivity, low cost, and was economical and practical, and was expected to become a new type of efficient uranium-removing adsorbent.

[0102] The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, several simple deductions and substitutions can also be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. Application of aerogel beads in treatment of uranium-containing wastewater, characterized in that, the aerogel beads are polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads, namely PVA-SA / CS-SH aerogel beads; when the aerogel beads are used for treating uranium-containing wastewater, the pH value of the wastewater is 5-7, the wastewater treatment temperature is 35-50 °C, and the adsorption treatment time is more than 5 h; the PVA-SA / CS-SH aerogel beads are prepared by the following method and steps: Step A: First, prepare benzaldehyde-chitosan, namely BCS, using benzaldehyde and chitosan; specifically, in Step A, dissolve chitosan in an acetic acid solution, add methanol, and after the chitosan is completely dissolved, add a mixed solution of benzaldehyde and methanol; react fully at room temperature to obtain a light yellow gel; wash the gel several times with methanol and absolute ethanol respectively to wash away the unreacted benzaldehyde, and then dry it to obtain amino-protected chitosan, namely BCS; Step B: Then prepare benzaldehyde-mercapto-functionalized chitosan, namely BCS-SH; specifically, in Step B, disperse the BCS prepared in Step A in toluene, add (3-mercaptopropyl)trimethoxysilane to obtain a mixture, ultrasonically disperse the mixture, and then carry out a hydrothermal reaction; after the reaction is complete, naturally cool to room temperature, wash the obtained solid reactant several times with toluene and absolute ethanol respectively, and dry it to obtain BCS-SH; Step C: Then prepare mercapto-functionalized chitosan, namely CS-SH; specifically, in Step C, hydrolyze the BCS-SH prepared in Step B with hydrochloric acid to remove the benzaldehyde used to protect the amino group and the chitosan that has not been successfully grafted with mercapto, and then wash it several times with absolute ethanol and deionized water, and dry it to obtain CS-SH; Step D: Use polyvinyl alcohol, sodium alginate and the CS-SH prepared in Step C to prepare polyvinyl alcohol-sodium alginate / mercapto-functionalized chitosan aerogel beads, namely PVA-SA / CS-SH; specifically, in Step D, first dissolve polyvinyl alcohol and sodium alginate in deionized water, heat and stir to form a uniform mixture; then transfer it to a low-temperature water bath, add the CS-SH prepared in Step C and stir, and after complete dissolution, naturally cool to room temperature to obtain a mixture; drop the mixture into a calcium chloride solution to crosslink and form gel beads; soak the gel beads in the calcium chloride solution, and then wash them several times with deionized water to remove the uncrosslinked calcium chloride; then freeze and vacuum freeze-dry the gel beads to obtain the PVA-SA / CS-SH.

2. The application according to claim 1, characterized in that, when the aerogel beads are used for treating uranium-containing wastewater, the wastewater treatment temperature is 40-50 °C.

3. The application according to claim 1, characterized in that, when the aerogel beads are used for treating uranium-containing wastewater, the adsorption treatment time is 5-10 h.

4. The application according to claim 1, characterized in that, When the aerogel beads are used for treating uranium-containing wastewater, the initial concentration C of uranyl ions in the wastewater 0 is 30 to 70 mg / L.

5. The application according to claim 1, characterized in that, When the aerogel beads are used for treating uranium-containing wastewater, the solid-liquid ratio of PVA-SA / CS-SH aerogel beads to the wastewater is 0.1-0.15 g / L.

6. According to the application described in any one of claims 1-5, characterized in that When the aerogel beads are used for treating uranium-containing wastewater, Cu in the wastewater is removed first. 2+ , Fe 3+ , CO 3 2- and SO 4 2- , and then the aerogel beads are brought into contact with the uranium-containing wastewater for uranium adsorption.

7. According to the application described in any one of claims 1-5, characterized in that When the aerogel beads are used for treating uranium-containing wastewater, the PVA-SA / CS-SH aerogel beads are added to the wastewater, and reciprocating oscillation adsorption is adopted.

8. According to the application described in claim 7, characterized in that The speed of oscillation adsorption is 100-150 rpm.

9. According to the application described in claim 1, characterized in that In step A, the concentration of the acetic acid solution is 1-3%, the reaction time for fully reacting at room temperature to obtain a light yellow gel is more than 24 h, the drying temperature of the gel is 60-70 °C, and the drying time is more than 10 h; In step B, the ultrasonic dispersion time of the mixture is more than 10 min. The hydrothermal reaction includes loading the mixture into a hydrothermal reaction kettle with a polytetrafluoroethylene liner and reacting at 105-120 °C for more than 12 h. The drying is carried out at 60-70 °C, and the drying time is more than 6 h; In step C, the concentration of hydrochloric acid is 0.2-1 mol / L, the temperature for hydrolyzing with hydrochloric acid is 50-70 °C, the hydrolysis time is more than 6 h. The drying is carried out at 60-70 °C, and the drying time is more than 6 h; In step D, water bath heating is used during heating and stirring, and the water bath heating temperature is 85-95 °C and the time is more than 1 h; the temperature of the low-temperature water bath is 75-82 °C, the time for adding CS-SH and stirring is more than 3 h, the concentration of the calcium chloride solution is 2-5 wt%, the soaking time of the gel beads in the calcium chloride solution is more than 5 h, the freezing temperature is -35 °C to -25 °C, the freezing time is more than 2 h, and the vacuum freeze-drying temperature is -55 °C to -45 °C and the time is more than 6 h.

10. According to the application described in claim 9, characterized in that In step D, a syringe is used to drop the mixed solution into the calcium chloride solution.

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