Method for preparing sulfydryl-functionalized and high-performance chitosan homogeneous membrane as binding phase of thin film diffusion gradient technology

Abstract

The invention discloses a sulfydryl-functionalized high-performance chitosan homogeneous membrane which is prepared by taking chitosan and polyvinyl alcohol as substrates, L-cysteine as a modifier and tris (2-carboxyethyl) phosphine hydrochloride as a reducing agent through a non-solvent induced phase inversion method. Therefore, the inventor also establishes a preparation method of the chitosan homogeneous membrane as a binding phase of a thin film diffusion gradient technology. Finally, the thin film material with high adsorption capacity and rapid adsorption on mercury is innovatively prepared, and the DGT device taking the thin film material as a binding phase can rapidly, highly selectively and stably enrich mercury in a water body and is not interfered by other heavy metal ions; the technical problem that a biological membrane grows on the surface of a traditional DGT device due to long-time enrichment to interfere with accurate determination of mercury is successfully solved, the DGT technology is expanded to the field of complex determination of the concentration of trace Hg (II) in water, and the purpose of efficient, rapid and accurate determination is achieved.

Description

Technical Field

[0001] This invention belongs to the field of environmentally friendly material preparation and heavy metal enrichment technology, and particularly relates to a method for preparing a thiol-functionalized and high-performance chitosan homogeneous membrane as a binding phase for thin film diffusion gradient technology. Background Technology

[0002] Dissolved mercury (both free and complexed forms, collectively referred to as Hg(II)) is the form in water most readily converted into methylmercury. Increased concentrations can lead to exponential accumulation in surrounding organisms, posing a threat to ecosystems and human health. Therefore, monitoring the current state and dynamic changes of mercury is crucial for assessing its impact on human health. Conventional trace mercury determination involves water sample collection, transportation and preservation, purging-gold tube trapping-thermal desorption-cold atomic fluorescence spectrometry determination. These processes inevitably lead to changes in the chemical speciation of trace Hg(II) and may introduce numerous potential pollutants. Diffusive gradients in thin films (DGT) technology is an in-situ passive sampling technique that can quantitatively accumulate unstable metallic speciations in the aquatic environment through a concentration-dependent dynamic process, achieving pre-enrichment of analytes. These advantages can facilitate the accurate determination of trace Hg(II).

[0003] Existing DGT devices typically use commercially available resins as the binding phase, which have limited adsorption capacity and rate for Hg(II), resulting in a low diffusion coefficient for Hg(II). Therefore, the devices usually require a long deployment time to enrich sufficient Hg(II) and achieve accurate measurement. However, prolonged deployment of DGT devices can easily lead to biofilm growth on the surface, affecting the accuracy of the method. Therefore, there is a need to develop a thin film material that can rapidly and efficiently enrich Hg(II) as the binding phase to overcome these technical bottlenecks.

[0004] Chitosan is a natural biopolymer rich in amino and hydroxyl groups. Its low preparation cost, environmental friendliness, and ease of modification endow chitosan-based films with outstanding advantages such as low toxicity, environmental friendliness, ease of molding, and rich structure, making it a preferred material for adsorbent substrates and occupying an important position in the removal of toxic and harmful heavy metals from wastewater. However, traditional chitosan hydrogel membrane materials have limited adsorption capacity and poor selectivity for mercury. Therefore, it is necessary to graft thiol groups with high affinity and strong coordination for mercury to enhance the adsorption driving force. Furthermore, traditional chitosan-based membranes have a dense structure, making it difficult for mercury to penetrate from the outer surface into the internal pores. Modifiers also have difficulty entering the internal pores and remain only on the surface, resulting in a low density of active functional groups and hindering rapid mass transfer. Therefore, it is necessary to create a permeable porous structure through the preparation process to provide a rapid transport channel for mercury from the bulk solution to the effective functional groups. Meanwhile, the thickness of chitosan membranes prepared by traditional methods is difficult to control effectively, which will affect the parallelism of adsorption properties between membranes and ultimately affect the accuracy of the measurement. Therefore, the preparation method needs to accurately control the thickness of the membrane. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing a thiol-functionalized and high-performance chitosan homogeneous membrane as a binding phase for thin film diffusion gradient technology.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A high-performance chitosan homogeneous membrane with thiol functionalization was prepared by a non-solvent-induced phase inversion method using chitosan and polyvinyl alcohol as the base, L-cysteine ​​as the modifier, and tris(2-carboxyethyl)phosphonic acid hydrochloride as the reducing agent.

[0008] The chitosan homogeneous membrane described above has a sponge-like three-dimensional porous network structure.

[0009] The chitosan homogeneous membrane described above is used as the binding phase in thin film diffusion gradient technology.

[0010] The aforementioned chitosan homogeneous membrane or thin film diffusion gradient technology is used to enrich and adsorb mercury in water.

[0011] A method for preparing a thiol-functionalized and high-performance chitosan homogeneous membrane as a binding phase in thin-film diffusion gradient technology includes the following steps:

[0012] 1) Preparation of CS / PVA base film

[0013] a) Prepare two portions of adipic acid solution, one portion of which is prepared by adding chitosan (CS) powder to prepare a CS solution, and the other portion of which is prepared by adding polyvinyl alcohol (PVA) particles to prepare a PVA solution. Then, dissolve sodium chloride particles in the cooled PVA solution.

[0014] b) A CS / PVA casting solution was prepared by mixing a CS solution and a PVA solution; the casting solution was coated onto a clean glass plate to form a hydrogel of a certain thickness; the hydrogel and the glass plate were then frozen together and immersed in anhydrous ethanol to obtain a CS / PVA membrane; this membrane was stabilized by curing in sodium hydroxide solution and washed with ultrapure water; wherein, the mass concentration of CS in the polymer (w) t The concentration is generally 70%-90%, with 90% CS concentration yielding the best results.

[0015] 2) Fabrication of phases using thin-film diffusion gradient technology

[0016] a) Use a mold to cut the CS / PVA membrane into thin films, immerse them in glutaraldehyde solution to react and fully crosslink, and rinse with pure water;

[0017] b) L-cysteine ​​particles were dissolved in ultrapure water to prepare a modifier solution; then a membrane reaction was carried out to prepare a CS-S / PVA modified membrane; the pH of the modified solution was generally 3-9, and the modification effect was best when the pH was 5.

[0018] c) The CS-S / PVA modified membrane was immersed in a tris(2-carboxyethyl)phosphine hydrochloride solution to reduce the sulfur-containing groups to thiol groups, and then rinsed with ultrapure water to obtain the final thiol-functionalized chitosan homogeneous membrane (CS-CYS / PVA membrane).

[0019] In step 1)a): Adipic acid particles were dissolved in ultrapure water at 35°C to prepare two 50 mL adipic acid solutions with a concentration of 0.2 mol / L. In one solution, 2.7 g of chitosan (CS) powder was added and stirred for 12 h until fully dissolved to prepare a CS solution. In the other solution, 0.3 g of polyvinyl alcohol (PVA) particles were added and heated to 90°C and stirred for 3 h until fully dissolved to prepare a PVA solution. Subsequently, 2.7 g of sodium chloride particles were dissolved in the cooled PVA solution. The role of adipic acid is to act as a dissolving medium, providing a stable dissolution environment for CS and PVA, while also helping to regulate the polarity of the system and promote subsequent membrane formation. The role of sodium chloride is to regulate the osmotic pressure of the system through salt ions, optimize the pore structure of the subsequent membrane, and improve the membrane formation stability.

[0020] In step 1)b): the CS solution and PVA solution are mixed and stirred at room temperature for 6 h to prepare the CS / PVA casting solution; after ultrasonic degassing, the casting solution is coated onto a clean glass plate with a scraper to form a hydrogel of a certain thickness; the hydrogel and glass plate are then placed together in a -80℃ freezer for 30 min, and then slowly immersed in anhydrous ethanol for 1 h to allow for complete phase inversion, thus obtaining the CS / PVA membrane; this membrane is further solidified and stabilized in a 3% (wt%) sodium hydroxide solution and washed multiple times with ultrapure water. The purpose of mixing and stirring for 6 hours is to fully integrate the CS solution and PVA solution, avoid membrane structure delamination, and ensure the uniformity of the internal structure of the membrane. Ultrasonic venting removes air bubbles from the casting solution, preventing partial pore collapse after membrane formation and ensuring a smooth membrane surface. The scraper precisely controls the base membrane thickness to match the size requirements of the DGT binding phase, ensuring uniform subsequent modification effects. The -80℃ freezing process induces rapid freezing of the solvent (water) in the casting solution, creating a regular and porous structure for subsequent phase transformation and increasing the specific surface area of ​​the membrane. Anhydrous ethanol immersion utilizes solvent displacement, replacing frozen water in the membrane with ethanol, while simultaneously promoting the rearrangement of polymer chains to form a stable porous sponge-like structure. The sodium hydroxide solution neutralizes residual adipic acid in the membrane, deprotonating the amino groups of CS and enhancing the membrane's chemical stability. Multiple rinses with ultrapure water remove residual sodium hydroxide and unreacted small molecules from the membrane surface, preventing impurities from affecting subsequent modification reactions and adsorption performance.

[0021] In step 2)a): The CS / PVA membrane is cut into 25 mm diameter films using a mold, immersed in a 2% (v / v%) glutaraldehyde solution, and reacted at pH 8 and 50℃ for 4 h to achieve full crosslinking. The membrane is then rinsed multiple times with pure water. The glutaraldehyde solution acts as a crosslinking agent, forming a stable crosslinked network through condensation reactions with the amino groups on CS molecules and the hydroxyl groups on PVA molecules. The pH 8 and 50℃ conditions are used to optimize the crosslinking efficiency, preventing over-crosslinking that could clog membrane pores, ultimately improving the membrane's swelling resistance and operational stability. Multiple rinses with pure water remove residual glutaraldehyde from the membrane surface, ensuring biocompatibility and the specificity of subsequent reactions.

[0022] In step 2)b): 0.06 g of L-cysteine ​​particles (CYS) were dissolved in ultrapure water to prepare a modifier solution; then a membrane was added and reacted at pH 5 and 60℃ for 2 h to obtain a CS-S / PVA modified membrane; wherein, L-cysteine, as a thiol precursor, can undergo a Schiff base reaction with the aldehyde group on the membrane surface to graft sulfur-containing groups onto the membrane surface; the reaction temperature was increased to 60℃ to increase the reaction rate, and 2 h was used to ensure sufficient grafting to form a CS-S / PVA modified membrane.

[0023] In step 2)c): The CS-S / PVA modified membrane was immersed in a 10 mmol / L tris(2-carboxyethyl)phosphine hydrochloride solution for 2 h to fully reduce the sulfur-containing groups to thiol groups. It was then rinsed multiple times with ultrapure water to obtain the final thiol-functionalized chitosan homogeneous membrane (CS-CYS / PVA membrane). Immersing the membrane in the tris(2-carboxyethyl)phosphine hydrochloride solution for 2 h reduces the sulfur-containing groups grafted onto the membrane surface to free thiol groups, giving the membrane its core function of specifically adsorbing mercury. Multiple rinsing with ultrapure water removes residual tris(2-carboxyethyl)phosphine hydrochloride solution and reaction byproducts from the membrane surface, preventing the reducing agent from affecting mercury adsorption in subsequent DGT experiments and ensuring the purity of the bound phase.

[0024] To address the problems existing in current thin-film diffusion gradient technology, the inventors have developed a high-performance chitosan homogeneous membrane with thiol functionalization. Using chitosan and polyvinyl alcohol as the base, L-cysteine ​​as the modifier, and tris(2-carboxyethyl)phosphonic acid hydrochloride as the reducing agent, the membrane is prepared using a solvent-inducible phase inversion method. Based on this, the inventors have also established a method for preparing a chitosan homogeneous membrane as the binding phase in thin-film diffusion gradient technology. First, L-cysteine ​​is used as the modifier to load abundant sulfur-containing groups onto chitosan. Then, tris(2-carboxyethyl)phosphonic acid hydrochloride is used to fully reduce the sulfur-containing groups into highly active thiol groups. Second, a solvent-inducible phase inversion method is used to form the membrane, allowing for precise control of the thickness through a physical coating process. Ethanol is used as the conversion solvent, and sodium chloride is used as a pore-forming agent to form a loose, porous, and regularly spaced sponge-like structure. Finally, a novel thin film material with high adsorption capacity and rapid adsorption of mercury was prepared. The DGT device with this material as the binding phase can rapidly and selectively enrich mercury in water without interference from other heavy metal ions. This successfully overcomes the technical problem of biofilm growth on the surface of traditional DGT devices, which interferes with the accurate determination of mercury due to long-term enrichment. This extends DGT technology to the determination of trace Hg(II) concentration in complex water bodies (such as seawater in estuaries), achieving the goal of efficient, rapid and accurate determination. Attached Figure Description

[0025] Figure 1 The figure shows the optimization results of the parameters of the CS-CYS / PVA membrane prepared in this invention. In the figure: a) the effect of CS concentration, b) the effect of reaction pH, and c) the effect of tris(2-carboxyethyl)phosphine hydrochloride solution.

[0026] Figure 2 The image shows the color development results of thiol groups in the CS-CYS / PVA membrane prepared in this invention.

[0027] Figure 3Scanning electron microscope images of the CS / PVA membrane and CS-CYS / PVA membrane prepared for this invention. In the figure: a CS / PVA membrane (2.00KX), b CS / PVA membrane (20.00KX), c CS-CYS / PVA membrane (2.00KX), d CS-CYS / PVA membrane (20.00KX).

[0028] Figure 4 The graph shows the change in the adsorption capacity of the CS-CYS / PVA membrane prepared in this invention for Hg(II) over time.

[0029] Figure 5 The figures show the fitting curves of the isothermal model of the CS-CYS / PVA film prepared in this invention at different temperatures. In the figure: a 298K, b 308K, c 318K.

[0030] Figure 6 The figure shows the selective adsorption results of the CS-CYS / PVA membrane prepared in this invention when eight cationic metals coexist. In the figure: a is a binary system, b is a multi-component system.

[0031] Figure 7 The diffusion coefficient of Hg(II) in different water bodies is shown in the figure for a DGT device using the CS-CYS / PVA membrane prepared in this invention as the binding phase. In the figure: a is ultrapure water, b is river water, and c is seawater. Detailed Implementation

[0032] Example 1: Preparation method of thiol-modified chitosan homogeneous membrane

[0033] 1) Preparation of CS / PVA base film

[0034] a) First, adipic acid particles were dissolved in ultrapure water at 35°C to prepare two 50 mL adipic acid solutions with a concentration of 0.2 mol / L. In one solution, 2.7 g of chitosan (CS) powder was added and stirred for 12 h until fully dissolved to prepare a CS solution. In the other solution, 0.3 g of polyvinyl alcohol (PVA) particles were added and heated to 90°C and stirred for 3 h until fully dissolved to prepare a PVA solution. Subsequently, 2.7 g of sodium chloride particles were dissolved in the cooled PVA solution.

[0035] b) The CS solution and PVA solution were mixed and stirred at room temperature for 6 h to prepare a CS / PVA casting solution. After ultrasonic degassing, the casting solution was coated onto a clean glass plate using a film scraper to form a hydrogel with a certain thickness (approximately 1.8 mm). The hydrogel and glass plate were then placed together in a -80°C freezer for 30 min, and then slowly immersed in anhydrous ethanol for 1 h to allow for complete phase inversion, resulting in a CS / PVA membrane. This membrane was further solidified and stabilized in a 3% (wt%) sodium hydroxide solution and washed multiple times with ultrapure water.

[0036] 2) Fabrication of phases using thin-film diffusion gradient technology

[0037] a) Use a mold to cut the CS / PVA membrane into circular films with a diameter of 25 mm, immerse them in a 2% (v / v%) glutaraldehyde solution, and react them in an environment of pH 8 and 50℃ for 4 h to achieve full cross-linking. Rinse the membrane surface with pure water multiple times to remove residual glutaraldehyde.

[0038] b) A modifier solution was prepared by dissolving 0.06 g of L-cysteine ​​particles (CYS) in 50 mL of ultrapure water. The solution was then added to a membrane and reacted at pH 5 and 60 °C for 2 h to obtain a CS-S / PVA modified membrane.

[0039] c) The CS-S / PVA modified membrane was immersed in a 10 mmol / L tris(2-carboxyethyl)phosphine hydrochloride solution for 2 h to fully reduce the sulfur-containing groups to thiol groups. Then, it was rinsed multiple times with ultrapure water to obtain the final thiol-functionalized chitosan homogeneous membrane (CS-CYS / PVA membrane).

[0040] Example 2: Determination of optimal parameters for CS-CYS / PVA membrane (prepared in Example 1)

[0041] Polymers, as the main component of the membrane-forming solution, not only form the basic structure through molecular chain arrangement, but their chemical composition also provides active sites for functional modification, thus becoming a key parameter that needs to be controlled in membrane material preparation. In the experiment, the total mass concentration of polymer in the membrane-forming solution was fixed at 3%, and the mass ratio of CS to PVA was adjusted to achieve the desired CS mass concentration (w / w) in the polymer. t The concentration gradient parameters were set to 70%, 80%, and 90% as the optimization index. Simultaneously, water flux tests were conducted on CS / PVA membranes prepared with different CS concentrations.

[0042] Figure 1 a indicates that the corresponding water flux values ​​for the three CS concentrations are 1534 L / m³. 2 / h / bar, 1591 L / m 2 / h / bar、11866 L / m2 / h / bar. Among them, the water flux value at a CS concentration of 90% is significantly higher than that at other concentrations, reflecting that the CS / PVA base membrane prepared under this condition forms a more abundant pore network structure.

[0043] This study also used glutaraldehyde as a crosslinking agent and CYS as a modifier to achieve covalent grafting of sulfur-containing groups via Schiff base reaction. Key parameters of the grafting stage were optimized using a single-factor variable method. CS-CYS / PVA membranes prepared under different conditions were placed in 50 mL of 200 mg / L Hg(II) solution and subjected to isothermal shaking for 24 h to evaluate the adsorption effect.

[0044] Figure 1 b reveals that the pH of the modified solution significantly affects the removal efficiency of CS-CYS / PVA for Hg(II), with the adsorption capacity showing a trend of first increasing and then decreasing, reaching a peak at pH 5. The grafting effect is not ideal under acidic conditions (pH<5) possibly because the amino group on L-cysteine ​​is easily protonated to -NH3⁺, affecting its nucleophilic addition reaction with the aldehyde group on glutaraldehyde; the main reason for the decrease in adsorption efficiency under strongly alkaline conditions is that excessively high pH may cause the Schiff base bonds of the pre-crosslinked chitosan network to break, releasing free aldehyde groups to crosslink with cysteine, reducing the cysteine ​​grafting efficiency.

[0045] The above modifications significantly improved the adsorption capacity of the modified membrane for Hg(II). However, the thiol group (-SH), as a highly efficient coordination site for Hg(II), may undergo oxidation during modification or long-term storage to form a disulfide bond (-SS-), which reduces the number of active sites and weakens its affinity for low concentrations of Hg(II).

[0046] Tris(2-carboxyethyl)phosphonic acid hydrochloride, as a strong reducing agent, can specifically cleave disulfide bonds and stabilize thiol groups. CS-CYS / PVA membranes were immersed in a tris(2-carboxyethyl)phosphonic acid hydrochloride solution for 2 h for reduction stabilization. To evaluate the effect of tris(2-carboxyethyl)phosphonic acid hydrochloride treatment on improving the adsorption performance of low-concentration Hg(II), the membrane materials before and after treatment were placed in 100 mL of 100 μg / L Hg(II) solution, and the adsorption kinetic curves were compared and analyzed within 1 h. Figure 1 The results showed that the untreated CS-S / PVA membrane had limited adsorption capacity for low concentrations of Hg(II), possibly due to the oxidation of thiol groups during the modification process to form disulfide bonds or sulfur oxides. In contrast, the CS-CYS / PVA membrane treated with tris(2-carboxyethyl)phosphonic acid hydrochloride exhibited a twofold increase in adsorption capacity and rapid adsorption kinetics, with rapid adsorption occurring within 1 min and reaching adsorption equilibrium within 5 min.

[0047] Example 3: Determination of thiol content in CS-CYS / PVA membrane (prepared in Example 1)

[0048] Since thiol groups react with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) to form a yellow color, the Ellman test is used to determine the concentration of thiol groups. The specific method is as follows: First, prepare a phosphate buffered solution (PBS) containing 1 mmol / L ethylenediaminetetraacetic acid reagent and adjust the pH of the solution to 8.0. Second, prepare 4 g / L DTNB reagent and L-cysteine ​​standard solutions of different concentrations. For sample testing, the sample is added to the PBS solution and fully dissolved, then 50 μL of DTNB reagent is added for the colorimetric reaction. After standing for 30 min, the result is measured using a UV spectrophotometer. The concentrations of cysteine ​​standard curves were set to 0.1, 0.2, 0.3, 0.4, and 0.6 g / L, and the amount of material sample added was 0.015 g. When measuring on the instrument, the wavelength of the ultraviolet spectrophotometer was set to 412 nm. After diluting the standard curve solution and the sample solution by 6 times, a standard curve of cysteine ​​was established, and then quantification was performed.

[0049] according to Figure 2 As shown, the CS-CYS / PVA membrane exhibits a more significant colorimetric reaction. Quantitative calculations revealed that the thiol content of the membrane increased from 0.347 mmol / g (untreated) to 1.528 mmol / g after treatment with tris(2-carboxyethyl)phosphonic acid hydrochloride solution. This fully demonstrates that the reduction treatment with tris(2-carboxyethyl)phosphonic acid hydrochloride solution effectively increases the number of thiol active sites in the membrane, significantly enhances its ability to capture low concentrations of Hg(II), and improves the membrane's adsorption capacity.

[0050] Example 4: Microstructure of CS / PVA base film and CS-CYS / PVA film (prepared in Example 1)

[0051] The membrane was characterized using a scanning electron microscope, and the results are as follows: Figure 3 As shown. Among them, Figure 3 Abstract analysis revealed that the CS / PVA-based membrane exhibits a typical sponge-like three-dimensional porous network structure, with a regular pore size distribution and interconnected pore walls. This unique structure can be attributed to the rapid exchange of solvent (water) and non-solvent (anhydrous ethanol) during solvent-induced phase separation. Under this thermodynamic imbalance, the polymer solution undergoes liquid-liquid phase separation, driving the formation of numerous uniformly sized, interconnected pore structures. Simultaneously, polymer chains on the pore wall surfaces undergo in-situ cross-linking through intermolecular interactions, forming a dense and mechanically strong three-dimensional network that provides a stable skeletal support system for the membrane material. This structural feature not only ensures the membrane's high permeability but also endows it with excellent anti-swelling properties, laying the structural foundation for subsequent functionalization modifications.

[0052] Figure 3 The CD shows that the pore walls and surface texture of the modified CS-CYS / PVA membrane are rougher and more complex than those of the CS / PVA base membrane. This change stems from the chemical reaction between the functionalizing agents and chitosan molecular chains during the crosslinking and grafting process. The significantly increased roughness of the pore walls effectively expands the exposed area of ​​active sites, creating better interfacial conditions for the adsorption reaction.

[0053] Example 5: Change in the adsorption capacity of CS-CYS / PVA membrane (prepared in Example 1) for Hg(II) over time.

[0054] A certain volume of 100 μg / L Hg(II) concentration solution was prepared, the pH was adjusted to 7, and a cut membrane was added to carry out the adsorption reaction. Three 2 mL water samples were taken at certain time points within the 0-1 h time range, filtered, and the Hg(II) concentration was determined by hydrogen generation-atomic fluorescence spectrophotometer after digestion.

[0055] Figure 4 The study revealed that the CS-CYS / PVA membrane can achieve rapid adsorption within 1 min and reach adsorption equilibrium within 5 min. The final residual Hg(II) concentration can be reduced to below the drinking water standard (<1 μg / L). The adsorption process conforms to the pseudo-second-order model (R²=0.9998), highlighting its rapid and efficient adsorption characteristics.

[0056] Example 6: Adsorption isotherm performance evaluation of CS-CYS / PVA membrane (prepared in Example 1)

[0057] Prepare several 50 mL portions of Hg(II) solution with a concentration range of 0-500 mg / L. Add a membrane to each portion of the solution and control the experimental temperature at 25℃, 35℃ and 45℃ using a constant temperature air bath shaker. After shaking until adsorption equilibrium is reached, take three portions of water sample for filtration and digestion, and determine the remaining concentration of Hg(II).

[0058] like Figure 5As shown, the adsorption behavior of the membrane for Hg(II) at three temperatures highly conforms to the Langmuir isotherm adsorption model, rather than the Freundlich model, with R² values ​​of 0.9936–0.9983, exhibiting a higher correlation coefficient, indicating that the adsorption is a monolayer chemisorption. Fittings revealed that the maximum theoretical saturation adsorption capacities at 25℃, 35℃, and 45℃ were 15.39 mg / tablet (732.85 mg / g), 16.82 mg / tablet (800.95 mg / g), and 17.12 mg / tablet (815.24 mg / g), respectively, demonstrating excellent adsorption capacity. This is mainly due to the uniform sponge-like network structure formed within the membrane, which promotes the regular distribution of active functional groups such as thiol groups, providing uniform and high-density adsorption sites for Hg(II), ensuring that the coordination complexation reaction between Hg(II) and functional groups occurs at a stable interface.

[0059] Example 7: Selectivity analysis of CS-CYS / PVA membrane (prepared in Example 1)

[0060] Eight common metallic cations—Na(I), K(I), Ca(II), Mg(II), Zn(II), Cu(II), Pb(II), and Cd(II)—were selected, and binary and multi-component mixed systems of Hg(II) and the aforementioned metal cations were prepared. The concentration of Hg(II) was set at 10 mg / L, while the concentrations of other interfering metal cations were set at twice the Hg(II) concentration (20 mg / L). Three copies were prepared, and the residual concentrations of the other metals were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) to obtain the membrane's removal efficiency for Hg(II) and other metal cations.

[0061] Depend on Figure 6 As can be seen, the CS-CYS / PVA membrane consistently achieves a Hg(II) removal efficiency exceeding 97% in the binary system, while the removal rates for other cations such as Na(I), K(I), Ca(II), Mg(II), and Zn(II) are all below 5%, revealing high selectivity and affinity. According to the hard-soft acid-base theory, the sulfur atom in the thiol group is rich in lone pair electrons, allowing it to act as a soft base and preferentially form stable coordination bonds with the soft acid ion Hg(II) rather than the empty orbitals of other cations, thus achieving preferential capture of Hg(II). (Multi-component mixed system) Figure 6 The results in b) further confirm the selectivity advantage of CS-CYS / PVA, with its allocation coefficient K for Hg(II) being [missing information]. d The concentration of Hg(II) is significantly higher than that of other metal cations, indicating that even in complex environments where multiple ions coexist, the CS-CYS / PVA membrane can still efficiently identify and enrich Hg(II).

[0062] Example 8: Diffusion coefficient of Hg(II) in water by the CS-CYS / PVA@DGT device

[0063] The CS-CYS / PVA membrane prepared in Example 1 was used to assemble a DGT device (3.14 cm). 2 The window exposure area (purchased from Nanjing Zhigan Environmental Technology Co., Ltd.) was used to determine the diffusion coefficient of Hg(II) in water, as detailed below:

[0064] Several 2 L spiking solutions with a concentration of 100 μg / L were prepared in ultrapure water, river water, and seawater, respectively, and equilibrated at 25°C for at least 24 h. Subsequently, three DGT devices were added to each prepared solution for enrichment. At different time points (1, 2, 3, and 4 h), the three DGT devices were removed, immediately disassembled, and the bound phase was extracted. After rinsing with ultrapure water, the solution was eluted with digestion buffer. The eluent was filtered, and the Hg(II) concentration in the filtrate was measured. Additionally, 2 mL samples were taken before and after DGT enrichment to determine the Hg(II) concentration, ensuring no significant change in concentration, i.e., controlled within 10%.

[0065] The mass of Hg(II) enriched in the bound phase gel can be calculated from the concentration and volume of Hg(II) in the digestion solution. A linear graph of the mass M of mercury enriched in the bound phase gel versus the enrichment time t is then obtained, and the diffusion coefficient D is calculated using the following formula 1.

[0066]

[0067] In the formula, A is the diffusion area; Δg is the diffusion thickness; and C is the initial concentration of the Hg(II) to be measured in the original solution.

[0068] like Figure 7 The figure shows the diffusion coefficients of Hg(II) in ultrapure water, river water, and seawater using the CS-CYS / PVA@DGT device prepared in Example 1. The values ​​are as follows: The diffusion coefficient of CS-CYS / PVA@DGT is significantly higher than that of commercially available DGT devices using resin as the binding phase. Furthermore, the diffusion coefficient values ​​in river water and seawater only decreased by 6.5% and 9.0% respectively compared to the ultrapure water system, both falling within the error range. This result fully confirms the outstanding diffusion coefficient of CS-CYS / PVA@DGT, effectively shortening the time for in-situ enrichment of trace Hg(II), avoiding biofilm growth, and also exhibiting excellent anti-interference characteristics.

Claims

1. A thiol-functionalized and high-performance chitosan homogeneous membrane, characterized in that: The product was prepared by a non-solvent-induced phase inversion method using chitosan and polyvinyl alcohol as bases, L-cysteine ​​as a modifier, and tris(2-carboxyethyl)phosphonic acid hydrochloride as a reducing agent.

2. The chitosan homogeneous membrane according to claim 1, characterized in that... It has a sponge-like three-dimensional porous network structure.

3. The chitosan homogeneous membrane of claim 1 is used as a binding phase in thin film diffusion gradient technology.

4. The chitosan homogeneous membrane of claim 1 or the thin film diffusion gradient technology combined with the phase of claim 3 is used to enrich and adsorb mercury in water.

5. A method for preparing a thiol-functionalized and high-performance chitosan homogeneous membrane as a binding phase in thin-film diffusion gradient technology, characterized in that... Includes the following steps: 1) Preparation of CS / PVA base film a) Prepare two portions of adipic acid solution. Add chitosan powder to one portion to prepare a CS solution, and add polyvinyl alcohol particles to the other portion to prepare a PVA solution. Then dissolve sodium chloride particles in the cooled PVA solution. b) Prepare a CS / PVA casting solution by mixing a CS solution with a PVA solution; The casting solution was coated onto a clean glass plate to form a hydrogel; The hydrogel was then frozen together with the glass plate and then immersed in anhydrous ethanol to obtain a CS / PVA membrane. This membrane is stable after curing in sodium hydroxide solution and can be cleaned with ultrapure water. 2) Fabrication of phases using thin-film diffusion gradient technology a) Use a mold to cut the CS / PVA membrane into thin films, immerse them in glutaraldehyde solution to react and fully crosslink, and rinse with pure water; b) L-cysteine ​​particles were dissolved in ultrapure water to prepare a modifier solution; then added to the membrane reaction to prepare a CS-S / PVA modified membrane; c) The CS-S / PVA modified membrane was immersed in a tris(2-carboxyethyl)phosphine hydrochloride solution to reduce it to obtain thiol groups, and then rinsed with ultrapure water to obtain the final thiol-functionalized chitosan homogeneous membrane.

6. The preparation method according to claim 5, characterized in that... In step 1) a): Adipic acid particles were dissolved in ultrapure water at 35°C to prepare two 50 mL adipic acid solutions with a concentration of 0.2 mol / L. 2.7 g of chitosan powder was added to one solution and stirred for 12 h until fully dissolved to prepare a CS solution. 0.3 g of polyvinyl alcohol particles were added to the other solution and heated to 90°C and stirred for 3 h until fully dissolved to prepare a PVA solution. Subsequently, 2.7 g of sodium chloride particles were dissolved in the cooled PVA solution.

7. The preparation method according to claim 5, characterized in that... In step 1)b): the CS solution and PVA solution were mixed and stirred at room temperature for 6 h to prepare the CS / PVA casting solution; after ultrasonic degassing, the casting solution was coated onto a clean glass plate with a scraper to form a hydrogel; the hydrogel and the glass plate were then placed together in a -80℃ freezer for 30 min, and then slowly immersed in anhydrous ethanol for 1 h to allow for complete phase inversion, thus obtaining the CS / PVA membrane; this membrane was further solidified and stabilized in a 3% sodium hydroxide solution and washed multiple times with ultrapure water.

8. The preparation method according to claim 5, characterized in that... In step 2) a): the CS / PVA membrane is cut into films with a diameter of 25 mm using a mold, immersed in a 2% glutaraldehyde solution, and reacted in an environment of pH 8 and 50℃ for 4 h to achieve full cross-linking, followed by multiple rinses with pure water.

9. The preparation method according to claim 5, characterized in that... In step 2)b): 0.06 g of L-cysteine ​​particles were dissolved in ultrapure water to prepare a modifier solution; then the membrane was added and reacted at pH 5 and 60℃ for 2 h to obtain a CS-S / PVA modified membrane.

10. The preparation method according to claim 5, characterized in that... In step 2)c): the CS-S / PVA membrane is immersed in a 10 mmol / L tris(2-carboxyethyl)phosphine hydrochloride solution for 2 h to fully reduce the sulfur-containing groups to thiol groups, and then rinsed multiple times with ultrapure water to obtain the final thiol-functionalized chitosan homogeneous membrane CS-CYS / PVA.

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