Grafting cellulose-based separation membrane and method for preparing the same

Abstract

The application discloses a grafted cellulose-based separation membrane and a preparation method thereof. The separation membrane comprises a cellulose-containing membrane substrate and double grafting chains grown on both sides of the membrane; the double grafting chains comprise Block A chains grafted on the membrane substrate and Block B chains grafted on the ends of the Block A chains; the structure of the separation membrane is: substrate-[Block A]-[Block B], the Block A chains are block copolymer chains containing thiourea groups and amine oxime groups, and the Block B chains are block hydrophobic chains containing vinyl imidazole onium salt grafting products. The Block A chains can improve the adsorption capacity for heavy metal ions through the bidentate coordination of the thiourea groups and the amine oxime groups; the Block B chains can remove organic matters through the electrostatic repulsion of the vinyl imidazole onium salt for harmful anions and the hydrophobic-hydrophobic interaction. The preparation method of the separation membrane comprises double-sided activation of the membrane substrate, fixing of initiators and double-sided growth of the Block A chains and the Block B chains, the obtained grafted cellulose membrane has a Zeta potential of greater than or equal to +40 mV, a water contact angle of greater than or equal to 105 degrees and a carbon residue rate of greater than or equal to 30% at 600 DEG C, and is suitable for treatment of complex contaminated wastewater such as electroplating wastewater.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of separation membranes and their preparation, and particularly relates to a grafted cellulose-based separation membrane and a preparation method thereof. BACKGROUND

[0002] As a high-efficiency and low-energy separation method, separation membrane technology has been widely used in water treatment, resource recovery, environmental protection and other fields. Among them, cellulose-based separation membranes have become a research hotspot due to their wide raw material sources (such as cotton pulp and wood pulp), excellent biocompatibility, low cost and easy modification. However, the large number of hydroxyl groups in the cellulose molecular chain can easily form hydrogen bonds with water molecules, resulting in excessive hydrophilicity on the membrane surface. When treating wastewater containing proteins, colloids and other substances, irreversible adsorption of membrane pollution phenomenon easily occurs. In addition, heavy metal ions (such as Pb 2+ , Cr 3+ ), toxic anions (such as CrO4 2- , AsO4 3- ) and persistent organic matter (such as phenol and dye) often coexist in industrial wastewater. The traditional cellulose membrane only relies on the weak adsorption of surface hydroxyl groups, and has limited functions, which is difficult to meet the deep treatment and separation needs of complex systems (such as mixed wastewater containing heavy metals, anions and organic matter). Therefore, developing a cellulose-based separation membrane with efficient multi-pollutant removal capacity, strong anti-pollution property and high stability has become the key to solving the problem of complex wastewater treatment. SUMMARY

[0003] To solve the above problems, this part aims to provide a grafted cellulose-based separation membrane and a preparation method thereof.

[0004] To achieve this purpose, on the one hand, the present application provides a grafted cellulose-based separation membrane.

[0005] A grafted cellulose-based separation membrane includes a cellulose-containing membrane substrate and double-grafted chains grown on both sides of the membrane surface; the double-grafted chains include BlockA chains grafted on the substrate in turn and BlockB chains grafted on the ends of the BlockA chains; the structure of the separation membrane is: membrane substrate-[BlockA]-[BlockB];

[0006] Among them, the BlockA chain is a block copolymer chain containing thioureido groups and amine oxime groups; the molar ratio of the thioureido groups to the amine oxime groups is 1:3-5, and the density of the thioureido groups is ≥1.2 mmol / g, based on the dry weight of the BlockA chain;

[0007] The Block B chain is a block hydrophobic chain containing a grafted product of a vinyl imidazolium salt; the vinyl imidazolium salt is a 1-alkyl-3-vinyl imidazolium halide prepared by quaternization of N-vinylimidazole with a haloalkane; the haloalkane includes a chloroalkane or a bromoalkane with a carbon chain of 11-17;

[0008] The cellulose base layer further contains 30-40 wt% of silk fibroin.

[0009] As a preferred technical solution of the present application, the Block A chain is a block copolymer chain formed by pre-grafting of acrylonitrile and propylene isothiocyanate at a molar ratio of 8-10:1 through atom transfer radical polymerization (ATRP) and then group transformation with 5-8 wt% hydroxylamine solution; wherein the coefficient of variation of thiourea group is ≤15%.

[0010] After the above pre-grafting process, hydroxylamine (NH2OH) provides two nucleophilic sites (-NH2 and -OH) at the same time, which can simultaneously convert the cyano group (-CN) and the isothiocyanate group (-NCS) to obtain amine oxime group and thiourea group, respectively. The specific mechanism is as follows:

[0011] (1) Hydroxylamine exists in the form of NH2OH·H2O in a neutral solution, and its -NH2 has strong nucleophilicity, preferentially attacking the carbon atom with partial positive charge in the high-polarity cyano group (the electron cloud of C≡N bond is biased towards N, making C show δ + ), forming an imine intermediate (-C(NH2)=NH); the N atom of the imine intermediate further combines with the proton H + in the solution, enhancing the electrophilicity of the adjacent C atom. While the -OH of hydroxylamine is partly combined with the N atom of the imine intermediate through hydrogen bonding, stabilizing the intermediate structure; the other part acts as a nucleophile, attacking the C atom to form a binary intermediate of -C(NH2)(OH)-NH2; the binary intermediate forms a stable amine oxime group (-C(NH2)=NOH) through intramolecular proton transfer.

[0012] (2) The middle C atom of the isothiocyanate group (-N=C=S) is a strong electrophilic center (δ + ), and the -NH2 of hydroxylamine can attack this C atom, forming an intermediate of -NH-C(S - )=NH2 + ; the S - in the intermediate rapidly combines with H + in the solution to form an -SH group; and the -NH2 + restores electrical neutrality by combining with OH- in the solution, finally forming a thiourea group (-NH-CS-NH2).

[0013] As a preferred technical scheme of the present application, the cellulose has a polymerization degree of 500-1500; the separation membrane has a Zeta potential of +40 mV or more, a water contact angle of 105° or more, and a carbon residue rate of 30% or more at 600°C.

[0014] In another aspect, the present application also provides a preparation method of the above-mentioned grafted cellulose-based separation membrane, which specifically comprises the following steps:

[0015] S1: double-side activation of the substrate: both sides of the cellulose-silk fibroin membrane substrate are subjected to plasma activation treatment until the carboxyl density of the membrane substrate surface is 0.8 mmol / g or more and the deviation of the carboxyl density of both sides of the membrane is 0.05 mmol / g or less;

[0016] S2: fixing of the initiator: the membrane substrate of S1 is immersed in anhydrous tetrahydrofuran, sealed and deoxygenated; under a nitrogen atmosphere, thionyl chloride is added and treated by reciprocating oscillation to complete the surface acyl chloride modification; then the initiator and anhydrous triethylamine are added and kept oscillating to complete the fixing of the initiator on both sides of the membrane substrate by esterification; the deviation of the Br element content of both sides is 0.02 mmol / g or less by XPS detection; the initiator is 2-bromoisobutyric acid hydroxyethyl ester;

[0017] S3: double-side growth of BlockA chain: the membrane substrate of S2 is vertically hung and immersed in an ion liquid / water mixed solvent with a volume ratio of 3-5:1, sealed and deoxygenated; acrylonitrile and acrylonitrile isothiocyanate monomers are added to grow the BlockA chain by atom transfer radical polymerization; then the membrane is immersed in a hydroxylamine, hydrazine or amino compound solution with a pH of 7.0±0.2, and treated at a constant temperature of 30-35°C for 4-6 h to simultaneously convert the cyano group into an amine oxime group and the isothiocyanate group into a thiourea group, thereby obtaining a separation membrane with double-side grown BlockA chain;

[0018] S4: double-side growth of Block B chain: the separation membrane of S3 is vertically hung and immersed in a DMF / water mixed solvent with a volume ratio of 3:2, sealed and deoxygenated; 1-alkyl-3-vinylimidazolium halide is added to grow the Block B chain from the end of the BlockA chain by atom transfer radical polymerization using the halogen at the end of the BlockA chain as an initiation site, thereby obtaining the grafted cellulose-based separation membrane.

[0019] In the preparation method provided by the application, Block A chains and Block B chains are sequentially in-situ polymerized and grafted on both sides of the film surface to form a covalently bonded structure of "substrate-[Block A]-[Block B]". Specifically, the carboxyl group of the activated substrate can be first converted into acyl chloride, and then esterification is performed with the initiator containing a hydroxyl group, i.e., 2-hydroxyethyl isobutyrate, to generate a bromoalkyl ester (structure: substrate-COOCH2CH2OC(CH3)2Br); the bromoalkyl group serves as an initiation site of ATRP. Under the catalysis of CuBr / pentamethyldiethylene triamine (PMDETA), the aforementioned bromoalkyl ester is uniformly split to generate radicals, which initiate the growth of Block A chains on the surface of the substrate based on ATRP, with the initiation site of the film substrate as the starting point; and the Block B grafting process further occurs grafting growth with the bromine atom at the end of the Block A chain as the initiation site.

[0020] It should be noted that the group transformation process described in step S3 must be strictly controlled in terms of pH. Overly acidic conditions can cause the protonation of hydroxylamine, which loses nucleophilicity and can cause the hydrolysis of isothiocyanate to carbamate (byproduct); and overalkaline conditions can cause the excessive hydrolysis of cyanide to carboxylic acid, which cannot form the amine oxime group, and can also cause the decomposition of thioureido group to amino and sulfide. In addition, this step needs to be performed before the growth of the Block B chain in step S4. If the order is reversed, the hydrophilic long chain of Block B will hinder the diffusion of hydroxylamine, resulting in incomplete group transformation of Block A chain.

[0021] As a preferred technical solution of the application, in step S1, the activation treatment is performed on the rotating sample table of the plasma treatment instrument, ensuring that both sides of the film substrate are exposed to the plasma atmosphere, and the rotation speed is 5±2 rpm; the activation parameters are as follows: the oxygen flow is 20-30 sccm, the vacuum degree is 0.5-1.0 Pa, the power is 100 W, and the activation treatment is performed for 5-8 min.

[0022] As a preferred technical solution of the application, in step S2, the thionyl chloride treatment conditions are as follows: first, immerse in an ice bath at 0℃ for 0.5-1 h, and then raise the temperature to 25℃ and continue to immerse for 2 h; the amount of thionyl chloride used is 60-90 times the molar amount of the carboxyl group of the film substrate.

[0023] As a preferred technical solution of the application, in step S2, the esterification reaction conditions are as follows: use anhydrous triethylamine as a catalyst, first react at 0℃ for 2 h; then raise the temperature to 25℃ and continue to react for 12-15 h; the ice bath can inhibit side reactions, and the extension of the reaction time at 25℃ ensures complete esterification and avoids solution polymerization;

[0024] The amount of 2-hydroxyethyl isobutyrate used is 5-10 times the molar amount of the carboxyl group, and the molar ratio of triethylamine to 2-hydroxyethyl isobutyrate is 0.8-1.0.

[0025] As a preferred technical solution of the present application, in step S3, the pre-grafting growth conditions of the Block A chain are: under a nitrogen atmosphere, in the presence of a catalyst and an initiator, under a water bath at 40-50°C for 2-3h; during the grafting process, ensure that the film is turned over 180° every 25±5min; and keep the magnetic stirring speed at 350±50rpm to ensure that the monomer concentration and catalyst concentration on both sides of the film are consistent in real time;

[0026] The catalyst is CuBr / pentamethyldiethylenetriamine with a molar ratio of 1:1-1.2, and the amount used is 1-2mol% of the total monomer molar amount; the initiator is ethyl α-bromoisobutyrate, and the amount used is 0.1-1.0wt% of the total monomer mass; and the amount of hydroxylamine used is 1.2-2.0 times the total monomer molar amount of acrylonitrile and propylene isothiocyanate.

[0027] As a preferred technical solution of the present application, in step S4, the growth conditions of the Block B chain are: under a nitrogen atmosphere, at 65-75°C for 3-4h; during the grafting process, ensure that the film is turned over 180° every 25±5min; and the amount of 1-alkyl-3-vinylimidazolium halide used is 2-3 times the total monomer molar amount of acrylonitrile and propylene isothiocyanate.

[0028] In addition, the present application also provides the use of the above-mentioned separation membrane in electroplating wastewater treatment. When treating electroplating wastewater containing Pb 2+ ≥500mg / L, CrO4 2- ≥100mg / L, the separation membrane has a Pb 2+ adsorption capacity ≥320mg / g, a CrO4 2- removal rate ≥96.5%.

[0029] Beneficial effects:

[0030] The grafted cellulose-based separation membrane of the present solution takes the double-sided distributed thiourea-amine oxime synergistic coordination and hydrophobic imidazolium double-functional grafting layer as the core innovation: the thiourea-amine oxime in the Block A chain forms a bidentate synergistic coordination: the -S- of the thiourea group has strong nucleophilicity and can strongly coordinate with Pb 2+ ; the -NOH of the amine oxime group forms a stable bidentate coordination through hydrogen bonding assistance. The hydrophobic alkyl imidazolium long chain of the Block B chain forms a dense hydrophobic layer, adsorbs phenol through hydrophobic-hydrophobic interaction, and reduces the adsorption of proteins and other pollutants on the membrane surface; the positively charged imidazolium makes the Zeta potential ≥+40mV, which can remove harmful anions such as CrO4 2- through electrostatic adsorption.

[0031] The present scheme converts the cyano-isothiocyanate group in one step through a synchronous conversion process, avoiding residues from step-by-step conversion; promotes uniform growth of the bifunctional grafting layer through ATRP-controlled polymerization to make the group distribution coefficient of variation ≤15%, avoiding the group aggregation defects of traditional free radical polymerization; and through equipment adaptation (rotary sample table, reciprocating oscillation) during preparation, dynamic contact control (vertical suspension, regular turning), oxygen removal and solvent atmosphere protection, ensures that synchronous grafting on both sides can be realized, further improving its excellent performance in multi-pollutant treatment efficiency and stability. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 Schematic diagram of preparation of grafted cellulose-based separation membrane in Example 1.

[0033] Figure 2 TGA curve of grafted cellulose-based separation membrane in Examples and Comparative Examples.

[0034] Figure 3 DTG curve of grafted cellulose-based separation membrane in Examples and Comparative Examples.

[0035] Figure 4 S2p peak spectrum of grafted cellulose-based separation membrane in Example 1 and Comparative Examples 1-2 and Comparative Example 5; the four cases are located in the upper left, upper right, lower left and lower right areas of the figure in turn.

[0036] Figure 5 N1s peak spectrum of grafted cellulose-based separation membrane in Example 1 and Comparative Examples 1-2 and Comparative Example 5; the four cases are located in the upper left, upper right, lower left and lower right areas of the figure in turn. DETAILED DESCRIPTION

[0037] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the following combines specific examples and comparative examples, and through structured performance data and chemical principle analysis, the feasibility and advancement of the present application are comprehensively verified.

[0038] The purpose of this part is to outline some aspects of the embodiments of the present application and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this part to avoid obscuring the purpose of this part, the abstract of the specification and the purpose of the invention, and such simplifications or omissions cannot be used to limit the scope of the present application.

[0039] All experimental raw materials are commercially available analytical reagents, the test method refers to national standards or industry general specifications, the data is verified by 3 or more parallel experiments, and the error is ≤5%.

[0040] Cotton pulp cellulose, degree of polymerization 800, purity 99%, National Pharmaceutical Group.

[0041] 1 -butyl-3-methylimidazolium chloride ([BMIM]CI), CAS No. 79917-90-1, purity 98%, BOLIN BIOTECH GROUP. 2-bromo-2-methylpropionic acid, CAS No. 189324-13-8, purity 95%, Sigma-Aldrich. N-vinylimidazole, CAS No. 1072-63-5, purity 99%, Shanghai Jizisheng Biotech Co., Ltd. Hydroxylamine, CAS No. 7803-49-8, 50% aqueous solution, Shanghai Maier Biotech Co., Ltd.

[0042] 1 -chlorooctane, CAS No. 111-85-3, purity 99%; 1 -chlorotetradecane, CAS No. 2425-54-9, purity 98%; 1 -chlorohexadecane, CAS No. 4860-03-1, purity 95%, Shanghai Maikelin Biotech Co., Ltd.

[0043] Silk fibroin, extracted from silk fibroin, molecular weight 100 kDa, self-made; the anti-swelling rate was 96% after soaking in deionized water at 80°C for 24 h.

[0044] The following cellulose-containing film substrate was self-made, and the preparation process was as follows: 5.0 g of NaOH and 12.0 g of urea were dissolved in 100 mL of deionized water, and stirred until completely dissolved, then pre-cooled in a refrigerator at -10°C for 30 min; 6.0 g of cotton pulp cellulose and 4.0 g of silk fibroin were added, and mechanically stirred at 500 rpm for 2 h to form a uniform transparent solution. The above solution was scraped with a coater to form a wet film (wet film thickness 300 μm, size 10 cm x 10 cm), immediately immersed in deionized water at 25°C to coagulate and form; washed with water until neutral, soaked for 2 h, and the deionized water was replaced 3 times during the soaking to remove residual NaOH and urea, until the film surface pH = 7.0, vacuum dried at 60°C for 12 h, to obtain a cellulose film substrate (dry weight about 1.0 g).

[0045] In particular, the film substrate used in Comparative Example 3 was a pure cellulose film substrate, and the raw material used was 10 g of cotton pulp cellulose, and the remaining raw materials and preparation process were the same as above.

[0046] The 1 -alkyl-3-vinylimidazolium salt used in the following cases was prepared by quaternary ammonium reaction of N-vinylimidazole and chloroalkane (C n H 2n+ 1 Cl). N-vinylimidazole provides a polymerizable vinyl group as an imidazole ring parent body, and chloroalkane provides a long-chain alkyl group. Chloroalkanes with carbon chains of 8, 14, and 16 were selected as representatives, i.e. 1 -chlorooctane, 1 -chlorotetradecane, and 1 -chlorohexadecane. The chloroalkanes used were all dried with anhydrous magnesium sulfate for 24 h before use.

[0047] Preparation of Tetradecyl-3-vinylimidazolium Chloride: In a 500 mL three-necked flask, 0.1 mol of N-vinylimidazole was dissolved in 200 mL of acetonitrile under stirring; 0.12 mol of 1-chlorotetradecane was added dropwise slowly, and a reflux condenser was installed. The system was protected by nitrogen, and the temperature was raised to 70 °C. The reaction was stirred for 24 h. After the reaction, the system was cooled to room temperature, and white solid was precipitated. The solid was washed with anhydrous ether (3 x 50 mL) to remove acetonitrile and unreacted 1-chlorotetradecane. The crude product was dissolved in 50 mL of petroleum ether, and the insoluble substance was removed by filtration. The filtrate was frozen at -20 °C for 12 h, and then filtered under vacuum at 60 °C for 12 h to obtain white powder of 1-tetradecyl-3-vinylimidazolium chloride with a yield of 85%. The structural formula is:

[0048]

[0049] The preparation of 1-alkyl-3-vinylimidazolium chloride using 1-chlorooctane or 1-chlorohexadecane as the raw material was the same as above.

[0050] Example 1

[0051] A grafted cellulose-based separation membrane was prepared according to the following process: Figure 1 , specifically as follows:

[0052] S1, Double-sided activation of the membrane substrate: The dry membrane substrate was fixed on the rotating sample table (rotation speed 5 ± 1 rpm) of the plasma treatment instrument (SKY Technology), and the membrane was ensured to be exposed to the plasma atmosphere on both sides. The oxygen flow was set to 25 sccm, the vacuum degree was set to 0.8 Pa, the power was set to 100 W, and the activation was performed for 5 min. XPS measurement showed that the carboxyl density was 0.85 mmol / g (the characteristic peak of -COOH was located at 288.5 eV), and the error (large value-small value) on both sides was 0.02 mmol / g.

[0053] S2, Immobilization of the initiator: The activated membrane substrate of S1 was immersed in 100 mL of anhydrous tetrahydrofuran (THF), and the system was sealed and subjected to three cycles of freezing (-80 °C)-vacuum pumping (<1 Pa)-thawing (25 °C) to ensure that the oxygen content was <0.1 ppm. The system was sealed and protected by nitrogen, and placed in an ice bath at 0 °C. 5.0 mL of thionyl chloride (SOCl2) was slowly added, and a reciprocating shaker (120 rpm, amplitude 5 cm) was started for 1 h. Then the ice bath was removed, and the temperature was raised to 25 °C for continued shaking for 2 h.

[0054] The THF-SOCl2mixture was poured off and the membrane substrate was washed with anhydrous THF three times. 100 mL of anhydrous THF was added, the oxygen was removed and nitrogen was introduced. 1.0 g of 2-bromoisobutyric acid hydroxyethyl ester and 0.6 mL of triethylamine were added and the reaction was carried out at 0°C with shaking (120 rpm) for 2 h. Then the temperature was raised to 25°C and the reaction was continued with shaking for 12 h. After the reaction was completed, the membrane substrate was washed with anhydrous THF three times, washed with ethanol until neutral, and dried at 60°C under vacuum for 12 h. XPS analysis showed that the Br content on both sides of the membrane was 0.27 mmol / g, and the error (max-min) on both sides was 0.01 mmol / g.

[0055] S3, Double-sided growth of Block A chain: 50 mL of [BMIM]Cl / water mixed solvent (v:v=4:1) was prepared, and its conductivity was measured to be 6.5 mS / cm. 0.1 mol of acrylonitrile and 0.01 mol of propylene isothiocyanate were ultrasonically dissolved. The membrane substrate of S2 was vertically hung in the center of the reaction bottle (polytetrafluoroethylene hook, not touching the bottle wall), and after being immersed in the above-mentioned solution, the oxygen was removed and nitrogen was introduced. Then 0.33 mmol of CuBr, 0.396 mmol of PMDETA, and 6.0 mg of ethyl α-bromoisobutyrate were added, and the system was uniformly mixed and then reacted at 45°C in an oil bath for 2.5 h (the membrane was turned 180° every 30 min), and a pre-grafted membrane was obtained.

[0056] 10.0 g of hydroxylamine (5 g of pure hydroxylamine) was dissolved in 10 mL of deionized water, diluted with ethanol / deionized water (1:1 by volume) to 100 mL to prepare a hydroxylamine solution, and the pH was adjusted to 7.0±0.2 with 0.1 mol / L NaOH. The pre-grafted membrane was immersed in it and treated with constant temperature water bath shaking at 30°C for 4 h at a shaking frequency of 80 rpm. It was washed with deionized water (50 mL x 3) and dried at 60°C under vacuum for 5 h to obtain a Block A grafted and grown separation membrane.

[0057] S4, Double-sided growth of Block B chain: 100 mL of DMF / water solvent (volume ratio=3:2) was prepared, 0.297 mol of 1-tetradecyl-3-vinylimidazolium chloride was added and stirred until dissolved. The Block A modified membrane was vertically hung in the solution. After sealing, oxygen removal and nitrogen introduction, the temperature was raised to 70°C and the reaction was stirred for 3.5 h (the membrane was turned 180° every 30 min). The unreacted monomer was removed by ethanol washing (50 mL x 3), and then deionized water washing (50 mL x 2) was used to remove ethanol. The grafted cellulose-based separation membrane (dry weight about 1.5 g) was obtained after drying at 60°C under vacuum for 12 h.

[0058] Example 2

[0059] The difference from Example 1 is that in step S4, the amount of tetradecyl-3-vinylimidazolium chloride is adjusted to 0.231 mol, and the reaction temperature is 65°C.

[0060] Example 3

[0061] The difference from Example 1 is that the concentration of hydroxylamine solution is adjusted in step S3: 12.0 g of hydroxylamine (6 g of pure hydroxylamine) is dissolved in 10 mL of deionized water, diluted to 100 mL with ethanol / deionized water (1:1 by volume), and prepared into a hydroxylamine solution. The pH is adjusted to 7.0±0.2 with 0.1 mol / L NaOH. The rest of the operations remain unchanged.

[0062] Example 4

[0063] The difference from Example 1 is that the molar ratio of acrylonitrile to prop-2- enylic isothiocyanate is adjusted to 9:1 in step S3 without changing the total amount of monomers.

[0064] Example 5

[0065] The difference from Example 1 is that the raw material 1-alkyl-3-vinylimidazolium halide in step S4 is replaced with an equal amount of 1-hexadecyl-3-vinylimidazolium chloride.

[0066] Comparative Example 1

[0067] The difference from Example 1 is that only acrylonitrile is used for atom transfer radical polymerization in step S3, i.e., there is no thiourea group precursor in the Block A chain.

[0068] Comparative Example 2

[0069] The difference from Example 1 is that the raw material 1-alkyl-3-vinylimidazolium halide in step S4 is replaced with an equal amount of 1-octyl-3-vinylimidazolium chloride.

[0070] Comparative Example 3

[0071] The difference from Example 1 is that the film substrate used is a pure cellulose film substrate without silk fibroin, and the anti-swelling rate is measured to be 72%.

[0072] Comparative Example 4

[0073] The difference from Example 1 is that the free radical polymerization under aqueous phase system initiated by ammonium persulfate is used instead of atom transfer radical polymerization in steps S3 and S4.

[0074] Comparative Example 5

[0075] The difference from Example 1 is that the 1-alkyl-3-vinylimidazolium salt used in step S3 is replaced with an equal amount of tetradecyl-3-vinylimidazolium iodide.

[0076] Test Example

[0077] 1. Functional group density and structure

[0078] Thiocarbonyl density: XPS (Thermo Fisher K-Alpha), calibrated at binding energy 162.5 eV, error ≤ 0.05 mmol / g.

[0079] Coefficient of variation: XPS area scan 50 μm x 50 μm, S, N element distribution coefficient of variation, coefficient of variation = standard deviation / average x 100%.

[0080] Amine oxime density: hydroxylamine hydrochloride-potassium bromide titration method (0.1 mol / L Br2solution titration, starch indicator end point, error ≤ 0.1 mmol / g).

[0081] 2. Membrane surface performance test

[0082] Zeta potential: Malvern Zetasizer Nano ZS90, sample cell method. 0.01 mol / L KCl solution, pH = 5.0, test temperature 25℃, 3 measurements for average value;

[0083] Water contact angle: DSA100 contact angle instrument, sessile drop method. 5 μL deionized water droplet, 5 different measurement points for average value, error ≤ 2°.

[0084] The above test results are shown in Table 1.

[0085] Table 1

[0086]

[0087] 3. Thermal stability

[0088] Thermogravimetric analyzer TGA (ATS-TGA-1200 thermogravimetric analyzer, Shanghai Ettis Instrument Technology Co., Ltd.) was used. Temperature resolution 0.01℃, thermogravimetric resolution 0.01 mg.

[0089] Test process: Before testing, blank baseline correction was performed with empty alumina crucible to eliminate the influence of crucible itself weight loss on the results. The sample crucible was placed in the instrument sample table, the test cabin was sealed, and the temperature was raised according to the preset program at a rate of 10±0.1℃ / min from room temperature to 600℃; after constant temperature at 600℃ for 1 h, it was gradually reduced to room temperature at a rate of 10±0.1℃ / min. The TGA (residual mass, %) and DTG (mass loss rate, % / min) curves were recorded in real time, respectively as Figure 2 and Figure 3 .

[0090] The TGA curve represents the change of the percentage of residual mass with temperature, and the DTG curve represents the change of the mass loss rate (% / min) with temperature. In Figure 2 andFigure 3 In Examples 1-5, the weight loss was only 5% in the low-temperature range (<260℃), indicating that silk fibroin has good anti-swelling effect and inhibits water evaporation. In the mid-temperature range (260-400℃), there was thiourea-amine oxime synergistic decomposition, and in the high-temperature range (>400℃), there was a main decomposition peak of the imidazolium long chain. Due to the presence of the imidazolium long chain, thermal stability was improved, and the char residue was ≥30%. In Comparative Example 2, the Block B chain was short, with a char residue of only 14%; double decomposition peaks appeared at 350℃ and 420℃, corresponding to disulfide bond breakage and amide decomposition peaks, respectively. Furthermore, the iodine salt scheme used in Comparative Example 5 showed a sharp peak (-15% / min) at 300℃ on the DTG curve, which is a characteristic peak of iodine volatilization; subsequently, a main decomposition peak (-10% / min) appeared at 345℃, indicating an earlier decomposition temperature and reduced stability of the imidazolium backbone. However, the carbon skeleton still has a certain degree of stability, so it converges smoothly to zero in the high-temperature region (above 550℃); the residual carbon rate is still 18% at 600℃, indicating that there are no side reactions or decomposition effects afterward.

[0091] 4. Elemental analysis of membrane surface

[0092] Surface chemical element analysis of the grafted cellulose-based separation membrane samples of Example 1, Comparative Examples 1-2, and Comparative Example 5 was performed using XPS (Thermo Fisher K-Alpha), and the results were as follows: Figure 4 and Figure 5 The characteristic peak spectra of S2p and N1s.

[0093] exist Figure 4 In Example 1, the S2p orbital showed only a sharp symmetrical single peak of 162.5 eV (half-peak width ≈ 1.2 eV), corresponding to the electron binding energy of the -C=S bond in the thiourea group, which was lower than the 163.2 eV of free thiourea. Its symmetrical peak shape proved that there were no oxidation byproducts and that sulfur existed entirely in the form of thiourea, indicating that the grafting reaction was successful.

[0094] Comparative Example 1 lacks a thiourea group, which prevents the S atom from forming a stable C=S structure and keeps it in a free state. Therefore, its S2p ​​orbital only has a weak thiolate (-S-) or disulfide (-SS-) characteristic peak at 164 eV, and the peak width is relatively large (about 2.5 eV), indicating that the sulfur atom is oxidized and multiple sulfur species coexist.

[0095] Comparative Example 2 shows a bimodal structure. The main peak at 162.5 eV is a thioether bond (CSC), accounting for 70%; the shoulder peak at 164.0 eV is a disulfide bond, accounting for 30%, confirming that short alkyl chains easily lead to the formation of intermolecular disulfide bonds.

[0096] Comparative Example 5 is a doublet structure of 164 eV and 168 eV, respectively attributed to residual thioureido group and sulfonic acid group peak oxidized by iodine catalysis. The doublet spacing of 4 eV is consistent with the spin splitting characteristics of S2p orbitals, but the intensity ratio is abnormal (164 eV: 168 eV ≈ 7:13), indicating that 65% of sulfur is oxidized. This indicates that the weak nucleophilicity of iodine ions can weaken the stability of the imidazolium ring through an induction effect, leading to the oxidation of thioureido group and the formation of sulfonic acid product.

[0097] Figure 5 In the N1s spectrum of Example 1, the three peaks are clearly separated. Among them, the main peak at 399.2 eV is the amine oxime nitrogen (-C(NH2)=NOH), accounting for 49%; the peak at 398.0 eV is the thioureido nitrogen (-NH-CS-NH2), accounting for 20%, both confirming the successful growth of Block A chain; and the peak at 401.1 eV corresponds to the cationic nitrogen of imidazolium ([C-N=C(CH3)-N + -CH=CH2]) with a half-peak width of ≈1.8 eV, accounting for 31%.

[0098] Comparative Example 1 shows a doublet structure in the range of 396-406 eV: 399.0 eV for free amino group and 402.1 eV shoulder peak for oxidized nitrogen. This indicates that the absence of thioureido group leads to the exposure of amine group, which is easy to oxidize to form nitrogen oxides such as nitroso (-N=O) or nitro (-NO2). The oxidized nitrogen destroys the molecular structure, so it can explain why the TGA residual carbon rate is only 22%.

[0099] Comparative Example 2 is a three-peak structure, in which the cyano nitrogen (398.1 eV) is the result of intramolecular cyclization induced by short alkyl chain, the appearance of amide nitrogen (400.8 eV) peak indicates the occurrence of hydrolysis side reaction; and the charge transfer state nitrogen peak (403.0 eV, accounting for 25%) is due to the localization of charges caused by steric hindrance.

[0100] Comparative Example 5 is a doublet structure, in which 400.5 eV is attributed to protonated imidazole nitrogen, and cyano nitrogen (398.1 eV) and amide nitrogen (400.8 eV) are the results of intramolecular cyclization induced by short alkyl chain; and the charge transfer state nitrogen peak (403.0 eV) is due to the localization of charges caused by steric hindrance. 402.5 eV is oxidized nitrogen (N + -O - ), because I - weakens the electrostatic effect of imidazolium ring, making the nitrogen atom more susceptible to oxidation.

[0101] Application Example

[0102] 1. Application of grafted cellulose-based separation membrane in wastewater treatment.

[0103] Configuration of simulated electroplating wastewater: 500 mg / L Pb(NO3)2, 100 mg / L K2CrO4 and 38 mg / L phenol, and containing Cu(NO3)2, ZnCl2 each 50 mg / L, adjust pH to 5.0.

[0104] Wastewater treatment process:

[0105] (1) Filtration: The grafted cellulose-based separation membrane of each example / comparative example above was cut to size of 5 cm x 5 cm and fixed in a membrane filtration assembly; under the conditions of 25 ± 1 °C and cross-flow rate of 200 rpm, the electroplating wastewater of the above composition was continuously filtered for 120 min.

[0106] (2) Elution: 0.1 M citric acid-sodium citrate buffer solution was prepared as eluent (pH ~ 5.0). The cyclic desorption method was used, with a flow rate of 0.3 L / min for 1 h; after elution, the membrane surface was rinsed with deionized water at a flow rate of 0.3 L / min for 10 min, and vacuum dried at 60 °C for 30 min.

[0107] The filtrate and the original wastewater were taken and tested for Pb 2+ concentration by ICP-MS (Agilent 7900); the phenol content was monitored by TOC-L CPH total organic carbon analyzer; the CrO4 2- absorbance was detected by UV-Vis (Shimadzu UV-2600, wavelength of 372 nm), and the Pb 2+ adsorption capacity, phenol TOC removal rate and CrO4 2- removal rate were calculated.

[0108] 2. Anti-pollution performance

[0109] Test method: 1 g / L bovine serum albumin (BSA) solution was used as the pollution liquid, and the flux decay rate (initial flux J0, flux J1 after 1 h, decay rate = (J0-J1) / J0 x 100%) was measured. The separation performance test results of all grafted cellulose-based separation membranes are shown in Table 2.

[0110] Table 2

[0111]

[0112] In combination with Table 1 and Table 2, in the treatment application of wastewater, the two functional zones of the grafted cellulose-based separation membrane are clear and synergistically improve the comprehensive treatment capacity of the membrane. The hydrophobic imidazolium chain of Block B chain can quickly capture phenol (TOC removal rate ≥ 82%); the positive imidazolium can adsorb CrO4 2- through electrostatic adsorption; the thiourea group in Block A chain can coordinate with Pb 2+ , so that Pb 2+Adsorption capacity ≥320 mg / g. It should be noted that Pb... 2+ The adsorption depends on the functional groups of the Block A chain. Although the positive charge of the Block B chain may affect Pb... 2+ There is slight repulsion, but the coordination effect of Block A chain is stronger; and Block B chain can promote the coordination of Pb in solution. 2+ The long-chain alkyl hydrophobic layer diffuses into the region of the Block A chain, further reducing interference from water molecules at the Block A chain coordination sites and enhancing Pb. 2+ The stability of binding with coordinating groups. Therefore, overall Pb 2+ Adsorption is not significantly affected by the Block B chain.

[0113] Comparative Example 1, due to the absence of the thiourea group, Pb 2+ The adsorption capacity decreased by 37.3% compared to Example 1; although Comparative Example 5 had a certain positive charge (+35mV), its thiourea group was lost due to iodine oxidation, resulting in a 16.4% decrease in adsorption capacity and a 111% increase in flux decay rate. Both examples confirm that the synergistic coordination between the thiourea group and the amine oxime group is key to improving the adsorption capacity. It should be noted that the absence of the thiourea group has little effect on the electrostatic interaction of the block B chain; therefore, CrO4... 2- The removal rate showed a relatively small downward trend.

[0114] Comparative Example 2 showed that due to the shorter hydrophobic end of the Block B chain, the water contact angle was reduced, and the phenol removal rate decreased by 21.9%. This indicates that long-chain hydrophobic imidazolium can balance hydrophobicity and mass transfer efficiency, and enhance the adsorption of phenol and improve the resistance to organic pollution through hydrophobic interactions.

[0115] Comparative Example 3, lacking silk fibroin, showed a decrease in substrate swelling resistance to 72%; the adsorption capacity retention rate after cycling decreased by 11.3%, confirming that silk fibroin can enhance the mechanical strength and swelling resistance of the substrate and extend membrane life.

[0116] Comparative Example 4, due to the use of traditional free radical polymerization, showed a 58.3% decrease in the uniformity of Block A chain group distribution, leading to a decrease in Pb... 2+ The adsorption capacity decreased by 19.4%, indicating that the controllability of atom transfer radical polymerization can improve the growth and distribution uniformity of functional groups and optimize adsorption performance.

[0117] In the antifouling performance test, the separation membrane exhibited good antifouling performance, with flux attenuation rates of ≤26% in each embodiment. In contrast, the attenuation rate of Comparative Example 2 reached 35.6%, confirming that long-chain hydrophobic imidazolium can reduce protein adsorption and improve antifouling performance through hydrophobic-hydrophobic repulsion.

[0118] The above data show that the present invention significantly improves the treatment performance of grafted cellulose-based separation membranes for electroplating wastewater through the innovative combination of thiourea-aminooxime synergistic coordination and long-chain hydrophobic imidazolium antifouling.

[0119] It should be noted that the above examples are only used to illustrate the technical solutions of the present application but not to limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the spirit and scope of the present application, and all modifications and equivalents should be included in the scope of the claims of the present application.

Claims

1. A grafted cellulose-based separation membrane, characterized by, The cellulose-containing membrane substrate and the double-grafted chains grown on both sides of the membrane; the double-grafted chains include grafted BlockA chains and BlockB chains grafted on the ends of the BlockA chains; the separation membrane structure is: membrane substrate-[BlockA]-[BlockB]; The BlockA chain is a block copolymer chain containing thioureido groups and amine oxime groups; the molar ratio of the thioureido groups to the amine oxime groups is 1:3-5, and the density of the thioureido groups is ≥1.2 mmol / g, based on the dry weight of the BlockA chain; The BlockB chain is a block hydrophobic chain containing a grafted product of a vinyl imidazolium salt; the vinyl imidazolium salt is a 1-alkyl-3-vinyl imidazolium halide prepared by quaternization reaction of N-vinylimidazole and a halogenated alkane; the halogenated alkane includes a chloroalkane or a bromoalkane with a carbon chain of 11-17; The cellulose-containing membrane substrate further contains 30-40 wt% of silk fibroin.

2. The separation membrane according to claim 1, characterized by The BlockA chain is a block copolymer chain formed by pre-grafting of acrylonitrile and propylene isothiocyanate at a molar ratio of 8-10:1 through atom transfer radical polymerization, and then group transformation process with 5-8 wt% of hydroxylamine solution; the coefficient of variation of the thioureido group distribution is ≤15%.

3. The separation membrane according to claim 1, characterized by, The cellulose has a polymerization degree of 500-1500; the Zeta potential of the separation membrane is ≥+40 mV, the water contact angle is ≥105°, and the carbon residue rate at 600°C is ≥30%.

4. The production method of the grafted cellulose-based separation membrane according to any one of claims 1 to 3, characterized by, Specifically includes the following steps: S1, double-sided activation of the substrate: both sides of the cellulose-silk fibroin membrane substrate are treated by plasma activation until the carboxyl group density on the surface of the membrane substrate is ≥0.8 mmol / g, and the deviation of the carboxyl group density on both sides of the membrane is ≤0.05 mmol / g; S2, fixation of the initiator: the membrane substrate of S1 is immersed in anhydrous tetrahydrofuran, sealed and deoxygenated; under a nitrogen atmosphere, sulfuric chloride is added and treated by reciprocating oscillation to complete the surface acyl chloride modification; then the initiator and anhydrous triethylamine are added, and the reciprocating oscillation is maintained to complete the fixation of the initiator on both sides of the membrane substrate through esterification reaction; the deviation of the Br element content on both sides detected by XPS is ≤0.02 mmol / g; the initiator is 2-bromo isobutyric acid hydroxyethyl ester; S3, double-sided growth of BlockA chain: the membrane substrate of S2 is vertically hung and immersed in an ionic liquid / water mixed solvent, sealed and deoxygenated; acrylonitrile and propylene isothiocyanate monomers are added to grow the BlockA chain through atom transfer radical polymerization; then the membrane is transferred to a solution of hydroxylamine, hydrazine or amino compound with pH=7.0±0.2, and treated at a constant temperature of 30-35°C for 4-6 h to simultaneously convert the cyano group to amine oxime group and the isothiocyanate group to thioureido group, thereby obtaining a separation membrane with double-sided growth of BlockA chain; the deviation of the thioureido group density on both sides is ≤0.08 mmol / g; S4, Double-side growth of Block B chain: vertically suspending the separated membrane of S3 in DMF / water mixed solvent, sealing and deoxidizing; adding 1-alkyl-3-vinylimidazolium halide, growing Block B chain from the end of Block A chain based on atom transfer radical polymerization with the end halogen of Block A chain as the initiation site, to obtain the grafted cellulose-based separated membrane.

5. The method of claim 4, wherein, In step S1, the activation treatment is performed on the rotating sample table of the plasma treatment instrument, ensuring that both sides of the membrane substrate are exposed to the plasma atmosphere at a rotation speed of 5±2 rpm; the activation parameters are as follows: oxygen flow rate of 20-30 sccm, vacuum degree of 0.5-1.0 Pa, and power of 100 W, and the activation treatment is performed for 5-8 min.

6. The method of claim 4, wherein, In step S2, the thionyl chloride treatment conditions are as follows: first, immerse in an ice bath at 0°C for 0.5-1 h, and then continue to immerse at 25°C for 2 h; the amount of thionyl chloride used is 60-90 times the molar amount of the carboxyl groups of the membrane substrate.

7. The method of claim 4, wherein, In step S2, the esterification reaction conditions are as follows: first, react at 0°C for 2 h with anhydrous triethylamine as the catalyst; then, continue to react at 25°C for 12-15 h; the amount of 2-bromoisobutyric acid hydroxyethyl ester used is 5-10 times the molar amount of the carboxyl groups, and the molar ratio of triethylamine to 2-bromoisobutyric acid hydroxyethyl ester is 0.8-1.

0.

8. The method of claim 4, wherein, In step S3, the pre-grafting growth conditions of the Block A chain are as follows: under a nitrogen atmosphere, in the presence of a catalyst and an initiator, react at a water bath temperature of 40-50°C for 2-3 h; during the grafting process, ensure that the membrane is flipped over by 180° every 25±5 min, and maintain magnetic stirring at a rotation speed of 350±50 rpm; The catalyst is CuBr / pentamethyldiethylenetriamine with a molar ratio of 1:1-1.2, and the total amount is 1-2 mol% of the total molar amount of the monomers; the initiator is ethyl α-bromoisobutyrate, and the amount used is 0.1-1.0 wt% of the total mass of the monomers; the amount of hydroxylamine used is 1.2-2.0 times the total molar amount of acrylonitrile and propylene isothiocyanate monomers.

9. The method of claim 4, wherein, In step S4, the growth conditions of the Block B chain are as follows: under a nitrogen atmosphere and at a temperature of 65-75°C, react for 3-4 h; during the grafting process, ensure that the membrane is flipped over by 180° every 25±5 min; the amount of 1-alkyl-3-vinylimidazolium halide used is 2-3 times the total molar amount of acrylonitrile and propylene isothiocyanate monomers.

10. Use of the separation membrane according to any one of claims 1 to 3 in the treatment of electroplating wastewater, characterized in that, In the treatment of Pb 2 + ≥500mg / L, CrO4 2- ≥100mg / L electroplating wastewater, its Pb 2+ adsorption capacity ≥320mg / g, CrO4 2- removal rate ≥96.5%.

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