Preparation method and application of a polyester loose nanofiltration membrane based on guar gum cross-linking structure

Abstract

The application discloses a kind of preparation method and application of polyester loose nanofiltration membrane based on guar gum crosslinking structure, belong to membrane separation technical field.The preparation steps of the polyester loose nanofiltration membrane are as follows: guar gum and organic base catalyst are dissolved in water to obtain the aqueous solution containing guar gum;Ultrafiltration membrane is sequentially immersed in the aqueous solution containing guar gum and the oil phase solution containing trimesoyl chloride, and is treated by heat to obtain the polyester loose nanofiltration membrane.Guar gum, a biological macromolecule with low price, unique branched structure and rich in hydroxyl group, is used as the aqueous monomer, and the prepared loose nanofiltration membrane has high permeation flux, excellent dye rejection rate and inorganic salt permeation rate, and good anti-pollution performance, which can realize high-selectivity separation of dye / inorganic salt in textile wastewater, and has high practical application value.The method provided by the application is simple and easy to operate, the raw material cost is low, the preparation process can be enlarged, and it is beneficial to large-scale production and market promotion in the field of nanofiltration membrane.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of membrane separation, and more particularly relates to a preparation method and application of a polyester loose nanofiltration membrane based on a guar gum cross-linking structure. BACKGROUND

[0002] Textile wastewater usually contains a large amount of carcinogenic organic dyes, inorganic salts (such as NaCl and Na2SO4) and other organic compounds. If directly discharged, it will pose a threat to the ecological environment and human health. Separation and recovery of these solutes can not only reduce costs and the demand for raw materials, but also reduce the impact of textile wastewater on the environment. The use of nanofiltration technology to treat high-salinity textile wastewater has become a promising resource recovery and reuse strategy. High-performance nanofiltration membrane materials are crucial for promoting the application of membrane technology in the field of textile wastewater treatment. Currently, commercial nanofiltration membranes are mainly polyamide membranes with a dense functional layer, which are prepared by interfacial polycondensation of amine monomers (water-phase monomers) and acyl chloride monomers (oil-phase monomers). These membranes have high rejection rates for most solutes (such as dyes and salts), making it difficult to achieve selective separation of dye molecules and inorganic salts. Therefore, the preparation of loose nanofiltration membranes with high permeability and selectivity, which can effectively reject dye molecules while allowing rapid permeation of water molecules and inorganic salts, is of great significance for the separation of dyes / inorganic salts in textile wastewater.

[0003] Compared with polyamide nanofiltration membranes, polyester nanofiltration membranes have the following advantages: on the one hand, ester bonds (-COO-) are more resistant to chlorine than amide bonds (-CONH-), which is beneficial to maintaining the stability of the membrane during chemical cleaning; on the other hand, compared with commonly used amine monomers such as piperazine, the reactivity of hydroxyl monomers is lower, and the interfacial polymerization process is more controllable, which is beneficial to reducing the cross-linking degree of the polyester structure and achieving high selectivity separation of dyes / inorganic salts. Currently, the water-phase monomers used to prepare polyester loose nanofiltration membranes include alcohol compounds (such as tannic acid, mannitol, maltitol, etc.), cyclodextrin, polyvinyl alcohol, sodium alginate, etc. However, the water flux and dye / inorganic salt selectivity of the prepared polyester nanofiltration membranes still need to be further improved. For example, when sodium alginate is used as the water-phase monomer to prepare polyester nanofiltration membranes, the permeation flux of the membranes is less than 20 L·m -2 ·h -1 ·bar -1 , and the rejection rate of Na2SO4 is more than 40%, which cannot meet the requirements of loose nanofiltration membranes for textile wastewater. Therefore, there is an urgent need to develop new high-permeability polyester loose nanofiltration membranes for high-selectivity separation of dyes / inorganic salts. SUMMARY

[0004] The application aims to provide a preparation method and application of a polyester loose nanofiltration membrane based on a guar gum cross-linking structure, so as to solve the problems in the prior art and realize high selective separation of dyes and inorganic salts.

[0005] To achieve the above-mentioned purposes, the application provides the following solutions.

[0006] One of the technical solutions of the application provides a preparation method of a polyester loose nanofiltration membrane based on a guar gum cross-linking structure, comprising the following steps.

[0007] The guar gum and an organic base catalyst are dissolved in water to obtain an aqueous solution containing the guar gum; an ultrafiltration membrane is sequentially immersed in the aqueous solution containing the guar gum and an oil phase solution containing trimesoyl chloride, and is subjected to heat treatment to obtain the polyester loose nanofiltration membrane based on the guar gum cross-linking structure.

[0008] The organic base catalyst comprises triethylamine, tripropylamine or N,N-diisopropyl ethylamine.

[0009] Preferably, the preparation method of the polyester loose nanofiltration membrane based on the guar gum cross-linking structure specifically comprises the following steps.

[0010] The guar gum and an organic base catalyst are dissolved in water to obtain an aqueous solution containing the guar gum; the aqueous solution containing the guar gum is poured on the surface of a support layer of an ultrafiltration membrane for immersion treatment to obtain an ultrafiltration membrane with distributed aqueous monomers; then, the excess aqueous solution containing the guar gum is removed, an oil phase solution containing trimesoyl chloride is poured on one side of the ultrafiltration membrane with the distributed aqueous monomers, subjected to interfacial polymerization, the excess oil phase solution containing trimesoyl chloride is removed, and heat treatment is performed to obtain the polyester loose nanofiltration membrane based on the guar gum cross-linking structure.

[0011] The organic base catalyst comprises triethylamine, tripropylamine or N,N-diisopropyl ethylamine.

[0012] In the application, a solution of triethylamine, tripropylamine or N,N-diisopropyl ethylamine is selected as a catalyst for the reaction of a hydroxyl-containing substance and an acyl chloride monomer, instead of sodium hydroxide, because sodium hydroxide is more suitable for the rapid esterification of primary alcohols and acyl chlorides, and is not ideal for the catalytic effect of the system, and triethylamine, tripropylamine or N,N-diisopropyl ethylamine is suitable for the esterification reaction of a hydroxyl-containing substance with steric hindrance.

[0013] Preferably, the concentration of the organic base catalyst in the aqueous solution containing the guar gum is 0.5-2 wt%, and the concentration of the guar gum in the aqueous solution containing the guar gum is 0.05-0.3 wt%.

[0014] Preferably, the solvent in the oil phase solution containing trimesoyl chloride comprises n-hexane, cyclohexane or Isopar G; the concentration of trimesoyl chloride in the oil phase solution containing trimesoyl chloride is 0.1-0.3 wt%.

[0015] Preferably, the guar gum comprises guar gum original powder and / or functional group derivatized guar gum.

[0016] Further, the functional group derivatized guar gum comprises carboxymethyl guar gum and / or hydroxypropyl guar gum.

[0017] Preferably, the ultrafiltration membrane comprises a polyether sulfone ultrafiltration membrane, a polysulfone ultrafiltration membrane or a polyacrylonitrile ultrafiltration membrane.

[0018] Preferably, the time of the immersion treatment is 2-10 min; the time of the interfacial polymerization reaction is 2-10 min.

[0019] Preferably, the temperature of the heat treatment is 40-90 DEG C, and the time is 1-30 min.

[0020] The water phase monomer, the oil phase monomer, the catalyst dosage, the heat treatment temperature and the time all exist a suitable range, these parameters are directly related to the thickness and the crosslinking degree of the polyester separation layer prepared by the interfacial polymerization, thereby affecting the permeation flux and the separation selectivity of the polyester membrane, i.e. the structure-activity relationship of the membrane.

[0021] The second technical scheme of the present application provides a polyester loose nanofiltration membrane based on a crosslinked structure of guar gum, which is prepared by the preparation method.

[0022] The third technical scheme of the present application provides an application of the polyester loose nanofiltration membrane based on the crosslinked structure of guar gum in textile wastewater treatment.

[0023] The technical mechanism of the present application is as follows:

[0024] Guar gum (GG) is a kind of environment-friendly high-purity natural polysaccharide extracted from guar beans, which has a unique branched structure, is soluble in cold / hot water, and has rich hydroxyl functional groups. Compared with the ionic crosslinking dependence of sodium alginate and the acid dissolution limitation of chitosan, the unique structure, universality and low cost of guar gum make it occupy an important position in bio-based macromolecules. In the present application, guar gum is used as a water phase monomer to prepare a high-flux, anti-pollution polyester loose nanofiltration membrane for the selective separation of dyes and inorganic salts in textile wastewater, which not only widens the utilization field of guar gum and its derivatives, but also promotes the development of high-performance membrane materials.

[0025] The application takes bio-based macromolecular guar gum as a water-phase monomer, and performs interfacial polymerization reaction with oil-phase monomer trimesoyl chloride (TMC) under the action of triethylamine catalyst to prepare a polyester loose nanofiltration membrane. The water-phase monomer guar gum adopted is a natural polysaccharide material with unique branched structure and low price, and has low reactivity, large molecular weight (about 220 kDa) and good hydrophilicity. The reaction with TMC can generate a loose nanofiltration membrane with low crosslinking degree and suitable pore size structure, which not only has good chlorine resistance of the polyester membrane, but also can obtain high permeation flux and dye / inorganic salt selectivity. Moreover, the polyester functional layer formed through the interfacial polymerization reaction has good hydrophilicity, and the prepared membrane has excellent anti-pollution performance, which can prolong the service life of the membrane and enhance the service performance of the membrane in treating textile wastewater. The polyester loose nanofiltration membrane disclosed in the application exhibits stable permeation flux, excellent dye rejection rate and inorganic salt permeation rate in the 12h dye / inorganic salt mixed solution separation process, which makes up for the deficiency that the existing commercial nanofiltration membrane cannot realize selective separation of organic small molecules and inorganic salts.

[0026] The application discloses the following technical effects:

[0027] (1) Guar gum is used as a new type of bio-based macromolecular water-phase monomer, and its low reactivity and branched structure make it easy to react with TMC to generate a polyester functional layer with low crosslinking degree and suitable pore size structure, so that efficient permeation of water molecules and inorganic salt molecules and efficient rejection of dye molecules are realized.

[0028] (2) The selected water-phase monomer guar gum has low cost, good water solubility, environmental friendliness and low dosage, thereby reducing the preparation cost of the polyester loose nanofiltration membrane.

[0029] (3) Benefiting from the good hydrophilicity of the polyester nanofiltration membrane, compared with a traditional polyamide nanofiltration membrane, the polyester nanofiltration membrane has higher permeation selectivity and better anti-pollution effect, is conducive to maintaining stable separation performance of the membrane in a long-time use process, and has high practical application value.

[0030] (4) The application adopts the method of interfacial polymerization to prepare the polyester loose nanofiltration membrane, the method can be scaled up, and combined with low raw material cost, the technology is suitable for large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 It is a scanning electron microscope graph of the polyether sulfone ultrafiltration membrane surface in Example 1 at different magnifications;

[0032] Figure 2 It is a scanning electron microscope graph of the polyester loose nanofiltration membrane prepared in Example 1 at different magnifications;

[0033] Figure 3ATR-FTIR spectra of the polyethersulfone ultrafiltration membrane and the prepared polyester loose nanofiltration membrane of Example 1;

[0034] Figure 4 Effect plots of permeation flux, Congo red rejection rate, and sodium chloride rejection rate of the polyester loose nanofiltration membrane prepared for Example 1, Example 3, and Comparative Example 5 when treating a Congo red / sodium chloride mixed solution;

[0035] Figure 5 Water contact angle plot of the polyester loose nanofiltration membrane prepared for Example 3;

[0036] Figure 6 Performance plot of the polyester loose nanofiltration membrane prepared for Example 3 when treating a Congo red / sodium chloride mixed solution for a long time. DETAILED DESCRIPTION

[0037] The detailed description particularly prefers certain aspects, features and embodiments of the present application, but it is to be understood that no limitation of the scope of the present application is intended thereby, but rather the objectives of the present application are to provide a more detailed description of certain aspects, features and embodiments of the present application.

[0038] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, for a range of values of a parameter, unless otherwise stated, each intervening value of the parameter is also specifically included within the scope of the present application. The intervening values of the parameter are combined with a stated value of the parameter in range form. These are only examples of the various values of the parameter, which are consistent with the present application. Other examples of the various values of the parameter, which are consistent with the present application, are also possible and encompassed within the scope of the present application.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All publications mentioned in the specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any reference is not construed as an admission that it is prior art with respect to the present application.

[0040] Many modifications and variations of this application can be made in the light of the above teachings without departing from the spirit and scope thereof, and it is to be understood that all such modifications and variations fall within the scope of the application. Additional embodiments of the application will readily occur to those skilled in the art. The disclosures of the specification and drawings are to be considered as illustrative only of the principles of the application and are to be considered not as limiting of the scope of the application as defined by the appended claims.

[0041] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" and the like are open-ended and do not exclude additional, unrecited elements or method steps.

[0042] It should be noted that the present application does not detail the conventional operation means in the art, and is not the focus of the present application.

[0043] The raw materials used in the following examples and comparative examples of the present application are commercially available products, and the source of the commercially available products does not affect the technical effects of the present application.

[0044] The room temperature involved in the present application is 25±5℃ unless otherwise specified.

[0045] Example 1

[0046] The present embodiment provides a preparation method of a polyester loose nanofiltration membrane based on guar gum cross-linked structure, and the steps are as follows:

[0047] 1) At room temperature, 0.15 g of guar gum and 0.5 g of triethylamine were dissolved in 49.35 g of deionized water to prepare a 0.3 wt% guar gum aqueous solution, and 0.1 g of trimesoyl chloride was dissolved in 49.9 g of n-hexane to prepare a 0.2 wt% trimesoyl chloride oil phase solution;

[0048] 2) The polyether sulfone ultrafiltration membrane (PES UF, molecular weight cut-off 150 kDa) was soaked with deionized water to remove surface residues, and the obtained guar gum aqueous solution was uniformly poured onto the support layer of the PES UF membrane, and after 2 min of immersion, the excess solution was poured out, and the residual aqueous solution on the surface of the membrane was removed by blowing to obtain an ultrafiltration membrane with distributed aqueous monomers; then the obtained trimesoyl chloride oil phase solution was uniformly poured onto the surface of the ultrafiltration membrane with distributed aqueous monomers, and after 5 min of interfacial polymerization, a polyester layer was formed, and the oil phase solution was poured out;

[0049] 3) The polyester composite membrane prepared in step 2) was heat treated at 65℃ for 10 min to obtain a polyester loose nanofiltration membrane based on guar gum cross-linked structure (GG / TMC-LNF), which was soaked in deionized water for standby.

[0050] The polyether sulfone ultrafiltration membrane used in the present embodiment and the obtained polyester loose nanofiltration membrane (GG / TMC-LNF) were characterized by scanning electron microscopy.

[0051] Figure 1 The scanning electron microscope images of the polyether sulfone ultrafiltration membrane surface at different magnifications in Example 1 are shown in Figure 1.

[0052] Figure 2 The scanning electron microscope images of the polyester loose nanofiltration membrane prepared in Example 1 at different magnifications are shown in Figure 2.

[0053] Figure 3Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the polyethersulfone ultrafiltration membrane and the prepared polyester loose nanofiltration membrane in Example 1.

[0054] It can be seen from Figure 1 , Figure 2 and Figure 3 that the obvious pore structure can be observed on the surface of the polyethersulfone ultrafiltration membrane, and after the formation of the polyester layer, the membrane surface becomes rough but more compact. The appearance of the 1730 cm -1 position-COO- functional group in the infrared spectrum indicates that the guar gum successfully undergoes interfacial polymerization with trimesoyl chloride to form a polyester crosslinked structure.

[0055] The permeation selectivity of the polyester loose nanofiltration membrane prepared in this example was evaluated by using a laboratory cross-flow filtration device. First, the polyester loose nanofiltration membrane was pre-pressed with deionized water at 2.5 bar for 30 min, and then deionized water, 100 ppm Congo red (CR) solution, 2000 ppm sodium sulfate solution, and 2000 ppm sodium chloride solution were used as feed liquid at 2 bar to test the pure water permeation flux (PWP) and retention rate (R) of the membrane. The PWP of the membrane was as high as 116.6 L·m -2 ·h -1 ·bar -1 , the Congo red retention rate was 99.9%, and the Na2SO4 and NaCl retention rates were 7.6% and 6.8%, respectively, showing excellent dye retention rate and inorganic salt permeability, and the NaCl / CR selectivity factor a was as high as 932.

[0056] Similarly, a mixed solution containing 2000 ppm sodium chloride and 100 ppm Congo red was used as the feed liquid, and other parameters were the same as above. The results are shown in Figure 4 .

[0057] Figure 4 The permeation flux, Congo red retention rate, and sodium chloride retention rate of the polyester loose nanofiltration membranes prepared in Example 1, Example 3, and Comparative Example 5 when treating a Congo red / sodium chloride mixed solution.

[0058] As can be seen from Figure 4 , when a mixed solution containing 2000 ppm sodium chloride and 100 ppm Congo red was used as the feed liquid, the prepared loose nanofiltration membrane showed high permeation selectivity to the dye / salt mixed solution, and the permeation flux of the membrane was 80.2 L·m -2 ·h -1 ·bar -1 , the Congo red retention rate was still as high as 99.9%, and the NaCl retention rate was 12.6%.

[0059] Example 2

[0060] The embodiment provides a preparation method of a polyester loose nanofiltration membrane based on a guar gum crosslinking structure, and steps are as follows:

[0061] 1) At room temperature, 0.1 g of guar gum and 0.6 g of tripropylamine are dissolved in 49.3 g of deionized water to prepare a 0.2 wt% guar gum aqueous solution, and 0.15 g of trimesoyl chloride is dissolved in 49.85 g of cyclohexane solution to prepare a 0.3 wt% trimesoyl chloride oil phase solution;

[0062] 2) The polysulfone ultrafiltration membrane (PS UF, molecular weight cut-off 100 kDa) is soaked with deionized water, and surface residues are removed, the obtained guar gum aqueous solution is uniformly poured on the support layer surface of the PS UF membrane, and after 5 min of immersion, the excess solution is poured out, and the water phase solution remaining on the membrane surface is removed by blowing; the obtained trimesoyl chloride oil phase solution is uniformly poured on the surface of the ultrafiltration membrane distributed with the water phase monomer, and after 3 min of interfacial polymerization, a polyester layer is formed, and the oil phase solution is poured out;

[0063] 3) The polyester composite membrane prepared in step 2) is subjected to heat treatment at 70 DEG C for 5 min to obtain a polyester loose nanofiltration membrane based on a guar gum crosslinking structure, and is soaked in deionized water for standby.

[0064] According to the test method of the membrane performance in Example 1, the pure water permeation flux (PWP) and the rejection rate (R) of the membrane are tested under 2 bar by using deionized water, 100 ppm Congo red (CR) solution, 2000 ppm sodium sulfate solution and 2000 ppm sodium chloride solution as feed liquid respectively, and the PWP of the membrane is up to 122.8 L·m -2 ·h -1 ·bar -1 , the Congo red rejection rate is 99.7%, and the Na2SO4 and NaCl rejection rates are 7.0% and 5.3% respectively, indicating that the membrane prepared under the condition also has high flux and dye / salt selective separation performance.

[0065] Example 3

[0066] The embodiment provides a preparation method of a polyester loose nanofiltration membrane based on a guar gum crosslinking structure, and steps are as follows:

[0067] The method is the same as that described in Example 1, except that:

[0068] In step 1), the guar gum is carboxymethyl guar gum (CMGG).

[0069] According to the test method of the membrane performance in Example 1, the pure water permeation flux (PWP) of the prepared membrane (CMGG / TMC-LNF) is 155.2 L·m -2 ·h -1· bar -1 When the mixed solution containing 2000 ppm sodium chloride and 100 ppm Congo red was used as the feed liquid, the permeation flux of the membrane was 105.5 L·m -2 · h -1 · bar -1 The Congo red rejection rate was 99.9%, and the NaCl rejection rate was 9.1% (as shown in Table 2), indicating that the functionalized guar gum can also be used as a bio-based water phase monomer to prepare a polyester loose nanofiltration membrane for high-selectivity separation of dye / salt mixtures, and the performance is further improved than when the guar gum raw powder is used as a water phase monomer. Figure 4

[0070] The water contact angle diagram of the polyester loose nanofiltration membrane prepared in Example 3 is shown in Figure 2, and the water contact angle of the obtained membrane is 59.3°. Figure 5

[0071] Figure 6 The performance diagram of the polyester loose nanofiltration membrane prepared in Example 3 for long-time treatment of a Congo red / sodium chloride mixed solution is shown in Figure 3.

[0072] Since the CMGG / TMC-LNF membrane has further improved permeation flux and dye / salt separation performance compared with the GG / TMC-LNF membrane, the long-term running stability of the membrane is further explored. Benefiting from the stable cross-linked structure and good hydrophilicity of the polyester functional layer when CMGG is used as a water phase monomer, the prepared membrane exhibits stable separation performance during the 12 h test of a 2000 ppm sodium chloride and 100 ppm Congo red mixed solution, and the stable permeation flux is about 60.0 L·m -2 · h -1 · bar -1 After 12 h, the Congo red rejection rate is still as high as 99.9%, and the NaCl rejection rate is as low as 7.5% (as shown in Table 3), indicating that it has good long-term stability and anti-pollution performance. Figure 6

[0073] Comparative Example 1

[0074] The method described in Example 1 was used, except that:

[0075] In step 1), 0.15 g of guar gum and 0.5 g of sodium hydroxide were dissolved in 49.35 g of deionized water to prepare a 0.3 wt% guar gum water phase solution. Sodium hydroxide was used instead of triethylamine as a catalyst for esterification.

[0076] ​​The PWP and R of the membrane were tested according to the test method of membrane performance in Example 1, using deionized water, 100 ppm Congo Red (CR) solution, 2000 ppm Na2SO4 solution and 2000 ppm NaCl solution as feed liquid at 2 bar, respectively. The PWP of the membrane was 340.4 L·m -2 ·h -1 ·bar -1 The R of Congo Red was only 75.9%, and the R of Na2SO4 and NaCl was 5.2% and 4.1%, respectively. The results showed that the PWP of the membrane prepared under this condition was close to that of the ultrafiltration membrane as the substrate, and it was difficult to achieve effective retention of Congo Red, indicating that guar gum could not react with trimesoyl chloride to form a better cross-linked structure when sodium hydroxide was used as a catalyst, indicating that triethylamine was a suitable catalyst for this system.

[0077] Comparative Example 2

[0078] The method described in Example 1 was used, except that:

[0079] In step 1), 0.25 g of guar gum and 0.5 g of triethylamine were dissolved in 49.25 g of deionized water to prepare a 0.5 wt% guar gum aqueous solution. The concentration of guar gum was increased.

[0080] The PWP and R of the membrane were tested according to the test method of membrane performance in Example 1, using deionized water, 100 ppm Congo Red (CR) solution, 2000 ppm Na2SO4 solution and 2000 ppm NaCl solution as feed liquid at 2 bar, respectively. The PWP of the membrane was 340.4 L·m -2 ·h -1 ·bar -1 Although the R of Congo Red was as high as 99.9%, the R of Na2SO4 and NaCl was also relatively high (13.3% and 9.8%, respectively). The results showed that when the concentration of the aqueous monomer guar gum was high, the separation layer formed was thick, increasing the permeation resistance when the liquid passed through, and the excessive cross-linked structure increased the salt retention rate, which was not conducive to the selective separation of dyes / salts. Therefore, the concentration of the aqueous monomer guar gum has a suitable range.

[0081] Comparative Example 3

[0082] The method described in Example 1 was used, except that:

[0083] In step 1), 0.025 g of trimesoyl chloride was dissolved in 49.975 g of n-hexane solution to prepare a 0.05 wt% trimesoyl chloride oil solution. The concentration of trimesoyl chloride was reduced.

[0084] The PWP and R of the membrane were tested according to the test method of membrane performance in Example 1, using deionized water, 100 ppm Congo Red (CR) solution, 2000 ppm Na2SO4 solution and 2000 ppm NaCl solution as feed liquid at 2 bar, respectively. The PWP of the membrane was 110.4 L-m2-h -2 ·h -1 ·bar -1 Although the R of Na2SO4 and NaCl were low (6.5% and 6.1% respectively), the R of Congo Red was also low (89.7%). The results showed that when the concentration of oil phase monomer trimesoyl chloride was low, the cross-linking degree was insufficient, which led to the formation of a separation layer with defects. Although the PWP was maintained at a high level, the insufficient cross-linking resulted in a low dye retention rate, which was not conducive to the selective separation of dyes / salts. Therefore, the concentration of oil phase monomer trimesoyl chloride had an appropriate range.

[0085] Comparative Example 4

[0086] The method described in Example 1 was used, except that:

[0087] In step 1), 0.15 g guar gum and 1.5 g triethylamine were dissolved in 48.35 g deionized water to prepare a 0.3 wt% guar gum aqueous solution. The concentration of catalyst triethylamine was increased to 3.0%.

[0088] The PWP and R of the membrane were tested according to the test method of membrane performance in Example 1, using deionized water, 100 ppm Congo Red (CR) solution, 2000 ppm Na2SO4 solution and 2000 ppm NaCl solution as feed liquid at 2 bar, respectively. The PWP of the membrane was 110.4 L-m2-h -2 ·h -1 ·bar -1 The R of Congo Red reached 99.9%, and the R of Na2SO4 and NaCl were also high (8.4% and 7.7% respectively). The results showed that when the concentration of catalyst triethylamine was high, the mechanism was similar to that of a high concentration of aqueous phase monomer, resulting in a thicker separation layer, which increased the penetration resistance when the liquid permeated, thus significantly reducing the permeation flux. Since this effect did not significantly change the membrane pore size, the Congo Red retention rate and inorganic salt permeation rate remained at a high level. Even so, the low flux was not conducive to the separation efficiency of the membrane in actual use, so the concentration of catalyst triethylamine had an appropriate range.

[0089] To better compare the performance of the membranes under different parameters, the separation performance of the membranes prepared in Examples 1-2 and Comparative Examples 1-4 was summarized in Table 1.

[0090] Table 1 Summary of separation performance of membranes prepared in Examples 1-2 and Comparative Examples 1-4

[0091]

[0092] Comparative Example 5

[0093] NF3 polyamide nanofiltration membrane produced by Zhongke Liyang Company (the membrane of this type is equivalent to imported membrane NF270) was used as a control group for the experiment.

[0094] According to the test method of membrane performance in Example 1, the pure water permeation flux (PWP) of the NF3 polyamide membrane was only 21.4 L·m -2 ·h -1 ·bar -1 When a mixed solution containing 2000 ppm of sodium chloride and 100 ppm of Congo red was used as the feed liquid, the permeation flux of the membrane was as low as 20.7 L·m -2 ·h -1 ·bar -1 Although the Congo red rejection rate was 99.9%, the NaCl rejection rate was as high as 26.8% (as shown in Figure 4 ), indicating that the current commercial nanofiltration membrane separation layer is relatively dense, the permeation flux is low, and both small molecule organic matter and inorganic salt have high rejection rate, which is not conducive to the treatment of textile wastewater. The present application provides a new type of membrane material, which overcomes the mutual restriction between permeability and selectivity, and realizes precise separation of dyes and inorganic salts.

[0095] Each of the embodiments in the specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other.

[0096] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a polyester loose nanofiltration membrane based on the cross-linked structure of guar gum, characterized in that, comprising the steps of: dissolving guar gum and an organic base catalyst in water to obtain an aqueous solution comprising guar gum; immersing a ultrafiltration membrane in the aqueous solution comprising guar gum and an oil phase solution comprising trimesoyl chloride in sequence, and then heat treating to obtain the polyester loose nanofiltration membrane based on the crosslinked structure of guar gum; the organic base catalyst comprises triethylamine, tripropylamine or N,N-diisopropyl ethyl amine.

2. The production method according to claim 1, characterized by, comprising the steps of: dissolving guar gum and an organic base catalyst in water to obtain an aqueous solution comprising guar gum; immersing a ultrafiltration membrane in the aqueous solution comprising guar gum and an oil phase solution comprising trimesoyl chloride in sequence, and then heat treating to obtain the polyester loose nanofiltration membrane based on the crosslinked structure of guar gum; the organic base catalyst comprises triethylamine, tripropylamine or N,N-diisopropyl ethyl amine.

3. The production method according to claim 1 or 2, characterized by, the concentration of the organic base catalyst in the aqueous solution comprising guar gum is 0.5-2wt%; and / or, the concentration of guar gum in the aqueous solution comprising guar gum is 0.05-0.3wt%.

4. The production method according to claim 1 or 2, characterized by, the solvent in the oil phase solution comprising trimesoyl chloride comprises n-hexane, cyclohexane or Isopar G; and / or, the concentration of trimesoyl chloride in the oil phase solution comprising trimesoyl chloride is 0.1-0.3wt%.

5. The production method according to claim 1 or 2, characterized by, the guar gum comprises guar gum powder and / or functional group derivatized guar gum.

6. The production method according to claim 1 or 2, characterized by, the ultrafiltration membrane comprises polyether sulfone ultrafiltration membrane, polysulfone ultrafiltration membrane or polyacrylonitrile ultrafiltration membrane.

7. The preparation method according to claim 2, characterized in that, the time of the immersing treatment is 2-10min; and / or, the time of the interfacial polymerization is 2-10min.

8. The production method according to claim 1 or 2, characterized by, the temperature of the heat treatment is 40-90℃, and the time is 1-30min.

9. The polyester loose nanofiltration membrane based on the crosslinked structure of guar gum prepared by the preparation method of any one of claims 1-8.

10. The application of the polyester loose nanofiltration membrane based on the crosslinked structure of guar gum of claim 9 in textile wastewater treatment.

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