TA-POMs composite nanofiltration membrane for efficient salt dyeing separation and preparation method and application of TA-POMs composite nanofiltration membrane

Abstract

The invention relates to the technical field of nanofiltration membranes, in particular to a TA-POMs composite nanofiltration membrane for efficient salt staining separation and a preparation method and application thereof.The preparation method of the composite nanofiltration membrane comprises the steps that firstly, TA and POMs are dissolved in deionized water and a buffer solution with the pH equal to 7 respectively, then the TA and the POMs are mixed to obtain a water-phase solution, and the water-phase solution is filtered to obtain the TA-POMs composite nanofiltration membrane; dissolving TMC in normal hexane to obtain an organic phase solution; and preparing the composite nanofiltration membrane by utilizing interfacial polymerization reaction of the mixed water-phase solution and oil-phase solution on the PES base membrane. The thickness and aperture of the membrane are regulated and controlled by adjusting the concentration of POMs, the separation performance of the composite membrane is improved, the preparation process is simple and convenient, and the technological process is simple and efficient. The inherent defect that a traditional nanofiltration membrane is poor in dye separation performance is overcome, the prepared composite nanofiltration membrane has higher water flux and dye salt separation factors, and the printing and dyeing wastewater treatment efficiency can be remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membranes, and particularly to a TA-POMs composite nanofiltration membrane for efficient separation of dye and salt, a preparation method thereof, and an application thereof. Background Art

[0002] As one of the pillar industries in China, the textile and dyeing industry has brought good economic benefits to China. However, while creating economic benefits for the country, the water pollution pressure brought by the textile and dyeing industry cannot be underestimated. Due to the requirements of the dyeing and printing processing technology, a large amount of wastewater containing various dyes and high salt concentrations will be generated during the processing. Textile and dyeing wastewater is characterized by complex composition, high salt content, and difficult biodegradation. Direct discharge of untreated dyeing wastewater poses great risks to human health and the environment, and in addition, it will also lead to waste of resources. Green and efficient separation of dyes and inorganic salts in dyeing wastewater not only protects water resources but also realizes the recycling of resources, which is of great significance for environmental protection and resource recycling.

[0003] Compared with other water treatment technologies, membrane separation technology has become the best choice for treating dyeing wastewater due to its superior separation selectivity, low operating pressure, and reasonable energy consumption. Membrane separation technology is driven by pressure or concentration and relies on size sieving, charge repulsion, and Donnan effect to achieve separation of solutes and solvents. According to the different membrane pore sizes, membrane separation technology is divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), etc., and their application directions also have different focuses. The pore size of NF is between UF and RO and is commonly used for the treatment of dyeing wastewater. Traditional nanofiltration membranes are prepared by the interfacial polymerization reaction of amine monomers / hydroxy monomers and acyl chloride monomers. The NF membranes prepared in this way usually have a dense and thick selective layer, which cannot effectively separate organic dyes and inorganic salts. At the same time, there are also many problems such as the balance between flux and retention and membrane fouling. To solve the above problems, researchers have proposed the concept of loose nanofiltration (LNF). LNF has unique properties ranging from nanofiltration to ultrafiltration, can achieve high salt permeability and high water flux while retaining dyes, and theoretically can avoid the accumulation of inorganic salts on the membrane surface and at the same time achieve retention of organic substances.

[0004] Tannic acid (TA) is a natural polyphenol widely present in plants such as tea, nettle, wood, and berries. The molecular structure of tannic acid contains multiple phenolic hydroxyl groups, enabling tannic acid to react with acyl chloride monomers to form a film, and to a certain extent, it can also regulate the structure and separation performance of the polyester selective layer. In addition, tannic acid also has antibacterial and hydrophilic properties, which make it have important applications in many fields. Therefore, using tannic acid as an aqueous monomer to prepare a loose nanofiltration membrane can effectively separate organic dyes and inorganic salts while achieving high salt permeability and high water flux. Su et al. prepared a composite nanofiltration membrane by interfacial polymerization using tannic acid as an aqueous monomer, which has high salt permeability and good dye selectivity. At the same time, the membrane has excellent anti-fouling performance and chemical stability. Yang et al. prepared a composite nanofiltration membrane using carbonate as a modifier for tannic acid aqueous monomer. This membrane achieved efficient dye / salt separation while ensuring high water flux. In addition, the polyester nanofiltration membrane exhibits high electronegativity and abundant oxygen-containing functional groups, which can effectively remove ions in water. However, modifying the loose nanofiltration membrane with carbonate will inevitably increase the crosslinking degree of the selective layer and thus reduce the selectivity. Therefore, it is necessary to find a suitable modifier to regulate the interfacial polymerization process to prepare a thinner selective layer.

[0005] Polyoxometalates (POMs) are a class of polyanionic oxides with a definite geometric structure and well-defined surface ligands, mainly composed of high oxidation state metals (such as tungsten (W), molybdenum (Mo), niobium (Nb), antimony (Sb), and vanadium (V)) and transition heteroatoms such as phosphorus (P), boron (B), and silicon (Si). Due to their excellent catalytic selectivity and relatively high redox potential, they have extensive applications in fields such as photoelectrocatalysis and environmental protection. In addition, POMs also have good hydrophilicity, antibacterial properties, and negative charge, which make POMs have the potential to be applied in the field of membrane separation. As an electron-rich complex, POMs have extremely strong coordination ability, and the abundant hydroxyl groups in tannic acid can form coordination with tannic acid. In addition, the acidity of POMs can inhibit the diffusion process of the aqueous monomer into the oil phase, thereby forming a thinner selective layer to increase the flux, and at the same time, the coordination will also enhance the electronegativity and improve the rejection. Summary of the Invention

[0006] The object of the present invention is to solve the disadvantages existing in the prior art, and to propose a TA-POMs composite nanofiltration membrane for efficient dye / salt separation, its preparation method and application. The prepared composite membrane has higher water flux and dye / salt separation factor, and can significantly improve the treatment efficiency of printing and dyeing wastewater.

[0007] To achieve the above object, the present invention adopts the following technical scheme:

[0008] A preparation method of a TA-POMs composite nanofiltration membrane for efficient dye / salt separation, comprising the following steps:

[0009] Step 1, prepare the aqueous solution and the organic solution: Dissolve tannic acid (TA) and phosphomolybdic acid (PMo 12 ) in deionized water and phosphate buffer solution with pH = 7 respectively, and then mix the two to obtain the aqueous solution; dissolve trimesoyl chloride (TMC) in n-hexane to obtain the organic solution;

[0010] Step 2, prepare the nanofiltration membrane: Under room temperature conditions, soak the polyethersulfone (PES) membrane in deionized water, then fix it on the template, and sequentially soak the surface of the membrane with the aqueous solution and the organic solution prepared in Step 1. Subsequently, take out the membrane from the template and place it in an oven for heat treatment. The heat-treated membrane is soaked in deionized water overnight to obtain the TA-POMs composite nanofiltration membrane.

[0011] Preferably, in Step 1, the concentration of TA is 0.5 g / L, and the concentration of PMo 12 is 0.1 - 1.0 g / L, and the concentration of TMC is 0.1 g / L.

[0012] Preferably, in Step 2, the PES membrane is soaked in deionized water for 12 h.

[0013] Preferably, in Step 2, the soaking time of the aqueous solution is 10 min, the soaking time of the organic phase is 3 min, the heat treatment temperature is 60 °C, and the treatment time is 10 min.

[0014] Preferably, after the soaking treatment in deionized water and the aqueous solution in Step 2, the residual liquid on the surface of the membrane needs to be pushed dry with a glue stick before proceeding with the subsequent operations.

[0015] The TA-POMs composite nanofiltration membrane obtained by the above preparation method.

[0016] The present invention also provides an application of the above TA-POMs composite nanofiltration membrane in water treatment.

[0017] By adopting the above technical solutions: Through coordination, the binding of POMs and the aqueous monomer - tannic acid is achieved, and the optimization of key parameters of the membrane performance such as hydrophilicity and electronegativity is realized, so that both the salt permeability and the dye selectivity of the membrane are significantly improved. The prepared TA-POMs composite nanofiltration membrane can significantly improve the water flux, Congo red rejection rate, and dye / salt separation efficiency.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The present invention introduces POMs into the aqueous phase through coordination to regulate interfacial reactions. The prepared composite has a thinner selective layer and strong electronegativity. This method can prepare the TA-POMs composite nanofiltration membrane in only one step, with simple operation, a simple and efficient process. The obtained composite nanofiltration membrane is environmentally friendly and has the potential for large-scale applications.

[0020] 2. The introduction of POMs in the present invention enhances the electronegativity of the membrane surface, and has a strong repulsive effect on negatively charged organic dyes such as congo red. The rejection rate of the TA-POMs composite nanofiltration membrane for congo red is as high as 99.90%, the rejection of sodium chloride is 5.88%, and the separation factor is as high as 758.25, showing excellent selectivity between inorganic salts and organic dyes, superior to most of the current loose nanofiltration membranes. The introduction of POMs slows down the diffusion from the aqueous phase to the oil phase, making the selective layer thinner and thus improving the permeation flux. The permeation flux of the TA-POMs composite nanofiltration membrane is as high as 56.73 LMH / bar, which is 4.34 times that of the blank membrane. The excellent dye / salt selectivity and permeability of the composite nanofiltration membrane contribute to the improvement of the wastewater treatment efficiency during the actual use of the membrane. Brief Description of the Drawings

[0021] Figure 1 It is a scanning electron microscope image (FE-SEM) of the surface morphology of the TA-POMs composite polyester nanofiltration membrane in the present invention;

[0022] Figure 2 It is an atomic force microscope image (AFM) of the membrane in the present invention;

[0023] Figure 3 It is an infrared spectrum (FTIR) of the PES membrane, TA blank membrane, TA-0.1P membrane, TA-0.5P membrane and TA-1.0P membrane in the present invention;

[0024] Figure 4 It is an X-ray photoelectron spectroscopy (XPS) of the TA blank membrane, TA-0.1P membrane, TA-0.5P membrane and TA-1.0P membrane in the present invention;

[0025] Figure 5 It is the water contact angle (WCA) of the TA blank membrane, TA-0.1P membrane, TA-0.5P membrane and TA-1.0P membrane in the present invention;

[0026] Figure 6 It is the Zeta potential diagram of the blank membrane and the composite membrane in the present invention;

[0027] Figure 7 It is the molecular weight cut-off diagram (MWCO) of the blank membrane and the composite membrane in the present invention;

[0028] Figure 8 It is the diagram of pure water and dye permeability and dye selectivity of the composite membrane;

[0029] Figure 9 It is a graph of the salt permeability and salt selectivity of the composite membrane;

[0030] Figure 10 It is a graph of the separation performance of CR / Na2SO4 dyed salt, CR / NaCl dyed salt, and CR / MB separation performance;

[0031] Figure 11 It is a graph of the stability test of the TA-0.5P membrane in the present invention;

[0032] Figure 12 It is a graph of the long-term stability test of the TA-0.5P modified membrane in the present invention;

[0033] Figure 13 It is a graph of the filtration effect and dye removal effect of the NF-270 commercial membrane and the TA-0.5P modified membrane in the present invention on simulated printing and dyeing wastewater;

[0034] Figure 14 It is a graph of the anti-organic pollution performance test of the NF-270 commercial membrane and the TA-0.5P modified membrane in the present invention;

[0035] Figure 15 It is a graph of the chlorine resistance test of the commercial nanofiltration membrane NF-270 and the TA-0.5P modified membrane in the present invention. Detailed implementation mode

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings, so that those skilled in the art can better understand the advantages and features of the present invention, and thus make a clearer definition of the protection scope of the present invention. The embodiments described in the present invention are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work belong to the protection scope of the present invention.

[0037] Example 1:

[0038] A TA-POMs composite nanofiltration membrane for efficient separation of dyed salts is prepared as follows:

[0039] Step 1: Prepare an aqueous solution and an organic solution: Dissolve 0.01 g of tannic acid (TA) and 0.005 g of phosphomolybdic acid (PMo 12 ) in 5 mL of deionized water and 15 mL of phosphate buffer solution with pH = 7 respectively, and then mix the two to obtain 20 mL of aqueous solution; Dissolve 0.1 g of trimesoyl chloride (TMC) in 100 mL of n-hexane to obtain an organic solution.

[0040] Step 2: Preparation of composite nanofiltration membrane:

[0041] Under room temperature conditions, soak the polyethersulfone (PES) membrane in deionized water, then fix it on a template. Use a rubber roller to push and dry the water on the membrane surface for 2 min until it is air-dried. After fixing it on the template, pour the aqueous solution prepared in Step 1 and soak for 10 min. After pouring out the aqueous solution, use a rubber roller to push and dry the residual aqueous solution for 2 min until it is air-dried. Fix it on the template and pour the organic solution prepared in Step 1 and soak for 3 min. After pouring out the organic solution, place the membrane in an oven at 60 °C for heat treatment for 10 min. The heat-treated membrane is soaked in deionized water overnight to obtain the TA-POMs composite nanofiltration membrane, marked as TA-0.1.

[0042] Example 2:

[0043] Same as Example 1, except that 0.01 g of PMo 12 is dissolved in 15 mL of phosphate buffer solution with pH = 7. The prepared membrane is denoted as TA-0.5.

[0044] Example 3:

[0045] Same as Example 1, except that 0.02 g of PMo 12 is dissolved in 15 mL of phosphate buffer solution with pH = 7. The prepared membrane is denoted as TA-1.0.

[0046] Comparative Example 1:

[0047] Same as Example 1, except that PMo 12 is not added to the phosphate buffer solution. The prepared membrane is denoted as TA. The measurement methods for each evaluation index of TA, TA-0.1P, TA-0.5P, and TA-1.0P in the present invention are as follows:

[0048] (1) The surface structure of the membrane is determined by SEM images, and the measurement results are as Figure 1 shown.

[0049] (2) The roughness composition of the membrane surface is determined by AFM images, and the measurement results are as Figure 2 shown.

[0050] (3) The chemical properties of the membrane selective layer are determined by FTIR images, and the measurement results are as Figure 3 shown.

[0051] (4) The elemental composition of the membrane surface is determined by XPS, and the measurement results are as Figure 4 shown.

[0052] (5) The water contact angle (WCA) of the membrane was measured using a water contact angle meter. A 3 μL water droplet was dropped onto the membrane surface, and the angle formed between the water droplet and the membrane surface at 20 s was measured. The measurement results are shown as Figure 5 shown below.

[0053] (6) The charge characteristics of the membrane samples were determined by Zeta potential mapping. The measurement results are shown as Figure 6 shown below.

[0054] (7) The separation performance of the TA-POMs composite nanofiltration membrane for single dye solutions and single salt solutions was tested using a crossflow device. 0.1 g / L Congo Red (CR), Methylene Blue (MB), Rhodamine B (RhB), and Indigo Disulfonate Sodium (IC) solutions and 1.0 g / L Sodium Chloride (NaCl), Sodium Sulfate (Na2SO4), Magnesium Chloride (MgCl2), and Magnesium Sulfate (MgSO4) solutions were used as feed solutions. The membrane was pre-pressed at 2 bar for 10 min to stabilize the membrane structure and performance, and then the membrane performance was tested at the same pressure. The concentration of the dye before and after filtration was measured using an ultraviolet spectrophotometer, and the concentration of the inorganic salt before and after was measured using a conductivity meter (DDSJ-308F). The flux (J, LMH)

[0055] and the rejection (R, %) of the dye were calculated using the following formulas:

[0056]

[0057] where V is the volume of the permeate (L), S is the effective filtration area (m 2 2), Δt is the filtration time (h), C p and C f are the concentrations of the solute in the permeate and the feed solution (g / L), respectively.

[0058] The test results are shown as Figure 8 shown below.

[0059] (8) The dye-salt separation performance was reflected by the separation effect on the dye-salt mixed solution. 0.1 g / L CR and 1.0 g / L NaCl, 0.1 g / L CR and 1.0 g / L Na2SO4 mixed solutions were used as simulated printing and dyeing wastewater to test the dye-salt separation performance at 2 bar pressure. The concentrations of the salt and the dye before and after filtration were measured using a conductivity meter and an ultraviolet spectrophotometer, and the rejection rates of the salt and the dye were calculated separately, denoted as R1 and R2. The dye-salt separation factor (α) was calculated using the following formula:

[0060]

[0061] The test results are shown as Figure 10 shown below.

[0062] (9) The dye-dye separation performance is reflected by the separation effect on the mixed dye solution. Using a mixed solution of 0.1 g / L CR and 0.1 g / L MB as the simulated printing and dyeing wastewater, the dye separation performance was tested under a pressure of 2 bar. The concentrations of the dyes before and after filtration were measured using a UV spectrophotometer, and the rejection rates of MB and CR were calculated and denoted as R1 and R2 respectively. The dye-salt separation factor (α) was calculated by the following formula:

[0063]

[0064] The test results are as Figure 10 shown.

[0065] Figure 1 is the scanning electron microscopy image (FE-SEM) of the surface morphology of the TA-POMs composite polyester nanofiltration membrane in the present invention. In Figure 1 , (a1-d1), (a2-d2), and (a3-d3) are the surfaces of the TA blank membrane, TA-0.1P membrane, TA-0.5P membrane, and TA-1.0P membrane at low magnification, high magnification, and cross-section respectively. It can be seen from Figure 1 that compared with the TA blank membrane, the thickness of the selective layer of the TA-0.1P membrane, TA-0.5P membrane, and TA-1.0P membrane has been significantly reduced, which may be related to the coordination between PMo 12 and TA reducing the reaction sites between TA and TMC. At the same time, the strong acidity of PMo 12 consumes a large amount of phosphate buffer solution, thus reducing the promoting effect of the phosphate buffer solution on the interfacial reaction process.

[0066] Figure 2 is the atomic force microscopy image (AFM) of the membrane in the present invention. It can be seen from Figure 2 the changes in the roughness of the TA blank membrane, TA-0.1P membrane, TA-0.5P membrane, and TA-1.0P membrane, indicating that the surface roughness of the membrane increases with the increase in the concentration of PMo12. The rougher membrane surface increases the contact area with water molecules. Therefore, the loose nanofiltration membrane modified by PMo12 has good permeability.

[0067] Figure 3 is the Fourier transform infrared spectroscopy image (FTIR) of the PES membrane, TA blank membrane, TA-0.1P membrane, TA-0.5P membrane, and TA-1.0P membrane in the present invention. It can be seen from Figure 3 that except for the PES support membrane, characteristic peaks are shown at 1725-1728 cm -1 (C=O), indicating that TA and TMC have successfully reacted to form a polyester selective layer; a relatively wide characteristic peak appears at 3425-3217 cm -1 , which is the stretching vibration of O-H, indicating that there may be unreacted TA; 1725-1728 cm-1 The intensity of the characteristic peak at [location] decreases with the increase of PMo 12 concentration, which may be due to the coordination between PMo 12 and TA reducing the polymerization reaction with TMC; the lower intensity of the characteristic peak may also indicate the formation of a thinner selective layer.

[0068] Figure 4 are the X-ray photoelectron spectroscopy (XPS) diagrams of the TA blank membrane, TA-0.1P membrane, TA-0.5P membrane and TA-1.0P membrane in the present invention. It can be seen from Figure 4 that compared with the TFC blank membrane, there is an obvious Mo characteristic peak in the composite membrane, indicating that PMo 12 has been loaded on the composite membrane. At the same time, with the increase of PMo 12 concentration, the peak area of Mo in TA-1.0P is larger than that in the TA-0.1P membrane, which means that more PMo 12 coordinates with TA, which can effectively improve the performance of the membrane.

[0069] Figure 5 are the water contact angles (WCA) of the TA blank membrane, TA-0.1P membrane, TA-0.5P membrane and TA-1.0P membrane in the present invention. It can be seen from Figure 5 that the WCA decreases from 35.0 ± 0.9° of the TA blank membrane to 31.8 ± 0.3° of TA-0.1P. With the increase of PMo 12 concentration, the WCA further decreases, indicating that the addition of PMo 12 is beneficial to enhancing the hydrophilicity of the membrane surface. Figure 6 is the zeta potential diagram of the TFC blank membrane and the composite membrane in the present invention. It can be seen from Figure 5 , Figure 6 that compared with the TFC blank membrane, due to the modification of the negatively charged PMo 12 on the surface of the composite membrane, the electronegativity of the membrane surface is enhanced, thus enhancing the electrostatic repulsion effect on the membrane surface and further improving the rejection rate of the composite membrane to sodium sulfate.

[0070] Figure 6 is the Zeta potential diagram of the blank membrane and the composite membrane in the present invention. It can be seen from Figure 6 that compared with the blank membrane, due to the modification of the negatively charged PMo 12 on the surface of the composite membrane, the electronegativity of the membrane surface is enhanced and increases gradually with the increase of the PMo 12 loading amount.

[0071] Figure 7 is the molecular weight cut-off diagram (MWCO) of the blank membrane and the composite membrane in the present invention. It can be seen from Figure 7 that with the increase of PMo 12With the increase in concentration, the pore size of the loose nanofiltration membrane also increases, and the pore size of the membrane follows the order of TA-1.0P modified membrane > TA-0.5P modified membrane > TA-0.1P modified membrane > TA blank membrane.

[0072] Figure 8 It is the pure water and dye permeability of the composite membrane and the selectivity diagram of the dye. As Figure 8 (a) shows, with the addition of PMo 12 , the pure water flux of the composite nanofiltration membrane further increases with the increase in the concentration of PMo 12 . At the same time, a similar pattern is also shown for the four fuel solutions. As Figure 8 (b) shows, the rejection rate of the composite membrane for the dye is higher than that of the blank membrane.

[0073] Figure 9 It is the salt permeability and salt selectivity of the composite membrane. As Figure 9 a shows, after being modified by PMo 12 , the permeability of the modified membrane is greatly improved. As Figure 9 b shows, the rejection pattern of the blank membrane and the modified membrane for the four salts is R(MgCl2) < R(NaCl) < R(MgSO4) < R(Na2SO4).

[0074] Figure 10 It is the separation performance of CR / Na2SO4 dye-salt, CR / NaCl dye-salt, and CR / MB. As Figure 10 ab shows, the separation effect of the blank membrane on the mixed solutions of CR and Na2SO4 and CR and NaCl is not ideal, while after being modified by PMo 12 , the separation effect is greatly improved while maintaining a relatively high flux. As Figure 10 c shows, the rejection rate of the modified membrane for CR in the binary mixed dye is slightly lower than that for single CR. And the rejection rate of the composite membrane for MB in the binary mixed dye is greater than that for single MB.

[0075] Figure 11 It is the stability test of the TA-0.5P membrane in the present invention, respectively exploring the concentration of CR, the pH value of the feed liquid, different operating pressures, and the performance of the loose nanofiltration membrane when the operating pressure is greater than 4 bar. As Figure 11 can be seen, after being modified by PMo 12The modified loose nanofiltration membrane has a good treatment effect on high-concentration CR solution and good acid and alkali resistance. Although with the increase of the operating pressure, the flux (LMH) of the TA-0.5P modified membrane increases while the rejection rate decreases, and when the operating pressure is greater than 4 bar, both the rejection rate and the permeability (LMH / bar) decrease, but when the operating pressure is 2 bar, it still has good permeability and rejection rate.

[0076] Figure 12 This is the long-term stability test of the TA-0.5P modified membrane in the present invention. Using 0.1 g / L CR solution as the feed liquid, the flux and rejection rate of the TA-0.5P modified membrane within 4000 min are tested. From Figure 12 It can be seen that within the initial 1000 min, the flux of the TA-0.5P modified membrane fluctuates and decreases slightly due to the interception of CR on the membrane surface and the compaction of the selective layer. However, in the subsequent tests, the flux gradually stabilizes, about 80% of the initial flux, and the rejection rate of the TA-0.5P modified membrane for the CR solution remains stable above 99% during the test time. This shows that the TA-0.5P modified membrane has good long-term stability.

[0077] Figure 13 This is the test of the percolation effect and dye removal effect of the NF-270 commercial membrane and the TA-0.5P modified membrane in the present invention on simulated printing and dyeing wastewater. Using simulated printing and dyeing wastewater with CR:NaCl = 0.1 g / L:2.0 g / L and an initial volume of 500 mL as the feed liquid, during the separation process, with the increase of the number of cycles, the rejection rates of both NF-270 and the TA-0.5P modified membrane for CR are higher than 99%, indicating that the loss of dyes can be ignored. In addition, during the separation process, NF-270 consumed a total of 2000 mL of pure water (four times the feed volume) to reach the standard of NaCl concentration <0.1 g / L in the feed liquid, while the TA-0.5P modified membrane only requires 1500 mL of water (three times the feed volume). At the same time, during the test of the TA-0.5P modified membrane, the difference in salt concentration between the feed liquid and the permeate is less than that of NF-270, which indicates that the looser and rougher selective layer of the TA-0.5P modified membrane makes it have higher permeability and lower NaCl rejection rate. Using the TA-0.5P modified membrane for testing, the entire cycle process takes about 1440 min, which is less than the 2400 min required by NF-270, indicating that the TA-0.5P modified membrane has a shorter running time and lower water consumption during the separation process.

[0078] Figure 14It is the test of the anti-organic fouling performance of the commercial NF-270 membrane and the TA-0.5P modified membrane in the present invention. The present invention uses 1.0 g / L negatively charged bovine serum albumin (BSA) and 1.0 g / L positively charged lysozyme (LZM) solutions for the test. Due to the attachment and adhesion of the simulated pollutants, the water fluxes of the TA blank membrane and the TA-0.5P modified membrane both decrease. After physically cleaning the membrane surface, the water fluxes of both the commercial NF-270 membrane and the TA-0.5P modified membrane are recovered to a certain extent, and the flux recovery efficiency of the TA-0.5P modified membrane for organic pollutants is better than that of the commercial NF-270 membrane. The main reason is the coordination of negatively charged and hydrophilic PMo 12 with TA, which delays the fouling process of organic pollutants, thereby enhancing the anti-organic fouling performance of the TA-0.5P modified membrane. In addition, the electronegative PMo 12 strengthens the electrostatic repulsion of negatively charged organic pollutants, making the normalized permeability of the TA-0.5P modified membrane to the LZM solution (electropositive) lower than that of the BSA solution. Therefore, the TA-0.5P modified membrane has strong anti-organic fouling performance.

[0079] Figure 15 It is the test of the chlorine resistance of the commercial nanofiltration membrane NF-270 and the TA-0.5P modified membrane in the present invention. Using 0.5 g / L CR solution as the simulated pollutant and 2.0 g / L sodium hypochlorite (NaClO) as the chemical cleaning agent, after three cycles of pollution and chemical cleaning, the permeability of NF-270 drops to 50% of the initial value, and the rejection rate drops to 98%. Compared with traditional commercial membranes, the decrease in the rejection rate of the TA-0.5P modified membrane is not obvious. After three cycles of pollution and chemical cleaning, the rejection rate still remains above 99.2%, while the permeability only drops by 10%. Therefore, the TA-0.5P modified membrane has good chlorine resistance.

[0080] In summary, the present invention adjusts the concentration of POMs to control the thickness and pore size of the membrane, achieving the improvement of the separation performance of the composite membrane. The preparation process is simple, and the technological process is simple and efficient. The present invention solves the inherent defect of the poor dye separation performance of traditional nanofiltration membranes. The prepared composite membrane has higher water flux and dye-salt separation factor, and can significantly improve the treatment efficiency of printing and dyeing wastewater.

[0081] The descriptions and practices disclosed in the present invention are easy to think about and understand for ordinary technicians in the technical field. Without departing from the principle of the present invention, several improvements and refinements can also be made. Therefore, the modifications or improvements made without deviating from the spirit of the present invention should also be regarded as within the protection scope of the present invention.

Claims

1. A preparation method of a TA-POMs composite nanofiltration membrane for efficient separation of dye salts, characterized in that, It includes the following steps: Step 1, prepare the aqueous solution and the organic solution: Dissolve tannic acid (TA) and phosphomolybdic acid (PMo) 12 in deionized water and phosphate buffer solution with pH = 7 respectively, and then mix the two to obtain the aqueous solution; Dissolve trimesoyl chloride (TMC) in n-hexane to obtain the organic solution; Step 2: Prepare a nanofiltration membrane. At room temperature, soak a polyethersulfone (PES) membrane in deionized water, then fix it on a template. Subsequently, soak the surface of the membrane with the aqueous solution and organic solution prepared in Step 1 in sequence. Then, take the membrane out of the template and place it in an oven for heat treatment. After heat treatment, soak the membrane in deionized water overnight to obtain a TA-POMs composite nanofiltration membrane.

2. The preparation method of a TA-POMs composite nanofiltration membrane for efficient separation of dye salts according to claim 1, characterized in that, In Step 1, the concentration of the TA is 0.5 g / L, and the concentration of PMo 12 is 0.1 - 1.0 g / L, and the concentration of TMC is 0.1 g / L.

3. The preparation method of a TA-POMs composite nanofiltration membrane for efficient separation of dye salts according to claim 1, characterized in that, In Step 2, the PES membrane is soaked in deionized water for 12 h.

4. The preparation method of the TA-POMs composite nanofiltration membrane for efficient separation of dye salts according to claim 1, characterized in that, In Step 2, the soaking time of the aqueous solution is 10 min, the soaking time of the organic phase is 3 min, the heat treatment temperature is 60 °C, and the treatment time is 10 min.

5. The preparation method of a TA-POMs composite nanofiltration membrane for efficient separation of dye salts according to claim 1, characterized in that, In Step 2, after the soaking treatment with deionized water and the aqueous solution, the residual liquid on the surface of the membrane needs to be pushed dry with a glue stick before proceeding with the subsequent operations.

6. A TA-POMs composite nanofiltration membrane obtained by the preparation method according to any one of claims 1-5.

7. An application of the TA-POMs composite nanofiltration membrane according to claim 6 in water treatment.

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