Composite floc modified ultrafiltration membrane, and preparation method and application thereof

Abstract

The application relates to the field of water treatment membrane separation technology and discloses a composite flocculus modified ultrafiltration membrane and a preparation method and application thereof. The preparation method comprises the following steps: (1) under the condition that the pH value is 8.0-9.0, a cationic polyacrylamide solution and a polyaluminum chloride solution are subjected to a contact reaction in the presence of ultrapure water to obtain a composite flocculus suspension; and (2) under the action of a transmembrane pressure, the composite flocculus suspension passes through an ultrafiltration membrane, the composite flocculus in the composite flocculus suspension is pre-deposited on the surface of the ultrafiltration membrane to form a functional separation layer, and the composite flocculus modified ultrafiltration membrane is obtained. The composite flocculus pre-deposited modified ultrafiltration membrane of the cationic polyacrylamide and the polyaluminum chloride has a perfluorinated compound interception rate of not less than 70 wt%, stable performance under complex water quality conditions, simple preparation process, low cost and a good industrial application prospect.

Description

Technical Field

[0001] This invention relates to the field of water treatment membrane separation technology, specifically to a composite floc-modified ultrafiltration membrane, its preparation method, and its application. Background Technology

[0002] Perfluorinated compounds (per- and polyfluoroalkyl substances, PFAS) are a class of artificially synthesized persistent organic pollutants. The carbon-fluorine bonds in their molecular structure have extremely high bond energy (about 485 kJ / mol), which endows them with excellent chemical stability, hydrophobicity, oleophobicity and heat resistance. Therefore, they are widely used in fire-fighting foam, waterproof and oil-proof materials, food packaging and textiles.

[0003] However, it is precisely this extreme stability that makes PFAS difficult to degrade in the natural environment, exhibiting bioaccumulation, long-distance migration, and potential carcinogenic and endocrine-disrupting toxic effects. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), as typical examples, have been widely detected in groundwater, surface water, drinking water, and even human blood, posing a serious threat to the ecological environment and human health. Therefore, developing efficient, economical, and applicable PFAS removal technologies for complex water quality conditions has become a research hotspot in the field of environmental engineering.

[0004] Currently, the main technologies for removing PFAS from water include adsorption, advanced oxidation, coagulation and sedimentation, and membrane separation. While nanofiltration (NF) and reverse osmosis (RO) membranes can effectively retain PFAS through size sieving and electrostatic repulsion, their high operating pressure, high energy consumption, severe membrane fouling, and difficulty in treating the concentrate limit their large-scale application. Ultrafiltration (UF) membranes have advantages such as low operating pressure, high flux, and low operating costs, but their larger pore size traditionally makes them difficult to effectively retain smaller PFAS molecules; ultrafiltration alone typically has a very low PFAS rejection rate.

[0005] To enhance the PFAS retention capacity of ultrafiltration membranes, researchers have explored methods such as surface coating, graft polymerization, and pre-deposition of functional materials to modify the membranes. Among these methods, pre-depositing flocs on the ultrafiltration membrane surface to form a functional separation layer, thereby enabling the ultrafiltration membrane to acquire specific PFAS retention capacity, is a promising technological approach.

[0006] However, the flocs used in existing technologies are mostly single inorganic flocs with limited specific surface area and surface charge density, making it difficult to further improve their adsorption and retention capacity for PFAS. They mostly rely on natural sedimentation, resulting in a loose pre-deposited layer structure that lacks synergistic enhancement from organic polymer components, leading to a low retention rate. After floc formation, PFAS filtration is usually performed directly without forming an independent and stable modified membrane product. The membrane modification process is not decoupled from the filtration process, resulting in poor process controllability.

[0007] In summary, developing a composite floc-modified ultrafiltration membrane that can significantly improve the PFAS rejection rate of ultrafiltration membranes, has good resistance to interference from coexisting ions and organic matter, and has a simple preparation process is of great practical significance and application value. Summary of the Invention

[0008] The present invention aims to overcome the problems of existing pre-deposited modified ultrafiltration membranes having low rejection rate of perfluorinated compounds, unstable performance under complex water quality, and difficulty in obtaining independent and stable modified membrane products.

[0009] To achieve the above objectives, the first aspect of the present invention provides a method for preparing a composite floc-modified ultrafiltration membrane, comprising the following steps: (1) In the presence of ultrapure water, cationic polyacrylamide solution and polyaluminum chloride solution are reacted at a pH of 8.0-9.0 to obtain a suspension of composite flocs; (2) The suspension of the composite flocs is passed through an ultrafiltration membrane under transmembrane pressure, so that the composite flocs are pre-deposited on the surface of the ultrafiltration membrane to form a functional separation layer, thus obtaining the composite floc modified ultrafiltration membrane.

[0010] The second aspect of the present invention provides a composite floc-modified ultrafiltration membrane, which is obtained by the preparation method described in the first aspect. The surface of the composite floc-modified ultrafiltration membrane is pre-deposited with a three-dimensional network composite floc functional separation layer formed by cationic polyacrylamide and polyaluminum chloride.

[0011] The third aspect of this invention provides the application of the composite floc-modified ultrafiltration membrane described in the second aspect in the treatment of water containing perfluorinated compounds.

[0012] Compared with the prior art, the present invention has at least the following beneficial effects: 1. This invention involves pre-forming a dense three-dimensional network composite floc with cationic polyacrylamide (CPAM) and polyaluminum chloride (PAC) under specific pH conditions, and then pre-depositing this composite floc onto the surface of an ultrafiltration membrane to construct a functional separation layer. Compared to pre-depositing and modifying with PAC alone, the long-chain molecules of cationic polyacrylamide (CPAM) can effectively penetrate and entangle within the PAC floc framework, significantly increasing the specific surface area and surface positive charge density of the floc layer. Simultaneously, the flexible segments of CPAM can fill the pores between the PAC flocs, making the separation layer more dense and uniform. Experimental results show that the ultrafiltration membrane modified with the composite floc of this invention achieves a rejection rate of over 70% for perfluorinated compounds (up to 91.7%), far exceeding that of the original ultrafiltration membrane (rejection rate less than 10%) and the membrane modified with single PAC pre-deposition.

[0013] 2. This invention employs a pre-preparation composite floc-pre-deposition modification strategy, separating the membrane modification process from the perfluorinated compound filtration process. This avoids the competitive adsorption and floc structure alteration problems that may result from direct contact between perfluorinated compounds and coagulants. The CPAM@PAC composite floc separation layer formed by pre-deposition possesses multiple removal mechanisms, including physical sieving, electrostatic adsorption, and hydrophobic partitioning. It can achieve highly efficient removal of perfluorinated compounds through size sieving, electrostatic attraction between cationic groups and anionic head groups of perfluorinated compounds, and hydrophobic interaction between hydrophobic regions and fluorocarbon chains of perfluorinated compounds.

[0014] 3. In different anions and cations (such as Na) + Ca 2+ Cl - SO4 2- HCO3 - Under complex water quality conditions where perfluorinated compounds (such as humic acid, bovine serum albumin, sodium alginate, etc.) coexist, the composite floc-modified ultrafiltration membrane of this invention can still maintain a rejection rate of no less than 70 wt% for perfluorinated compounds. This overcomes the problem of performance degradation of traditional membrane treatment technology under actual complex water quality conditions and is suitable for the deep treatment of various water bodies such as groundwater, surface water, industrial wastewater and drinking water.

[0015] 4. The entire pre-deposition process of this invention is carried out at room temperature and pressure, without the need for complex chemical grafting or expensive nanomaterials. The functional modification of the ultrafiltration membrane can be achieved simply through pH adjustment, stirring, and filtration deposition. In addition, the pre-deposited composite floc layer has good stability, and can be regenerated by pre-deposition after the performance of the separation layer degrades, showing good practical application prospects and industrialization value. Attached Figure Description

[0016] Figure 1 This is a photograph of the composite floc pre-deposition modified ultrafiltration membrane prepared in a preferred embodiment of the present invention. Figure 2 This is a SEM image of the composite floc pre-deposition modified ultrafiltration membrane prepared in a preferred embodiment of the present invention. Detailed Implementation

[0017] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0018] As mentioned above, the first aspect of the present invention provides a method for preparing a composite floc-modified ultrafiltration membrane, comprising the following steps: (1) In the presence of ultrapure water, cationic polyacrylamide solution and polyaluminum chloride solution are reacted at a pH of 8.0-9.0 to obtain a suspension of composite flocs; (2) The suspension of the composite flocs is passed through an ultrafiltration membrane under transmembrane pressure, so that the composite flocs are pre-deposited on the surface of the ultrafiltration membrane to form a functional separation layer, thus obtaining the composite floc modified ultrafiltration membrane.

[0019] In a preferred embodiment, in step (1), the concentration of the cationic polyacrylamide solution is 0.1-0.4 g / L; the concentration of the polyaluminum chloride solution, calculated as Al, is 10-100 mg / L; and the weight ratio of the amount of polyacrylamide in the cationic polyacrylamide solution to the weight ratio of the amount of Al in the polyaluminum chloride solution is 0.007-0.090:1.

[0020] More preferably, the basicity of the polyaluminum chloride solution is 0.5-2.0.

[0021] More preferably, the basicity of the polyaluminum chloride solution is 1.0-2.0.

[0022] Preferably, the polyaluminum chloride solution is prepared by a method comprising the following steps: adding a 0.1-0.4 mol / L NaOH solution dropwise to a 0.1-0.2 mol / L AlCl3 solution using a peristaltic pump at a rotation speed of not less than 300 rpm and a water bath temperature of 40-60℃, wherein the rotation speed of the peristaltic pump is 0.5-1.5 rpm, thereby obtaining the polyaluminum chloride solution.

[0023] It should be noted that in this invention, the amount of NaOH solution used is adjusted to prepare polyaluminum chloride solutions with different alkalinity.

[0024] Preferably, in step (1), the preparation method of the cationic polyacrylamide solution includes: adding cationic polyacrylamide particles to ultrapure water, stirring under a water bath heating condition of 40-60 ℃ until completely dissolved, and then cooling and adjusting the volume to the required concentration of 0.1-0.4 g / L.

[0025] In a preferred embodiment, in step (1), the contact reaction is carried out under stirring conditions, and includes at least: stirring at a speed of 200-400 rpm for 0.5-5 min, and then stirring at a speed of 20-60 rpm for 5-30 min.

[0026] In a preferred embodiment, in step (2), the material of the ultrafiltration membrane is selected from polyvinylidene fluoride, polyethersulfone or polysulfone, and its molecular weight cutoff is 10-100 kDa.

[0027] According to a preferred embodiment, in step (2), the transmembrane pressure is 0.05-0.2 MPa.

[0028] Preferably, the method of the present invention further includes: pre-treating the ultrafiltration membrane before performing the pre-deposition, including: soaking the ultrafiltration membrane in ultrapure water for more than 8 hours, changing the ultrapure water every two days, and then storing it in ultrapure water.

[0029] As mentioned above, the second aspect of the present invention provides a composite floc-modified ultrafiltration membrane, which is obtained by the preparation method described in the first aspect. The surface of the composite floc-modified ultrafiltration membrane is pre-deposited with a three-dimensional network composite floc functional separation layer formed by cationic polyacrylamide and polyaluminum chloride.

[0030] As previously stated, the third aspect of this invention provides the application of the composite floc-modified ultrafiltration membrane described in the second aspect in the treatment of water containing perfluorinated compounds.

[0031] In a preferred embodiment, the application of the present invention includes: placing the composite floc-modified ultrafiltration membrane in a filtration device, and allowing the water containing perfluorinated compounds to pass through the composite floc-modified ultrafiltration membrane under a pressure of 0.05-0.2 MPa, so as to achieve the retention of perfluorinated compounds, and the retention rate is not less than 70 wt%.

[0032] Preferably, the specific operation of "passing the water containing perfluorinated compounds through the composite floc-modified ultrafiltration membrane under a pressure of 0.05-0.2 MPa" includes: dead-end filtration or cross-flow filtration, using nitrogen pressurization or peristaltic pump drive. According to a preferred embodiment, the perfluorinated compound is selected from at least one of perfluorooctanoic acid, perfluorononanoic acid, and perfluorohexanoic acid.

[0033] In a preferred embodiment, the concentration of perfluorinated compounds in the water containing perfluorinated compounds is 0.05-100 mg / L.

[0034] In a preferred embodiment, the water containing perfluorinated compounds also contains Na. + Ca 2+ Cl - SO4 2- HCO3 - At least one of humic acid, bovine serum albumin, and sodium alginate.

[0035] The present invention will be described in detail below through examples. Unless otherwise specified, the raw materials used are all commercially available products.

[0036] Cationic polyacrylamide: molecular weight 500,000, ionicity 20%, analytical grade, purchased from Shanghai Dibai Biotechnology Co., Ltd. Ultrafiltration membrane: Sold by Shanghai Mopeng Industrial Co., Ltd., with a molecular weight cutoff of 100 kDa and an effective membrane area of ​​30.07 cm². 2 Ultrafiltration membrane; Perfluorooctanoic acid (PFOA): purity ≥98%, purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Humic acid: Sodium humate, analytical grade, purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Bovine serum albumin: purity ≥98%, purchased from Shanghai Dibai Biotechnology Co., Ltd.

[0037] Pretreatment of ultrafiltration membrane: Soak the ultrafiltration membrane in ultrapure water for 8 hours, and change the water once every two days.

[0038] Preparation of cationic polyacrylamide solution: Add cationic polyacrylamide particles to ultrapure water, stir until completely dissolved under water bath heating at 50 ℃, cool to 25 ℃ and then adjust the volume to a concentration of 0.4 g / L.

[0039] Preparation of polyaluminum chloride solutions with different basicities: Under magnetic stirring at 320 rpm and a water bath temperature of 50℃, a 0.1 mol / L NaOH solution was slowly added dropwise to a 0.1 mol / L AlCl3 solution using a peristaltic pump. The peristaltic pump speed was controlled between 0.1 and 1.0 rpm. The basicities (B = [OH-]) were then obtained. - ] / [Al 3+ A polyaluminum chloride solution with a concentration of 1.0 was prepared and named PAC. 1.0 .

[0040] Example 1

[0041] This embodiment illustrates the preparation method of the composite floc-modified ultrafiltration membrane provided by the present invention, which includes the following steps: (1) Mix cationic polyacrylamide solution with polyaluminum chloride solution (PAC) 1.0 The polyacrylamide in the cationic polyacrylamide solution was added to 200 mL of ultrapure water at a weight ratio of 0.007:1 to Al in the polyaluminum chloride solution. The pH was adjusted to 8.5 using hydrochloric acid or sodium hydroxide solution. Then, a contact reaction was carried out: the mixture was stirred at 300 rpm for 3 min and then at 40 rpm for 15 min to obtain a suspension of composite flocs. (2) The suspension of the composite flocs is passed through a pretreated ultrafiltration membrane under a transmembrane pressure of 0.1 MPa, so that the composite flocs are pre-deposited on the surface of the ultrafiltration membrane to form a functional separation layer, thus obtaining the composite floc modified ultrafiltration membrane, named P1.

[0042] Figure 1 A physical image of the composite floc pre-deposition modified ultrafiltration membrane P1 is shown. Figure 2 A scanning electron microscope (SEM) image of the composite floc pre-deposition modified ultrafiltration membrane P1 is shown, showing that the membrane surface is uniformly covered with a three-dimensional network of composite flocs.

[0043] Example 2

[0044] The method was similar to that in Example 1, except that in step (1), the ratio of the weight of polyacrylamide in the cationic polyacrylamide solution to the weight of Al in the polyaluminum chloride solution was 0.022:1. Finally, the composite floc-modified ultrafiltration membrane was obtained and named P2.

[0045] Example 3

[0046] The method was similar to that in Example 1, except that in step (1), the ratio of the weight of polyacrylamide in the cationic polyacrylamide solution to the weight of Al in the polyaluminum chloride solution was 0.059:1. Finally, the composite floc-modified ultrafiltration membrane was obtained and named P3.

[0047] Example 4

[0048] The method was similar to that in Example 1, except that in step (1), the ratio of the weight of polyacrylamide in the cationic polyacrylamide solution to the weight of Al in the polyaluminum chloride solution was 0.089:1. Finally, the composite floc-modified ultrafiltration membrane was obtained and named P4.

[0049] Application Example 1 The composite floc-modified ultrafiltration membrane and the modified ultrafiltration membrane prepared in the above examples were placed in an ultrafiltration device, with the pretreated ultrafiltration membrane serving as a control group. Simulated wastewater containing 50 mg / L of perfluorooctanoic acid (PFOA) was prepared as the water to be treated and filtered under a nitrogen pressure of 0.1 MPa at a filtration temperature of 25 ± 1℃. Effluent samples were collected, and the PFOA concentrations in the influent and effluent were determined by ultra-high performance liquid chromatography (UPLC). The PFOA rejection rate was calculated according to formula (I): Formula (I); Where R is the rejection rate, C0 is the influent PFOA concentration, and C p This represents the PFOA concentration in the effluent.

[0050] The results are shown in Table 1.

[0051] Table 1 Application Example 2 This application example uses a method similar to that of Application Example 1. The difference is that the composite floc-modified ultrafiltration membrane P2 prepared in the above example is used, and simulated wastewater containing perfluorooctanoic acid (PFOA) concentrations of 10, 20, 30, and 40 mg / L is prepared as the water body to be treated, and the PFOA rejection rate is calculated.

[0052] The results are shown in Table 2.

[0053] Table 2 Application Example 3 This application example uses a method similar to Application Example 1. The difference is that the composite floc-modified ultrafiltration membrane P2 prepared in the previous examples is used, and multiple groups of simulated wastewater containing 50 mg / L perfluorooctanoic acid (PFOA) are set as the water bodies to be treated. Each water body also contains Cl at a concentration of 10 mmol / L. - / Na + Ca 2+ SO4 2- and HCO3 - , as well as humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) at a concentration of 10 mg / L.

[0054] Calculate the PFOA retention rate.

[0055] The results are shown in Table 3.

[0056] Table 3 Application Example 4 This application example uses a method similar to that of Application Example 1. The difference is that the composite floc-modified ultrafiltration membrane P2 prepared in the above example is used, and simulated wastewater containing PFNA concentration of 50 mg / L and simulated wastewater containing PFHxS concentration of 50 mg / L are prepared as the water bodies to be treated. The results are shown in Table 4.

[0057] Table 4 The above experimental results show that the ultrafiltration membrane modified by pre-deposition of CPAM and PAC composite flocs in this invention has a perfluorinated compound rejection rate of not less than 70 wt%, and can reach up to 92.9 wt%. It also exhibits stable performance under complex water quality conditions, and the preparation process is simple and low-cost, showing good prospects for industrial application.

[0058] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a composite floc-modified ultrafiltration membrane, characterized in that, Includes the following steps: (1) In the presence of ultrapure water, cationic polyacrylamide solution and polyaluminum chloride solution are reacted at a pH of 8.0-9.0 to obtain a suspension of composite flocs; (2) The suspension of the composite flocs is passed through an ultrafiltration membrane under transmembrane pressure, so that the composite flocs are pre-deposited on the surface of the ultrafiltration membrane to form a functional separation layer, thus obtaining the composite floc modified ultrafiltration membrane.

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of the cationic polyacrylamide solution is 0.1-0.4 g / L; the concentration of the polyaluminum chloride solution, calculated as Al, is 10-100 mg / L; and, The weight ratio of polyacrylamide in the cationic polyacrylamide solution to Al in the polyaluminum chloride solution is 0.007-0.090:

1.

3. The preparation method according to claim 1 or 2, characterized in that, In step (1), the alkalinity of the polyaluminum chloride solution is 0.5-2.

0.

4. The preparation method according to claim 1 or 2, characterized in that, In step (1), the contact reaction is carried out under stirring conditions, and includes at least: stirring at a speed of 200-400 rpm for 0.5-5 min, and then stirring at a speed of 20-60 rpm for 5-30 min.

5. The preparation method according to claim 1 or 2, characterized in that, In step (2), the transmembrane pressure is 0.05-0.2 MPa.

6. A composite floc-modified ultrafiltration membrane, characterized in that, The composite floc-modified ultrafiltration membrane is obtained by the preparation method according to any one of claims 1-5, wherein a three-dimensional network composite floc functional separation layer formed by cationic polyacrylamide and polyaluminum chloride is pre-deposited on the surface.

7. The application of the composite floc-modified ultrafiltration membrane according to claim 6 in the treatment of water containing perfluorinated compounds.

8. The application according to claim 7, characterized in that, The application includes placing the composite floc-modified ultrafiltration membrane in a filtration device, allowing the water containing perfluorinated compounds to pass through the composite floc-modified ultrafiltration membrane under a pressure of 0.05-0.2 MPa, so as to achieve the retention of perfluorinated compounds, with a retention rate of not less than 70 wt%.

9. The application according to claim 8, characterized in that, The perfluorinated compound is selected from at least one of perfluorooctanoic acid, perfluorononanoic acid, and perfluorohexanoic acid; and / or In the water containing perfluorinated compounds, the concentration of perfluorinated compounds is 0.05-100 mg / L.

10. The application according to claim 8, characterized in that, The water containing perfluorinated compounds also contains Na. + Ca 2 + Cl - SO4 2- HCO3 - At least one of humic acid, bovine serum albumin, and sodium alginate.

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