The application discloses a method for improving membrane pollution and synchronously removing perfluorooctanoic acid in water bodies by using an aluminum-based coagulant coupled with C18 hydrophobic cationic coagulant aid reinforced coagulation

Abstract

The application relates to a method for improving membrane pollution of an ultra-low-pressure ceramic membrane filtration system and synchronously removing perfluorooctanoic acid in water by coupling aluminum coagulant and C18 hydrophobic cationic coagulant strengthening coagulation, and belongs to the field of water pollution treatment. The application aims to solve the problems of low removal rate of perfluorooctanoic acid, serious membrane pollution and short membrane service cycle in the existing coagulation-membrane filtration process. The method comprises the following steps: firstly, an aluminum coagulant is added into raw water through a dosing pump and stirred rapidly, then a C18 hydrophobic cationic coagulant is added and stirred slowly, and coagulated water is obtained; secondly, the coagulated water is introduced into an ultra-low-pressure ceramic membrane filtration unit through gravity and subjected to gravity or micro-pressure filtration, and the filtered clean water is introduced into a clean water tank. The application is used for improving membrane pollution of an ultra-low-pressure ceramic membrane filtration system and synchronously removing perfluorooctanoic acid in water by coupling aluminum coagulant and C18 hydrophobic cationic coagulant strengthening coagulation.

Description

Technical Field

[0001] This invention belongs to the field of water pollution control. Background Technology

[0002] Perfluorooctanoic acid (PFOA) is a synthetic organic compound with high chemical and thermal stability, widely used in waterproof coatings, fire-fighting foams, and electronics manufacturing. Due to its extremely high carbon-fluorine bond energy, PFOA is almost non-degradable under natural conditions and has been widely detected in surface water, groundwater, soil, and drinking water systems. PFOA exhibits persistence, bioaccumulation, and potential reproductive toxicity.

[0003] In existing water treatment processes, coagulation and sedimentation is the most economical and widely used water purification method. It typically relies on aluminum- or iron-based coagulants (such as polyaluminum chloride) to aggregate and remove colloids and suspended particles through double-layer compression and charge neutralization. However, PFOA molecules have an amphipathic structure with hydrophobic segments and hydrophilic head groups, a particle size much smaller than the colloidal scale, a negatively charged surface, and significant interfacial activity, making them difficult for traditional coagulants to effectively capture. Especially at low concentrations (ng / L~µg / L), the removal rate of PFOA by polyaluminum chloride is usually less than 20%.

[0004] Furthermore, in areas where centralized water treatment is inconvenient, decentralized membrane treatment is often necessary. Currently, only nanofiltration membranes have a significant removal effect on perfluorooctanoic acid (PFOA), but due to their small pore size, they suffer from low membrane flux, easy clogging, and frequent operation and maintenance. When microfiltration or ultrafiltration membranes are used, the pore size is too large, resulting in no interception of PFOA. Additionally, organic matter in the water can rapidly reduce membrane flux, and its corrosion significantly shortens membrane lifespan. Another method involves coagulation and sedimentation followed by ultrafiltration or nanofiltration to remove residual PFOA. This process involves two stages of treatment, is complex, energy-intensive, and unsuitable for decentralized or low-pressure membrane systems.

[0005] In summary, there is an urgent need to develop a method for removing perfluorooctanoic acid (PFOA) from water that has a high removal rate, low energy consumption, is applicable to diverse wastewater types, and can be used in decentralized water treatment. Summary of the Invention

[0006] This invention aims to address the problems of low perfluorooctanoic acid (PFOA) removal rate, severe membrane fouling, and short membrane service life in existing coagulation-membrane filtration processes. It provides a method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing PFOA from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid.

[0007] A method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid (PFOA) from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, comprises the following steps:

[0008] 1. Raw water flows into the pre-coagulation tank of the ultra-low pressure ceramic membrane filtration system. In the pre-coagulation tank, aluminum-based coagulant is added to the raw water through a dosing pump and stirred rapidly. After the rapid stirring is completed, C18 hydrophobic cationic coagulant is quickly added and stirred slowly to obtain a coagulated water sample.

[0009] The C18 hydrophobic cationic coagulant is a trihydroxysilylpropyloctadecyldimethylammonium coagulant solution;

[0010] 2. The coagulated water sample flows into the constant water tank to reach the set constant water level, and then enters the ultra-low pressure ceramic membrane filtration unit by gravity. Under the condition that the transmembrane pressure difference is not higher than 0.02MPa, gravity or micro-pressure filtration is carried out, and the purified water after membrane filtration enters the clear water tank.

[0011] The beneficial effects of this invention are:

[0012] (1) Two-stage removal regulation mechanism: First, aluminum-based coagulant is added to achieve charge neutralization and removal of coexisting natural organic matter and suspended particles, which significantly reduces the competitive adsorption of complex water matrix on the active sites of trihydroxysilylpropyloctadecyldimethylammonium, thereby improving its targeted adsorption efficiency for PFOA.

[0013] (2) The addition of trihydroxysilylpropyloctadecyl dimethylammonium in the later stage helps coagulation and promotes directional adsorption: Trihydroxysilylpropyloctadecyl dimethylammonium is slowly added in the later stage so that it is uniformly coated on the surface of the Al-floc during the slow stirring stage, forming an ordered hydrophobic-hydrophilic interface layer, which not only enhances the hydrophobicity and adsorption selectivity of the floc surface, but also avoids the self-polymerization and deactivation of trihydroxysilylpropyloctadecyl dimethylammonium in a high ionic strength environment.

[0014] (3) Multifunctional groups enhance interfacial synergistic adsorption and regulate floc structure: The Si-OH group contained in trihydroxysilylpropyloctadecyl dimethylammonium, as a coagulant aid, can form hydrogen bonds with the Al-OH of aluminum flocs. At the same time, its C18 hydrophobic end forms hydrophobic association with the hydrophobic segments of perfluorooctanoic acid (PFOA) molecules. In addition, its cationic quaternary ammonium group has a charge attraction effect on the negatively charged PFOA. The three form a multi-interfacial adsorption mechanism of "hydrophilic anchoring - hydrophobic trapping - charge attraction", which greatly increases the probability of PFOA being trapped during coagulation. In particular, the presence of the Si-OH end enables the organic coagulant aid to be uniformly dispersed in the water and undergo a local co-assembly reaction with the aluminum flocs, which promotes the formation of a flexible, organically coated composite layer on the floc surface, improves the morphology and strength of the flocs, and thus forms a more stable filter cake layer.

[0015] (4) High stability of bifurcated floc structure: The floc growth path of "gradual slow stirring - later organosilicon cation-induced crosslinking" realizes a bifurcated network structure with low fractal dimension. This structure has high specific surface area, good permeability and water permeability of filter cake layer; thus enabling it to have good shear resistance and collection ability.

[0016] (5) "Membrane-based sedimentation" coupling enhances secondary pollutant removal: After coagulation, the material enters the ceramic microfiltration membrane unit directly without sedimentation. It forms a self-assembled filter cake layer through gravity drive without pumps. This layer has multi-level pores and high organic affinity, which significantly improves the PFOA rejection rate, inhibits membrane fouling, and reduces process length.

[0017] (6) Significant advantages in membrane operation and maintenance: Due to the high structural stability and reversibility of the formed filter cake layer, its bonding force with the membrane surface is moderate, and it can be effectively removed under low-pressure physical backwashing conditions. Most of the membrane fouling can be quickly desorbed, and the membrane flux is almost completely restored without relying on chemical cleaning. As a result, the system is almost free from chemical irreversible membrane fouling, membrane material loss is significantly reduced, operation and maintenance frequency is reduced, and membrane module service life is significantly extended, further reducing the operating cost of the decentralized membrane treatment system. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the ultra-low pressure ceramic membrane filtration system of the present invention;

[0019] Figure 2 This is a comparison chart of the membrane specific flux changes in the ultra-low pressure ceramic membrane filtration systems of Example 1, Comparative Examples 1 and 2;

[0020] Figure 3 The curves showing the change in floc particle size during coagulation in Example 4 and Comparative Example 3 are shown.

[0021] Figure 4 The changes in floc particle size and volume distribution at different coagulation stages in Example 4 and Comparative Example 3;

[0022] Figure 5 These are microscopic images showing the morphology of flocs after coagulation in Example 4 and Comparative Example 3.

[0023] Figure 6 Fourier transform infrared spectra of the dried powder of the trihydroxysilylpropyloctadecyldimethylammonium coagulant prepared in Example 1, and the dried floc samples in Example 4 and Comparative Example 3.

[0024] Figure 7 The figures show the water contact angle test results of the dried floc samples in Example 4 and Comparative Example 3. Detailed Implementation

[0025] Specific Implementation Method 1: This implementation method is a method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid (PFOA) from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid. It is carried out according to the following steps:

[0026] 1. Raw water flows into the pre-coagulation tank of the ultra-low pressure ceramic membrane filtration system. In the pre-coagulation tank, aluminum-based coagulant is added to the raw water through a dosing pump and stirred rapidly. After the rapid stirring is completed, C18 hydrophobic cationic coagulant is quickly added and stirred slowly to obtain a coagulated water sample.

[0027] The C18 hydrophobic cationic coagulant is a trihydroxysilylpropyloctadecyldimethylammonium coagulant solution;

[0028] 2. The coagulated water sample flows into the constant water tank to reach the set constant water level, and then enters the ultra-low pressure ceramic membrane filtration unit by gravity. Under the condition that the transmembrane pressure difference is not higher than 0.02MPa, gravity or micro-pressure filtration is carried out, and the purified water after membrane filtration enters the clear water tank.

[0029] In this embodiment, the coagulated water sample is not allowed to settle. After flowing into the constant water tank and reaching the set constant water level, it enters the ultra-low pressure ceramic membrane filtration unit by gravity.

[0030] This embodiment of the method includes the following steps: During the coagulation process, an aluminum-based coagulant is first added during a rapid stirring stage, followed by the addition of trihydroxysilylpropyloctadecyl dimethylammonium. Slow stirring then forms a multi-branched flocculant with a stable structure, large particle size, and low fractal dimension. The flocculated water directly enters an ultra-low pressure ceramic membrane filter unit for gravity-driven filtration, forming a filter cake layer with adsorption function, achieving synergistic removal of perfluorooctanoic acid (PFOA) and mitigation of membrane fouling. This embodiment significantly improves the PFOA removal efficiency and membrane flux stability through the sequential addition of coagulants and coagulant aids and the interfacial structure control mechanism. The formed filter cake layer can be effectively removed under low-pressure physical backwashing, the membrane flux is almost completely restored, there is almost no irreversible chemical fouling, and the membrane lifespan is significantly extended. This embodiment is applicable to decentralized water purification and the treatment of wastewater containing perfluorinated compounds.

[0031] This embodiment first adds an aluminum-based coagulant during the rapid coagulation and stirring stage, followed by the addition of trihydroxysilylpropyloctadecyl dimethylammonium as a coagulant aid. The coagulated water is then directly filtered through a ceramic membrane, achieving sequential addition of coagulant, interfacial structure regulation, and synergistic treatment of the filter cake layer. This results in the regulation of floc morphology, enhanced coagulation removal of perfluorooctanoic acid (PFOA) in the water, and enhanced adsorption removal by the filter cake layer. The operation is simple, membrane fouling is simultaneously alleviated, and the membrane service life is greatly extended.

[0032] The beneficial effects of this embodiment are:

[0033] (1) Two-stage removal regulation mechanism: First, aluminum-based coagulant is added to achieve charge neutralization and removal of coexisting natural organic matter and suspended particles, which significantly reduces the competitive adsorption of complex water matrix on the active sites of trihydroxysilylpropyloctadecyldimethylammonium, thereby improving its targeted adsorption efficiency for PFOA.

[0034] (2) The addition of trihydroxysilylpropyloctadecyl dimethylammonium in the later stage helps coagulation and promotes directional adsorption: Trihydroxysilylpropyloctadecyl dimethylammonium is slowly added in the later stage so that it is uniformly coated on the surface of the Al-floc during the slow stirring stage, forming an ordered hydrophobic-hydrophilic interface layer, which not only enhances the hydrophobicity and adsorption selectivity of the floc surface, but also avoids the self-polymerization and deactivation of trihydroxysilylpropyloctadecyl dimethylammonium in a high ionic strength environment.

[0035] (3) Multifunctional groups enhance interfacial synergistic adsorption and regulate floc structure: The Si-OH group contained in trihydroxysilylpropyloctadecyl dimethylammonium, as a coagulant aid, can form hydrogen bonds with the Al-OH of aluminum flocs. At the same time, its C18 hydrophobic end forms hydrophobic association with the hydrophobic segments of perfluorooctanoic acid (PFOA) molecules. In addition, its cationic quaternary ammonium group has a charge attraction effect on the negatively charged PFOA. The three form a multi-interfacial adsorption mechanism of "hydrophilic anchoring - hydrophobic trapping - charge attraction", which greatly increases the probability of PFOA being trapped during coagulation. In particular, the presence of the Si-OH end enables the organic coagulant aid to be uniformly dispersed in the water and undergo a local co-assembly reaction with the aluminum flocs, which promotes the formation of a flexible, organically coated composite layer on the floc surface, improves the morphology and strength of the flocs, and thus forms a more stable filter cake layer.

[0036] (4) High stability of bifurcated floc structure: The floc growth path of "gradual slow stirring - later organosilicon cation-induced crosslinking" realizes a bifurcated network structure with low fractal dimension. This structure has high specific surface area, good permeability and water permeability of filter cake layer; thus enabling it to have good shear resistance and collection ability.

[0037] (5) "Membrane-based sedimentation" coupling enhances secondary pollutant removal: After coagulation, the material enters the ceramic microfiltration membrane unit directly without sedimentation. It forms a self-assembled filter cake layer through gravity drive without pumps. This layer has multi-level pores and high organic affinity, which significantly improves the PFOA rejection rate, inhibits membrane fouling, and reduces process length.

[0038] (6) Significant advantages in membrane operation and maintenance: Due to the high structural stability and reversibility of the formed filter cake layer, its bonding force with the membrane surface is moderate, and it can be effectively removed under low-pressure physical backwashing conditions. Most of the membrane fouling can be quickly desorbed, and the membrane flux is almost completely restored without relying on chemical cleaning. As a result, the system is almost free from chemical irreversible membrane fouling, membrane material loss is significantly reduced, operation and maintenance frequency is reduced, and membrane module service life is significantly extended, further reducing the operating cost of the decentralized membrane treatment system.

[0039] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the aluminum-based coagulant mentioned in step one is aluminum chloride, aluminum sulfate, or polyaluminum chloride. Everything else is the same as in Specific Implementation Method One.

[0040] The aluminum-based coagulant described in this specific embodiment is an aluminum-based coagulant that works by generating aluminum ions or aluminum hydroxide colloids through a hydrolysis reaction, thereby adsorbing and aggregating suspended solids, colloidal substances, and dissolved organic matter in water.

[0041] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the dosage of Al in the aluminum-based coagulant mentioned in step one is 0.1 mM ~ 0.2 mM. Everything else is the same as in Specific Implementation Method One or Two.

[0042] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: in step one, under the condition of a stirring speed of 300 rpm to 400 rpm, the aluminum-based coagulant is added to the raw water in the pre-coagulation tank via a dosing pump and stirred rapidly for 1 to 3 minutes. After the rapid stirring is completed, under the condition of a stirring speed of 40 rpm to 150 rpm, the C18 hydrophobic cationic coagulant aid is quickly added and stirred slowly for 10 to 20 minutes. Everything else is the same as in Specific Implementation Methods One to Three.

[0043] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the trihydroxysilylpropyloctadecyldimethylammonium coagulant solution mentioned in step one is specifically prepared according to the following steps:

[0044] A solution of dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride, ethanol, and water were mixed and stirred for 6 to 24 hours at room temperature and a stirring speed of 100 to 200 rpm. The mixture was then dialyzed in an acidic aqueous solution to obtain a liquid trihydroxysilylpropyloctadecyldimethylammonium coagulant. The liquid trihydroxysilylpropyloctadecyldimethylammonium coagulant was then freeze-dried to obtain a powder. Finally, the powder was dissolved in water to obtain a trihydroxysilylpropyloctadecyldimethylammonium coagulant solution with a mass percentage of 2% to 4%.

[0045] The structural formula of the trihydroxysilylpropyloctadecyldimethylammonium coagulant liquid is as follows: Everything else is the same as in specific implementation methods one through four.

[0046] The CAS number of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride is 27668-52-6.

[0047] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in the following ways: the concentration of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution is 40 wt.%; the volume ratio of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution to ethanol is 1:(20~30); the volume ratio of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution to water is 1:(5~10); the acidic aqueous solution is specifically adjusted to pH 2.0~4.0 using 3M~12M hydrogen chloride; the dialysis is specifically performed using a 100Da~500Da dialysis membrane in a hydrogen chloride solution with a pH of 2.0~4.0 for 6h~24h; the freeze-drying is specifically performed by pre-freezing the trihydroxysilylpropyloctadecyldimethylammonium coagulant liquid at a temperature of -80℃~-20℃, and then freezing in a cold trap at a temperature of -50℃~ Freeze-dry at -40°C and a vacuum of 5 Pa to 40 Pa for 12 to 24 hours. Other procedures are the same as in specific embodiments one to five.

[0048] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that: after the rapid stirring in Step One, C18 hydrophobic cationic coagulant is added quickly within 1 minute; the molar ratio of Al to trihydroxysilylpropyloctadecyl dimethylammonium coagulant in the aluminum-based coagulant mentioned in Step One is (5~25):1. Everything else is the same as in Specific Implementation Methods One to Six.

[0049] Specific implementation method eight, combined with Figure 1 Specific description: The difference between this embodiment and one of the specific embodiments one to seven is that the ultra-low pressure ceramic membrane filtration system described in step one includes a pre-coagulation water tank, a constant water tank, an ultra-low pressure ceramic membrane filtration unit, a physical and chemical backwashing unit, and a clear water tank.

[0050] The pre-mixed coagulation tank, constant water tank, ultra-low pressure ceramic membrane filter unit, and clear water tank are connected in sequence through water purification pipelines; the physical and chemical backwashing units are connected to the ultra-low pressure ceramic membrane filter unit and clear water tank respectively through cleaning pipelines.

[0051] The ultra-low pressure ceramic membrane filtration unit operates under gravity drive and contains a flat ceramic membrane assembly. The flat ceramic membrane assembly is made of alumina, titanium dioxide, zirconium dioxide, or silicon carbide, and has a pore size of 0.1 μm to 10 μm. Other aspects are the same as in embodiments one through seven.

[0052] In this specific embodiment, the pre-coagulated water tank is equipped with a stirring paddle and first and second dosing ports for adding coagulants and coagulant aids to form primary flocs.

[0053] In this specific embodiment, the constant-level water tank is equipped with a stirring paddle and a float valve or other constant water level control device to maintain the liquid level height and provide a constant gravity driving force for the downstream membrane filtration unit;

[0054] In this specific embodiment, the ultra-low pressure ceramic membrane filtration unit operates by gravity drive, requiring no external pump pressure, and a flat ceramic membrane assembly is installed inside the membrane unit.

[0055] This specific embodiment can also be equipped with a dosing unit to dosing chemicals online into the membrane module pool, adding necessary coagulation aids or cleaning agents;

[0056] In this specific embodiment, the physical and chemical backwashing unit is used to perform periodic reverse pressure rinsing when the membrane module is fouled. It can also be used through the chemical rinsing dosing tank to add alkaline, acidic or oxidizing chemical cleaning agents to clean the membrane surface when the membrane fouling is severe.

[0057] In this specific embodiment, both the constant-position water tank and the pre-coagulation water tank are equipped with a stirrer to ensure uniform water flow and coagulation effect.

[0058] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that the transmembrane pressure difference of the flat ceramic membrane module is 0.004 MPa to 0.02 MPa. Everything else is the same as in Specific Implementation Methods One to Eight.

[0059] This specific implementation aims to provide stable gravity-driven pressure.

[0060] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that the physical and chemical backwashing unit specifically performs reverse pressurized backwashing under a backwashing pump pressure of 0.1 MPa to 0.2 MPa. Everything else is the same as in Specific Implementation Methods One to Nine.

[0061] The beneficial effects of the present invention are verified using the following embodiments:

[0062] Example 1 verifies the effect of trihydroxysilylpropyloctadecyl dimethylammonium coagulant aid combined with polyaluminum chloride coagulant on the removal of perfluorooctanoic acid in the experimental water and the mitigation of ceramic membrane fouling when used in the pre-coagulation section of an ultra-low pressure ceramic membrane filtration system.

[0063] A method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid (PFOA) from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, comprises the following steps:

[0064] 1. Raw water flows into the pre-coagulation tank of the ultra-low pressure ceramic membrane filtration system. Under the condition of a stirring speed of 400 rpm, aluminum-based coagulant is added to the raw water through a dosing pump and stirred rapidly for 1 minute. After the rapid stirring is completed, under the condition of a stirring speed of 100 rpm, a trihydroxysilylpropyloctadecyl dimethylammonium coagulant aid solution is rapidly added within 0.5 minutes and stirred slowly for 15 minutes to obtain a coagulated water sample.

[0065] The aluminum-based coagulant mentioned is polyaluminum chloride (using solid polyaluminum chloride produced by Comio Corporation, chemically pure, molecular formula Al). n (OH) m Cl 3n-m (0) <m>3n), with an Al2O3 content ω / %≥28.0% (the polyaluminum chloride is prepared as a 5g / L polyaluminum chloride mother liquor before use); the dosage of Al in the aluminum-based coagulant is 0.1mM; the molar ratio of Al to trihydroxysilylpropyloctadecyldimethylammonium coagulant in the aluminum-based coagulant is 10:1;

[0066] 2. The coagulated water sample flows into the constant water tank to reach the set constant water level. Then, the outlet valve of the constant water tank is opened, and the water enters the ultra-low pressure ceramic membrane filtration unit by gravity. Gravity filtration is carried out for 15 days under the condition of a transmembrane pressure difference of 78.5 mbar. The purified water after membrane filtration enters the clear water tank.

[0067] The trihydroxysilylpropyloctadecyldimethylammonium coagulant solution mentioned in step one is prepared according to the following steps:

[0068] Dimethyl octadecyl[3-trimethoxysilylpropyl]ammonium chloride solution, ethanol, and water were mixed and stirred for 6 hours at room temperature and a stirring speed of 100 rpm. Then, the mixture was dialyzed for 12 hours in a hydrogen chloride solution with a pH of 2.5 using a 500 Da dialysis membrane to obtain a trihydroxysilylpropyl octadecyl dimethylammonium coagulant liquid. The trihydroxysilylpropyl octadecyl dimethylammonium coagulant liquid was pre-frozen at a temperature of -20°C and then freeze-dried for 24 hours at a cold trap temperature of -40°C and a vacuum degree of 40 Pa to obtain a powder. Finally, the powder was dissolved in water to obtain a 4% (w / w) trihydroxysilylpropyl octadecyl dimethylammonium coagulant solution.

[0069] The concentration of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution is 40 wt.%; the volume ratio of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution to ethanol is 1:20; the volume ratio of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution to water is 1:5; the pH 2.5 hydrogen chloride solution is specifically prepared by adjusting the pH to 2.5 using 3M hydrogen chloride solution; the structural formula of the trihydroxysilylpropyloctadecyldimethylammonium coagulant liquid is [not specified]. .

[0070] The ultra-low pressure ceramic membrane filtration system described in step one includes a pre-coagulation water tank, a constant-level water tank, an ultra-low pressure ceramic membrane filtration unit, a physical and chemical backwashing unit, and a clear water tank.

[0071] The pre-mixed coagulation tank, constant water tank, ultra-low pressure ceramic membrane filter unit, and clear water tank are connected in sequence through water purification pipelines; the physical and chemical backwashing units are connected to the ultra-low pressure ceramic membrane filter unit and clear water tank respectively through cleaning pipelines.

[0072] The ultra-low pressure ceramic membrane filtration unit operates by gravity drive and is equipped with a flat ceramic membrane assembly. The flat ceramic membrane assembly is made of alumina ceramic membrane and has a pore size of 0.5μm.

[0073] Meanwhile, Comparative Example 1 was set up without the addition of trihydroxysilylpropyloctadecyl dimethylammonium coagulant, and only polyaluminum chloride was added for pre-coagulation treatment; Comparative Example 2 was set up with polyaluminum chloride followed by cationic polyacrylamide (a commonly used existing polymeric coagulant) for pre-coagulation treatment.

[0074] Comparative Example 1: This comparative example differs from Example 1 in that the addition of the trihydroxysilylpropyloctadecyldimethylammonium coagulant solution is omitted in step one. Everything else is the same as in Example 1.

[0075] Comparative Example 2: This comparative example differs from Example 1 in that the trihydroxysilylpropyloctadecyldimethylammonium coagulant solution in step 1 is replaced with a cationic polyacrylamide (CPAM) coagulant solution. The cationic polyacrylamide (CPAM) coagulant solution is prepared as follows: cationic polyacrylamide (CPAM) solid particles are added to pure water and stirred with a magnetic stirrer for 40 minutes to obtain a cationic polyacrylamide (CPAM) coagulant solution with a concentration of 0.5 g / L. The dosage of the cationic polyacrylamide (CPAM) coagulant solution during the pre-coagulation process in step 1 is 0.5 mg CPAM / L. Everything else is the same as in Example 1.

[0076] The experimental water was prepared using perfluorooctanoic acid (PFOA), kaolin, humic acid, NaCl, and NaHCO3 to simulate surface water with a PFOA content of 50 μg / L, a turbidity of 35 NTU, a humic acid content of 5 mg / L, a NaCl concentration of 1 mM, and a NaHCO3 concentration of 1 mM.

[0077] During the filtration process, the membrane flux was tested periodically using an electronic balance, and water samples were taken to test the PFOA content and other water quality indicators. The entire filtration system ran for 15 days (360 hours). After the 15-day operation, the final membrane specific flux was measured. Membrane flux (J·s) -1 ·m 2 ) is a parameter characterizing the degree of membrane fouling, that is, the effective filtration flow rate per unit membrane area per unit time. Since the initial flux varies due to differences in membrane production, specific flux is used to compare the filtration performance of different filtration processes. That is, the ratio of instantaneous membrane flux to initial pure water flux, J / J0, is used as an indicator to characterize the degree of membrane fouling, also known as specific flux or standardized permeate flux.

[0078] After 15 days of operation, a reverse physical pressure flush was performed using pure water. The backflushing pump pressure was 0.1 MPa. The backflushing process was repeated three times. Then, the membrane specific flux after physical cleaning was tested using pure water.

[0079] After 15 days of operation, the ceramic membrane was chemically cleaned: it was soaked in 0.1M HCl for 1 hour, then rinsed three times with pure water at normal pressure, then soaked in 0.1M NaOH for 1 hour, and finally rinsed three times with pure water at normal pressure. After chemical cleaning, the membrane specific flux was tested again with pure water.

[0080] The specific flux change of the ceramic membrane ultra-low pressure filtration system during the 15-day operation of this embodiment is as follows: Figure 2 As shown, Figure 2 The graph shows a comparison of the membrane specific flux changes in the ultra-low pressure ceramic membrane filtration systems of Example 1, Comparative Examples 1 and 2. Table 1 shows the final membrane specific flux, the membrane specific flux after physical cleaning, and the membrane specific flux after chemical cleaning. Tables 2-4 show the removal effects of perfluorinated compounds from water in Example 1 and Comparative Examples 1 and 2.

[0081] Table 1. Final membrane specific flux and membrane specific flux recovery after physical and chemical cleaning of the ultra-low pressure ceramic membrane filtration systems in Examples 1 and 2 (Comparative Examples 1 and 2)

[0082]

[0083] Table 2. Effluent water quality of the ultra-low pressure ceramic membrane filtration systems in Examples 1 and 2 (Day 1)

[0084]

[0085] Table 3. Effluent water quality of the ultra-low pressure ceramic membrane filtration systems in Examples 1 and 2 (Comparative Examples 1 and 2) on Day 5.

[0086]

[0087] Table 4. Effluent water quality of the ultra-low pressure ceramic membrane filtration systems in Examples 1 and 2 (Day 15)

[0088]

[0089] Example 2 verifies the coagulation removal effect of perfluorooctanoic acid (PFOA) in soil washing wastewater containing PFOA. The difference between this example and Example 1 is that: the dosage of Al in the aluminum-based coagulant in step 1 is 0.2 mM; the molar ratio of Al to trihydroxysilylpropyloctadecyldimethylammonium coagulant in the aluminum-based coagulant is 25:1; and gravity filtration is performed for 5 days under a transmembrane pressure difference of 78.5 mbar in step 2. Everything else is the same as in Example 1.

[0090] In this embodiment, the soil was taken from Harbin Institute of Technology. The soil sample was obtained from a depth of 0cm to 20cm below the ground. Soil animals, stones and plant litter were removed. The soil sample was air-dried, ground and passed through a 2mm sieve. 100g of the air-dried soil sample was added to 500mL of ultrapure water and stirred at room temperature (500rpm) for 24h to obtain soil washing water. After centrifugation, the soil washing water used in Example 2 was obtained. 50μg / L PFOA was added to the soil washing water and stirred at room temperature for 24h to obtain soil washing wastewater containing the target pollutant PFOA.

[0091] The effectiveness of this embodiment in removing perfluorinated compounds from water is shown in Tables 5 and 6:

[0092] Table 5. Effluent water quality of the ultra-low pressure ceramic membrane filtration system in Example 2, Day 1.

[0093]

[0094] Table 6. Effluent water quality of the ultra-low pressure ceramic membrane filtration system in Example 2, Day 5.

[0095]

[0096] After 5 days of operation, the final specific flux of the membrane was measured to be 0.42. After 5 days of operation, a reverse physical pressure flush was performed using pure water (the cleaning method was the same as in Example 1). The specific flux of the membrane after physical cleaning was tested with pure water and found to be 0.91. After 5 days of operation, the ceramic membrane was chemically cleaned (the cleaning method was the same as in Example 1). After chemical cleaning, the specific flux of the membrane was tested again with pure water and found to be 0.92.

[0097] The results show that, even under complex conditions of coexisting organic matter and high PFOA concentration, Example 2 still exhibits excellent PFOA removal performance, proving that the present invention can be applied to complex pollution situations and effectively remove high concentrations of PFOA in various environments.

[0098] Example 3 verifies the coagulation removal effect of perfluorooctanoic acid (PFOA) in actual surface water. The differences between this example and Example 1 are: the dosage of the aluminum-based coagulant in step 1 is 0.2 mM; the molar ratio of Al to trihydroxysilylpropyloctadecyldimethylammonium coagulant in the aluminum-based coagulant is 25:1; the pore size of the flat-plate ceramic membrane module in step 2 is 10 μm, and gravity filtration is performed for 5 days under a transmembrane pressure difference of 78.5 mbar. Everything else is the same as in Example 1.

[0099] The actual surface water in this embodiment was taken from the Harbin section of the Songhua River.

[0100] The effectiveness of this embodiment in removing perfluorinated compounds from water is shown in Tables 7 and 8:

[0101] Table 7. Effluent water quality of the ceramic membrane ultra-low pressure filtration system in Example 3, Day 1.

[0102]

[0103] Table 8. Effluent water quality of the ceramic membrane ultra-low pressure filtration system in Example 3, Day 5.

[0104]

[0105] After 5 days of operation, the final specific flux of the membrane was measured to be 0.48. After 5 days of operation, a reverse physical pressure flush was performed using pure water (the cleaning method was the same as in Example 1). The specific flux of the membrane after physical cleaning was tested with pure water and found to be 0.93. After 5 days of operation, the ceramic membrane was chemically cleaned (the cleaning method was the same as in Example 1). After chemical cleaning, the specific flux of the membrane was tested again with pure water and found to be 0.94.

[0106] The results show that, even under complex conditions of coexisting organic matter and low PFOA concentration, this embodiment still exhibits excellent PFOA removal performance.

[0107] Example 4: This example verifies the successful synthesis of the C18 hydrophobic cationic coagulant aid (trihydroxysilylpropyloctadecyl dimethylammonium) prepared in Example 1, and explores the changes in floc morphology during the coagulation stage.

[0108] The experimental water was prepared using perfluorooctanoic acid (PFOA), kaolin, humic acid, NaCl, and NaHCO3 to simulate surface water with a PFOA content of 50 μg / L, a turbidity of 35 NTU, a humic acid content of 5 mg / L, a NaCl content of 1 mM, and a NaHCO3 content of 1 mM.

[0109] Coagulation particle size distribution experiment: The coagulation process was tested using a Mastersizer 2000 laser particle size analyzer; the laser particle size analyzer performed dynamic particle size distribution tests at 20-second intervals, with a total test time of 20 minutes.

[0110] First, add raw water to a beaker and stir for 1 minute at a stirring speed of 400 rpm. Then, add polyaluminum chloride to the experimental water at a stirring speed of 400 rpm and stir rapidly for 1 minute. After the rapid stirring is completed, add trihydroxysilylpropyloctadecyl dimethylammonium coagulant solution within 0.1 minutes at a stirring speed of 100 rpm and stir slowly for 18 minutes. The dosage of Al in the aluminum-based coagulant is 0.1 mM. The molar ratio of Al to trihydroxysilylpropyloctadecyl dimethylammonium coagulant in the aluminum-based coagulant is 10:1.

[0111] Comparative Example 3: This comparative example differs from Example 4 in that the addition of the trihydroxysilylpropyloctadecyldimethylammonium coagulant solution is omitted; firstly, raw water is added to a beaker and stirred for 1 minute at a stirring speed of 400 rpm; then, polyaluminum chloride is added to the experimental water mixture and stirred rapidly for 1 minute at a stirring speed of 400 rpm; after the rapid stirring is completed, the mixture is stirred slowly for 18 minutes at a stirring speed of 100 rpm; the dosage of Al in the aluminum-based coagulant is 0.1 mM. Everything else is the same as in Example 4.

[0112] Figure 3 The figures show the particle size variation curves of flocs during the coagulation process in Example 4 and Comparative Example 3; Stage I corresponds to the raw water, Stage II corresponds to the rapid stirring stage, and Stage III corresponds to the slow stirring stage. As can be seen from the figures, this example can significantly increase the final particle size of the coagulated flocs.

[0113] The particle size distribution was recorded at 2 minutes after the start of the coagulation experiment (after adding polyaluminum chloride coagulant and stirring for 1 minute), when the particle size reached its maximum value (5.5 minutes in Example 4 and 4.5 minutes in Comparative Example 3), and at 20 minutes (the end of the coagulation experiment). Figure 4 . Figure 4 The figures show the changes in floc size and volume distribution at different coagulation stages in Example 4 and Comparative Example 3. As can be seen from the figures, this example can significantly increase the final floc size. Compared to the maximum particle size, the reduction in final particle size with slow stirring is relatively small, indicating that the flocs obtained in this example are not easily sheared and broken by mechanical stirring, and the floc strength is significantly enhanced.

[0114] The structure of flocculents can be represented by fractal dimension (D). f (This is represented by the symbol ) . Flocs with a multi-branched structure typically have a lower D. f Value, while higher D f Few value branches. D f Small-angle laser light scattering can be used for measurement. Dynamic particle size distribution is tested using a Mastersizer 2000 laser particle size analyzer. Fractal dimension is calculated and the average value is taken from data obtained at 18 min, 19 min, and 20 min of the coagulation experiment. The calculated D value for Example 4 is shown below. f The value is 1.79, and the D value in Comparative Example 3 is... f The value of 1.94 indicates that the floc branching structure obtained in this embodiment has increased.

[0115] After slow stirring, take 1 mL of water sample containing coagulated flocs from Example 4 and Comparative Example 3, respectively. Add the water samples onto glass slides using a dropper and observe the floc morphology using an optical microscope. Figure 5 The images show microscopic observations of the floc morphology after coagulation in Example 4 and Comparative Example 3. The images further demonstrate that the floc particle size obtained in this example is significantly increased and exhibits a multi-branched structure.

[0116] The flocs obtained after coagulation in Example 4 and Comparative Example 3 were centrifuged at 5000 rpm, the supernatant was discarded, and the flocs were washed with ultrapure water. This process was repeated five times to obtain separated flocs. The flocs were pre-cooled to -20°C and then freeze-dried (cold trap temperature -40°C, vacuum 40 Pa, drying time 24 h) to obtain dried floc samples. Fourier transform infrared spectroscopy was performed on the dried powder of the trihydroxysilylpropyloctadecyldimethylammonium coagulant prepared in Example 1, and the dried floc samples from Example 4 and Comparative Example 3. Figure 6 Fourier transform infrared spectra of the dried powder of trihydroxysilylpropyloctadecyl dimethylammonium coagulant prepared in Example 1, and the dried floc samples in Example 4 and Comparative Example 3; the infrared spectral analysis results show that the dried powder of the trihydroxysilylpropyloctadecyl dimethylammonium coagulant has a lower concentration at 3360 cm⁻¹. -1 A distinct absorption peak appears at 3465 cm⁻¹, which is attributed to the stretching vibration characteristic peak of the Si-OH group. In contrast, the dry polyaluminum chloride floc sample and its composite floc sample with added coagulant show an absorption peak at 3465 cm⁻¹. -1 -OH absorption peaks appeared at all locations, mainly originating from the stretching vibrations of the Al-OH groups. The coagulant sample and the polyaluminum chloride-coagulant composite floc sample showed absorption peaks at 2920 cm⁻¹. -1 2848cm -1 And 1468cm -1 The presence of distinct -CH3 and -CH2 absorption peaks at 915 cm⁻¹ indicates the presence of a hydrophobic alkyl carbon chain structure in the system. -1 The absorption peak at 720 cm⁻¹ is a characteristic absorption peak of Si-OH, while the absorption peak at 720 cm⁻¹ is a characteristic absorption peak of Si-OH. -1 This also corresponds to the vibrational characteristics of Si-OH. It is noteworthy that in the polyaluminum chloride-coagulant composite floc sample and the pure polyaluminum chloride floc sample, the original Si-OH related peak changed to 732 cm⁻¹. -1 The Si-O-Al stretching vibration peak at the location indicated that hydrogen bonding occurred between the Si-OH groups in the coagulant and the Al-OH groups in the polyaluminum chloride flocs, further forming a local co-assembled structure. This interaction promoted the composite between the aluminum-based flocs and the organosilicon coagulant, thereby altering the chemical environment and structural characteristics of the system.

[0117] Figure 7 The figures show the water contact angle test results of the dried floc samples in Example 4 and Comparative Example 3. The results show that the introduction of the coagulant significantly enhances the hydrophobicity of the flocs, proving the formation of a hydrophobic interface on the floc surface.< / m>

Claims

1. A method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid (PFOA) from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, characterized in that... It is done in the following steps:

1. Raw water flows into the pre-coagulation tank of the ultra-low pressure ceramic membrane filtration system. In the pre-coagulation tank, aluminum-based coagulant is added to the raw water through a dosing pump and stirred rapidly. After the rapid stirring is completed, C18 hydrophobic cationic coagulant is quickly added and stirred slowly to obtain a coagulated water sample. The C18 hydrophobic cationic coagulant is a trihydroxysilylpropyloctadecyldimethylammonium coagulant solution; 2. The coagulated water sample flows into the constant water tank to reach the set constant water level, and then enters the ultra-low pressure ceramic membrane filtration unit by gravity. Under the condition that the transmembrane pressure difference is not higher than 0.02MPa, gravity or micro-pressure filtration is carried out, and the purified water after membrane filtration enters the clear water tank.

2. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 1, is characterized in that... The aluminum-based coagulant mentioned in step one is aluminum chloride, aluminum sulfate, or polyaluminum chloride.

3. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid (PFOA) from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 1, is characterized in that... The amount of Al added in the aluminum-based coagulant mentioned in step one is 0.1 mM ~ 0.2 mM.

4. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 1, is characterized in that... In step one, under the condition of stirring speed of 300 rpm to 400 rpm, aluminum-based coagulant is added to the raw water in the pre-coagulated water tank by a dosing pump and stirred rapidly for 1 min to 3 min. After the rapid stirring is completed, under the condition of stirring speed of 40 rpm to 150 rpm, C18 hydrophobic cationic coagulant is quickly added and stirred slowly for 10 min to 20 min.

5. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid (PFOA) from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 1, is characterized in that... The trihydroxysilylpropyloctadecyldimethylammonium coagulant solution mentioned in step one is prepared according to the following steps: A solution of dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride, ethanol, and water were mixed and stirred for 6 to 24 hours at room temperature and a stirring speed of 100 to 200 rpm. The mixture was then dialyzed in an acidic aqueous solution to obtain a liquid trihydroxysilylpropyloctadecyldimethylammonium coagulant. The liquid trihydroxysilylpropyloctadecyldimethylammonium coagulant was then freeze-dried to obtain a powder. Finally, the powder was dissolved in water to obtain a trihydroxysilylpropyloctadecyldimethylammonium coagulant solution with a mass percentage of 2% to 4%. The structural formula of the trihydroxysilylpropyloctadecyldimethylammonium coagulant liquid is as follows: .

6. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 5, is characterized in that... The concentration of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution is 40 wt.%; the volume ratio of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution to ethanol is 1:(20~30); the volume ratio of the dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution to water is 1:(5~10); the acidic aqueous solution is specifically adjusted to pH 2.0~4.0 using 3M~12M hydrogen chloride; the dialysis is specifically performed using a 100Da~500Da dialysis membrane in a hydrogen chloride solution with pH 2.0~4.0 for 6h~24h; the freeze-drying is specifically performed by pre-freezing the trihydroxysilylpropyloctadecyldimethylammonium coagulant liquid at a temperature of -80℃~-20℃, and then freeze-drying for 12h~24h at a cold trap temperature of -50℃~-40℃ and a vacuum of 5Pa~40Pa.

7. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 1, is characterized in that... After the rapid stirring in step one is completed, C18 hydrophobic cationic coagulant is added quickly within 1 minute; the molar ratio of Al to trihydroxysilylpropyloctadecyl dimethylammonium coagulant in the aluminum-based coagulant mentioned in step one is (5~25):

1.

8. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 1, is characterized in that... The ultra-low pressure ceramic membrane filtration system described in step one includes a pre-coagulation water tank, a constant-level water tank, an ultra-low pressure ceramic membrane filtration unit, a physical and chemical backwashing unit, and a clear water tank. The pre-mixed coagulation tank, constant water tank, ultra-low pressure ceramic membrane filter unit, and clear water tank are connected in sequence through water purification pipelines; the physical and chemical backwashing units are connected to the ultra-low pressure ceramic membrane filter unit and clear water tank respectively through cleaning pipelines. The ultra-low pressure ceramic membrane filtration unit operates by gravity drive and is equipped with a flat ceramic membrane module. The flat ceramic membrane module is made of alumina, titanium dioxide, zirconium dioxide or silicon carbide, and the pore size of the flat ceramic membrane module is 0.1μm~10μm.

9. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 8, is characterized in that... The transmembrane pressure difference of the flat ceramic membrane module is 0.004MPa~0.02MPa.

10. The method for enhancing coagulation, improving membrane fouling in ultra-low pressure ceramic membrane filtration systems, and simultaneously removing perfluorooctanoic acid from water using an aluminum-based coagulant coupled with a C18 hydrophobic cationic coagulant aid, as described in claim 8, is characterized in that... The physical and chemical backwashing unit specifically performs reverse pressurized backwashing under a backwashing pump pressure of 0.1MPa~0.2MPa.

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