A membrane method and integrated device for seawater desalination

Abstract

The application relates to the technical field of water treatment, in particular to a membrane method for seawater desalination and an integrated device. The method comprises the following steps: disinfecting seawater, performing filtration treatment on the seawater through a pretreatment membrane device, and then performing desalination treatment on the seawater through a reverse osmosis membrane device through a safety filter and a high-pressure pump to obtain desalination water. The pretreatment membrane device adopts an immersed filtration membrane, which is a composite filtration membrane with an organic-metallic polyhedral structure material and a polyethylene imine complex modified layer on the surface and the body. The reverse osmosis membrane device adopts an anti-pollution reverse osmosis membrane, which comprises a porous base membrane and a composite separation layer on the surface of the porous base membrane; the porous base membrane is a polysulfone or polyethersulfone polymer layer cooperatively modified by micro-bubble coupling and covalent organic framework material, and the composite separation layer is a polyamide layer containing porous cage-shaped covalent polymerization material. The application reduces the organic pollution load of the reverse osmosis membrane from the source, prolongs the membrane cleaning cycle, has a short technological process, small land occupation, and is suitable for severe working conditions with limited space.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a membrane-based seawater desalination method and integrated device. Background Technology

[0002] Seawater desalination has become an important source of freshwater, especially for coastal and island regions. Compared to thermal desalination, which consumes more energy, membrane desalination requires less electricity because the process involves no phase change. Furthermore, membrane systems have a short construction cycle, are flexible in operation, and are easy to scale up for application.

[0003] In membrane-based seawater desalination, seawater is typically pretreated to remove impurities such as silt, colloids, suspended solids, and microorganisms before being desalinated using a reverse osmosis membrane. Common pretreatment methods include coagulation, sedimentation, and membrane filtration, but these methods suffer from drawbacks such as high reagent dosage (some of which are unsuitable for drinking water treatment), unstable impurity removal, low permeate production from filtration membranes, and long process flows. Furthermore, reverse osmosis membranes still experience issues such as unstable permeate quality, insufficient pressure resistance, and surface fouling. Additionally, conventional membrane-based seawater desalination processes involve numerous and dispersed treatment units, resulting in a long overall process flow, which is often limited by site constraints in their application.

[0004] Therefore, there is a need for a membrane-based seawater desalination method and integrated device that has a short process flow, good seawater impurity removal effect, stable operation and high integration. Summary of the Invention

[0005] (a) Technical problems to be solved In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a membrane-based seawater desalination method and integrated device, which can effectively improve the pretreatment effect and the quality of desalinated water while reducing the system footprint and shortening the seawater desalination process. It is especially suitable for harsh working conditions and environments.

[0006] (II) Technical Solution To achieve the above objectives, the main technical solutions adopted by the present invention include: In a first aspect, the present invention provides a membrane-based seawater desalination method, comprising the following steps: S1. Disinfect the seawater; S2. The disinfected seawater is passed into a pretreatment membrane device for filtration to obtain pretreatment membrane permeate. The pretreatment membrane device is an immersion filter membrane, which is a composite filter membrane with an organic-metal polyhedral structure material and a polyethyleneimine complex modified layer on its surface and body. S3. The pretreated membrane permeate is sequentially passed through a security filter and a high-pressure pump into a reverse osmosis membrane device for desalination treatment to obtain reverse osmosis permeate, which is seawater desalination water. The reverse osmosis membrane device employs an anti-fouling reverse osmosis membrane, which includes a porous bottom membrane and a composite separation layer on the surface of the porous bottom membrane. The porous bottom membrane is a polysulfone or polyethersulfone polymer layer synergistically modified with microbubble coupled covalent organic framework materials, and the composite separation layer is a polyamide layer containing porous cage-like covalent polymer materials.

[0007] Optionally, in step S2, the organometallic polyhedral material is a zirconium-based MOP material (Metal-Organic Polyhedra), prepared by reacting terephthalic acid and / or aminoterephthalic acid with an organozirconium metallocene salt at 50-70°C. The molar ratio of terephthalic acid and / or aminoterephthalic acid to zirconium ions is 1:(1-3), preferably 1:2. The organozirconium metallocene salt is one or a combination of several of the following: dicyclopentadienyl zirconium chloride, bis(methylcyclopentadienyl) zirconium chloride, bis(pentamethylcyclopentadienyl) zirconium chloride, bis(indenyl) zirconium chloride, and dicyclopentadienyl dimethyl zirconium. In the immersion filter membrane, the mass ratio of the organometallic polyhedral structure material to polyethyleneimine is 1.5-2:1.

[0008] The submersible filtration membrane relies on positively charged polyethyleneimine complexed on the membrane body and surface to enhance the targeted electrostatic adsorption and filtration interception of organic pollutants in seawater by the pretreatment membrane device, thereby improving the quality of the pretreated water. Since the zirconium-based MOP material is easily torn by stress during blending or coating, the zirconium-based MOP material is co-dissolved with polyethyleneimine in ethanol to prepare a modified solution, in which PEI acts as a flexible buffer.

[0009] Optionally, the method for preparing the submerged filter membrane includes: First, an organometallic polyhedral structure material and polyethyleneimine were co-dissolved in anhydrous ethanol and ultrasonically dispersed to prepare a modified pre-dispersed suspension, thereby coating the rigid zirconium-based MOP material with flexible long chains of polyethyleneimine. Then, a membrane substrate polymer and a hydrophilic pore-forming agent were added to an organic solvent and stirred at 60-80°C for 8-12 hours to prepare a homogeneous first casting solution. The polyethyleneimine was a branched molecule with a Mw of 10 kDa-30 kDa. The modified pre-dispersed suspension was added dropwise to the first casting solution, stirred for 2-4 hours, and then degassed under vacuum at room temperature for 12-24 hours to obtain the modified casting solution. The modified casting solution is used to prepare a nascent membrane by scraping with a doctor blade or spinning machine. The membrane is then immersed in a coagulation bath at 15-25°C for non-solvent-induced phase transformation and solidification. After cleaning, soaking and storage, the submerged filter membrane is obtained. The coagulation bath is pure water or an aqueous solution containing 2%-5% DMAc. The total mass content of the organic-metal polyhedral structure material and polyethyleneimine in the submerged filter membrane is 0.5-5 wt%.

[0010] Optionally, the modified casting solution comprises, by mass percentage: 12%-18% membrane substrate polymer, 70%-80% organic solvent, 2%-6% hydrophilic porogen, and 0.5%-5% inorganic-organic composite modifier; the inorganic-organic composite modifier is a composite of an organic-metal polyhedral structural material and polyethyleneimine (PEI); the membrane substrate polymer is polyvinylidene fluoride or polyethersulfone; the organic solvent is N,N-dimethylacetamide or N-methylpyrrolidone; and the hydrophilic porogen is polyvinylpyrrolidone or polyethylene glycol.

[0011] Optionally, in step S3, the raw materials for preparing the porous substrate include polysulfone / polyethersulfone, sodium bicarbonate, and Schiff base covalent organic framework material. The Schiff base covalent organic framework material is a COF functional filler prepared by polymerization of phthalaldehyde and aromatic amine monomers. The mass ratio of sodium bicarbonate to covalent organic framework material is 10:1-30:1, preferably 20:1, and the total amount of both added accounts for 0.5%-1.5% of the mass of the casting solution. The porous substrate is prepared by phase inversion through a slightly acidic coagulation bath system with a pH of 4.5-5.5 and a temperature of 15-25℃.

[0012] In a slightly acidic coagulation bath system, the growth and expansion of carbon dioxide microbubbles generated by the reaction of sodium bicarbonate in the casting solution are restricted by the occupancy of COF space, resulting in an orderly and uniform distribution of microbubbles. This increases the support strength of the substrate and resists high-pressure compaction. At the same time, the covalent organic framework material generates more interconnected effective pore structures under the physical confinement of microbubbles, thereby increasing the effective porosity of the porous substrate and reducing the permeation resistance of the porous substrate.

[0013] Optionally, the Schiff base type covalent organic framework material is prepared by a Schiff base condensation reaction of an aldehyde monomer and an amine monomer at room temperature for 3-5 hours; the aldehyde monomer is terephthalaldehyde and / or o-phthalaldehyde, and the amine monomer is trimesoamine and / or p-phenylenediamine; the molar ratio of the aldehyde monomer to the amine monomer is 1.2-1.5:1; the reaction solvent is any one of 1,4-dioxane, trimesoylbenzene, ethanol-water system, or acetic acid-water system.

[0014] Optionally, the porous cage-like covalent polymer is a ketone-enamine covalent polymer, prepared by a Schiff base condensation reaction of an oil phase solution of 2,4,6-tricarboxymethyl phloroglucinol (Tp) and an aqueous phase solution containing hexamethylenediamine and polylysine at room temperature for 6-10 h; the mass ratio of 2,4,6-tricarboxymethyl phloroglucinol, hexamethylenediamine, and polylysine is 1:(4-8):(0.01-0.1), preferably 1:6:0.05; the oil phase solvent is dichloromethane.

[0015] Optionally, the composite separation layer of the antifouling reverse osmosis membrane is prepared by interfacial polymerization, wherein the interfacial polymer material includes an aqueous phase material and an oil phase material; the aqueous phase material is an aqueous solution containing 1.0-3.5 wt% polyamine and 0.05-0.1 wt% porous cage-like covalent polymer material, wherein the polyamine is at least one selected from m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, and piperazine, preferably m-phenylenediamine, and the concentration is preferably 2.0 wt%. The oil phase material is a hydrophobic organic solvent solution containing 0.05-0.25 wt% polyacrylamide chloride, wherein the hydrophobic organic solvent is selected from at least one of n-hexane, cyclohexane, heptane, and isooctane, preferably n-hexane.

[0016] Optionally, the specific preparation steps of the antifouling reverse osmosis membrane are as follows: Step 1: Add 15%-18% polyethersulfone resin and 1%-3% polyvinylpyrrolidone to N,N-dimethylformamide solvent, stir at 60℃ for 6-8h to prepare a homogeneous initial liquid, add pre-dispersed sodium bicarbonate and Schiff base type covalent organic framework material mixed powder, stir and degas for 6-16h to obtain modified casting liquid; Step 2: The modified casting solution is coated onto the nonwoven fabric support layer and immersed in a slightly acidic coagulation bath for phase inversion to prepare a modified porous bottom membrane. Step 3: Immerse the porous substrate in an aqueous material containing aromatic diamine and porous cage-like covalent polymer, and blow off the excess liquid on the surface after immersion. Step 4: Cover the oil phase material with pyromellitic methyl chloride for interfacial polymerization. After polymerization, perform deep crosslinking by heat treatment at 70-120℃ for 3-8 minutes to obtain an antifouling reverse osmosis membrane.

[0017] In a second aspect, the present invention provides an integrated membrane-based seawater desalination device, comprising: The highly fouling-resistant and long-lasting seawater desalination process applied to any of the above embodiments includes a membrane tank, a pretreatment membrane device, a security filter, a high-pressure pump, and a fouling-resistant reverse osmosis membrane device connected in sequence by pipelines. The membrane tank is used to store raw seawater and is equipped with a dosing component for adding disinfectant to complete in-situ disinfection of seawater. The pretreatment membrane device is internally equipped with an immersion filter membrane, which is a composite filter membrane with an organic-metal polyhedral structure material and a polyethyleneimine complex modified layer on its surface and body. The antifouling reverse osmosis membrane device is internally equipped with an antifouling reverse osmosis membrane, which includes a polysulfone or polyethersulfone porous bottom membrane synergistically modified with microbubble coupled Schiff base type covalent organic framework material, and a polyamide composite separation layer containing porous cage covalent polymer material. The antifouling reverse osmosis membrane device is equipped with an energy recovery device. The seawater outlet of the membrane tank is connected to the inlet of the pretreatment membrane device. The product water outlet of the pretreatment membrane device is connected to the inlet of the antifouling reverse osmosis membrane device via a security filter and a high-pressure pump in sequence. The antifouling reverse osmosis membrane device is equipped with a freshwater production port and a concentrate discharge port. The freshwater production port is used to output desalinated seawater. The concentrate discharge port is connected to the high-pressure inlet of the energy recovery device. The low-pressure outlet of the energy recovery device is connected to the reverse osmosis concentrate subsequent treatment device. The high-pressure outlet of the energy recovery device is recirculated to the inlet side of the antifouling reverse osmosis membrane device to realize the recovery and recycling of energy from the residual pressure of the concentrate.

[0018] (III) Beneficial Effects To address the shortcomings of existing seawater desalination pretreatment methods, such as large dosage of chemicals, unstable impurity removal effect, low membrane permeate yield, and lengthy process flow, as well as the insufficient pressure resistance and easy fouling of reverse osmosis membranes, and the dispersed equipment and large footprint of the entire system, this invention provides a membrane-based seawater desalination process and integrated device. This method shortens the process flow, reduces the system footprint, improves pretreatment purification capacity and permeate quality, and is suitable for harsh operating conditions. Specific technical effects are as follows: 1. Pretreatment - Immersion Filtration Membrane (Zirconium-based MOP Material / PEI Complex Modification): High-density amino groups are introduced into the modified membrane body and surface, giving the membrane a strong positive charge. This allows for electrostatic targeting and adsorption of negatively charged organic pollutants such as humic acid, alginate, and extracellular polymeric substances (EPS) in seawater. This achieves efficient retention of large-molecule organic pollutants during the pretreatment stage, providing high-quality and stable feed water for reverse osmosis and ensuring long-term stable operation of the reverse osmosis membrane module. The porous structure of the MOP material itself increases the permeate flux of the filtration membrane. The submerged filtration mode simplifies the pretreatment unit, reduces reagent dosage, and avoids the drawbacks of traditional coagulation and sedimentation processes, such as high reagent consumption and the unsuitability of some reagents for drinking water treatment.

[0019] Among them, the organic-metal polyhedral structure material (zirconium-based MOP material) is pre-complexed and modified with polyethyleneimine. The flexible polyethyleneimine chain encapsulates the rigid MOP material particles, which improves the compatibility of MOP material with membrane substrate, alleviates the problem of stress breakage of MOP material particles during casting, coating / spinning, and enhances the overall mechanical toughness of the modified membrane.

[0020] 2. Reverse osmosis porous bottom membrane: The reverse osmosis porous bottom membrane utilizes a microbubble-coupled aromatic Schiff base-type COF synergistic modification to modify its pore structure. During the preparation of the porous bottom membrane, sodium bicarbonate in the casting solution generates CO2 microbubbles in situ under slightly acidic coagulation bath conditions. The COF material acts as a spatial confinement agent, restricting excessive bubble growth and resulting in a uniformly sized, orderly distributed submicron-sized bubble pore structure, preventing membrane damage caused by large pore defects. Using confined microbubbles as gaseous pore-forming templates, three-dimensional interconnected channels are constructed, significantly improving the effective porosity of the bottom membrane, reducing its permeation resistance, and thus enhancing overall membrane flux. The COF and the controlled-formation microbubble pores together construct a rigid support framework, effectively inhibiting membrane compaction under high-pressure conditions, enhancing the membrane's pressure resistance, and adapting it to the high-pressure operating conditions of seawater reverse osmosis.

[0021] 3. Reverse osmosis composite separation layer: The reverse osmosis composite separation layer is a polyamide layer loaded with a porous cage-like covalent polymer. This porous cage-like covalent polymer is integrated into the polyamide separation layer network. The internal cavities of the cage reduce the mass transfer resistance of water molecules permeating the separation layer, endowing the reverse osmosis membrane with high flux characteristics. The cage-like microporous structure enhances the ion hydration effect, increases the resistance to hydrated ion permeation, and improves the salt ion retention effect. The abundant free amino groups on the material surface enhance the hydrophilicity of the separation layer, alleviate the adhesion of pollutants to the membrane surface, and improve antifouling ability. The amino groups on the surface of the porous cage-like material can covalently cross-link with the polyamide generated by interfacial polymerization, firmly anchoring the cage-like packing material within the polyamide network, preventing packing material loss, and ensuring long-term stable membrane performance.

[0022] 4. Synergistic anti-pollution effect throughout the entire process: The pretreatment unit and the reverse osmosis desalination unit are not simply a superposition of filtration precision, but form a relay-style chemical anti-fouling synergy mechanism between upstream and downstream: the pretreatment submerged filter membrane preferentially adsorbs and retains most of the negatively charged organic macromolecular pollutants in seawater, reducing the pollutants entering the reverse osmosis unit from the source, protecting the downstream reverse osmosis membrane, inhibiting the deposition of organic pollutants on the surface of the cage-like polyamide separation layer, effectively extending the chemical cleaning cycle of the reverse osmosis membrane, and improving the overall service life of the membrane module and the seawater desalination unit.

[0023] 5. Technical effects at the integrated device level: The pipelines of each functional unit are integrated and arranged, and an integrated membrane tank is configured for chemical dosing and disinfection, and an energy recovery device is used to recover the residual pressure of the concentrate. On the one hand, this simplifies the entire process flow, reduces the footprint caused by the dispersed arrangement of equipment, and adapts to the harsh working conditions with limited space. On the other hand, the return of the residual pressure of the concentrate reduces the overall energy consumption of the seawater desalination system. Attached Figure Description

[0024] Figure 1 This is a flowchart of a membrane-based seawater desalination method.

[0025] Figure 2 This is a schematic diagram of the preparation process of an immersion filter membrane.

[0026] Figure 3 This is a schematic diagram of the preparation process for an antifouling reverse osmosis membrane.

[0027] Figure 4 This is a block diagram of an integrated membrane seawater desalination unit. Detailed Implementation

[0028] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1 The diagram illustrates a membrane-based seawater desalination method according to the present invention, comprising the following steps: S1. Disinfect the seawater.

[0030] To prevent membrane fouling caused by the attachment and proliferation of algae or bacterial microorganisms during long-term operation, the seawater disinfection treatment employs any one of the following methods, or any combination of two methods: (1) Oxidizing agents for disinfection: Add sodium hypochlorite, chloramine, chlorine dioxide, ozone, hydrogen peroxide or ferrate; (2) Non-oxidizing agents for disinfection: Add 2,2-dibromo-3-nitropropamide (DBNPA) or isothiazolinone bactericide; (3) Physical and advanced oxidation disinfection: Ultraviolet disinfection, ultrasonic microcavitation disinfection, ozone-ultraviolet combined advanced oxidation disinfection, or electrocatalytic in-situ oxidation disinfection.

[0031] S2. The disinfected seawater is passed into a pretreatment membrane device for filtration to obtain pretreatment membrane permeate. The pretreatment membrane device is an immersion filter membrane, which is a composite filter membrane with an organic-metal polyhedral structure material and a polyethyleneimine complex modified layer on its surface and body.

[0032] For the preparation method of the composite filter membrane, please refer to [link / reference]. Figure 2 The steps are as follows: (1) First, an organometallic polyhedral structure material is prepared. The organometallic polyhedral structure material in this invention is a zirconium-based MOP material, which is prepared by reacting terephthalic acid and / or aminoterephthalic acid with organozirconium metallocene salt (or metallocene complex) at 50-70°C. The molar ratio of terephthalic acid and / or aminoterephthalic acid to zirconium ions is 1:(1-3), preferably 1:2.

[0033] MOP (metal-organic polyhedral cage) is a zero-dimensional nanomolecule with a rigid spatial structure. In the complexed modification layer, MOP acts as a nanoscale anchoring scaffold. PEI polymer chains interpenetrate, entwine, or coordinate around the cage-like structure of MOP, forming a rigid-flexible three-dimensional network; PEI is a branched molecule with a molecular weight (Mw) of 10 kDa-30 kDa. MOP expands the flexible chains of PEI, preventing their collapse, thus exposing 100% of the extremely high density of active adsorption sites (amine groups) on PEI to the aqueous phase, greatly increasing the adsorption capacity for pollutants. At the same time, the discrete cage-like structure of MOP ensures the porous permeability of the modification layer itself, achieving efficient adsorption without water blockage, which is something that the infinitely extended polymeric framework materials of MOF materials cannot replace.

[0034] This invention utilizes the strong polycationic properties of PEI and the strong electrostatic attraction between it and macromolecular pollutants, combined with the nanoscale support and anti-collapse effect provided by the rigid cage structure of MOP and the Lewis acid-base coordination effect induced by the zirconium core, to construct a three-dimensional network modified layer with both high adsorption capacity and high water permeability. This effectively removes negatively charged organic pollutants such as humic acid, alginate, and extracellular polymeric substances (EPS) from seawater, and realizes an anti-fouling mechanism that shifts from "in-pore clogging to reversible surface interception," greatly reducing the clogging probability of submerged filter membranes.

[0035] (2) Organometallic polyhedral structural material and polyethyleneimine were co-dissolved in anhydrous ethanol and ultrasonically dispersed to prepare a modified pre-dispersed suspension, so that the flexible long chain of polyethyleneimine coated the rigid zirconium-based MOP material. The mass ratio of organometallic polyhedral structural material to polyethyleneimine was 1.5-2:1.

[0036] (3) Add the membrane substrate polymer and hydrophilic pore-forming agent to an organic solvent, and stir at a constant temperature of 60-80℃ for 8-12 hours to prepare a homogeneous first casting solution. Add the modified pre-dispersed suspension obtained in step (2) dropwise to the first casting solution, stir for 2-4 hours, and then degas under vacuum at room temperature for 12-24 hours to obtain the modified casting solution. Preferably, the modified casting solution comprises, by mass percentage: The membrane substrate consists of 12%-18% polymer, 70%-80% organic solvent, 2%-6% hydrophilic porogen, and 0.5%-5% inorganic-organic composite modifier. The membrane substrate polymer is polyvinylidene fluoride or polyethersulfone; the organic solvent is N,N-dimethylacetamide or N-methylpyrrolidone; and the hydrophilic porogen is polyvinylpyrrolidone or polyethylene glycol. The inorganic-organic composite modifier is the composite of the organic-metal polyhedral structure material obtained in step (2) and polyethyleneimine (mass ratio 1.5-2:1).

[0037] (4) The modified casting solution is used to prepare a nascent membrane by scraping with a doctor blade or spinning a spinning machine. The membrane is then immersed in a coagulation bath at 15-25°C for solvent-induced phase transformation and solidification. After cleaning, soaking, and storage, an immersion filter membrane is obtained. The total mass content of the organic-metal polyhedral structure material and polyethyleneimine in the immersion filter membrane is 0.5-5 wt%. The coagulation bath is pure water or an aqueous solution containing 2%-5% DMAc.

[0038] Submersible filtration membranes rely on positively charged polyethyleneimine complexed with the membrane body and surface to enhance the targeted electrostatic adsorption and filtration interception of organic pollutants in seawater, thereby improving the quality of pretreated water. Since zirconium-based MOP materials are easily torn by stress during blending or membrane coating, a modified solution is prepared by co-dissolving the aforementioned organic-metal polyhedral structure material with polyethyleneimine in ethanol, where PEI acts as a flexible buffer.

[0039] Specifically, the submersible filter membrane can be prepared according to the following steps: ① Raw material composition of polymer casting solution (by mass percentage): Membrane substrate polymer (12%-18%): preferably polyvinylidene fluoride (PVDF) or polyethersulfone (PES), providing the high tensile strength and chemical resistance required for submersible filtration; Organic solvent (70%-80%): selected from N,N-dimethylacetamide (DMAc) or N-methylpyrrolidone (NMP), used to completely dissolve the film substrate polymer; Hydrophilic porogen (2%-6%): Selected from polyvinylpyrrolidone (PVP, such as K30 or K90) or polyethylene glycol (PEG-400), used to induce the formation of highly interconnected macroporous structures during phase transformation; Inorganic-organic composite modifier (0.5%-5%): that is, the mixture of the zirconium-based MOP material and polyethyleneimine (PEI).

[0040] ② Polar co-solution and preparation of casting solution: Solution preparation process: First, accurately weigh zirconium-based MOP material and PEI according to the mass ratio (1.5-2):1, and ultrasonically disperse them together in a small amount of anhydrous ethanol to form a uniform modified pre-dispersed suspension; during this process, the flexible long chains of PEI will interpenetrate and coat the rigid particle surface of zirconium-based MOP material.

[0041] Film casting process: The above polymer substrate and pore-forming agent are added to an organic solvent and mechanically stirred at a constant temperature of 60℃-80℃ for 8-12 hours until completely dissolved to form a homogeneous first casting solution. Mixing and degassing: The modified pre-dispersed suspension is slowly added dropwise to the first casting solution, and stirring is continued for 2-4 hours to ensure that the PEI / zirconium-based MOP material modifier is uniformly dispersed in the polymer network; then, it is allowed to stand at room temperature or subjected to vacuum degassing for 12-24 hours to completely remove the tiny air bubbles in the film-forming system, and the final modified casting solution is obtained.

[0042] ③The specific process of phase inversion (NIPS) film formation: Coating / spinning: The fully degassed modified casting solution is coated onto the nonwoven support layer using a stainless steel coating blade (the blade gap is set to 150μm-250μm); or hollow fiber nascent fibers are formed by extrusion through a dual-hole spinneret on a spinning machine (extrusion pressure of spinning solution is 0.1-0.3MPa, and dry spinning distance of air section is set to 3-10cm).

[0043] Phase transformation solidification: The prepared liquid membrane or nascent hollow fiber is rapidly immersed in a coagulation bath for non-solvent-induced phase separation (NIPS). The coagulation bath is pure water at 15℃-25℃ or an aqueous solution with 2%-5% DMAc added.

[0044] Microscopic evolution mechanism: In the coagulation bath, the organic solvent (NMP / DMAc) in the casting solution undergoes strong bidirectional diffusion exchange with the water in the coagulation bath; due to the presence of trace amounts of ethanol in the system, local thermodynamic instability is accelerated, which promotes the rapid formation of an asymmetric sponge-like pore structure in the membrane body; at the same time, under the drag force of solvent replacement, the PEI / zirconium-based MOP material composite with hydrophilic amino groups will spontaneously accumulate on the coagulation surface of the membrane.

[0045] ④ Cleaning and storage: After curing and molding, thoroughly clean the immersion filter membrane with deionized water to replace the residual solvent on the surface, and finally store it in an aqueous solution containing 0.5%-1% sodium bisulfite for later use.

[0046] S3. The pretreated membrane permeate is passed through a security filter and a high-pressure pump in sequence into the reverse osmosis membrane unit for desalination treatment to obtain reverse osmosis permeate, which is seawater desalination water.

[0047] The reverse osmosis membrane device uses an anti-fouling reverse osmosis membrane, which includes a porous bottom membrane and a composite separation layer on the surface of the porous bottom membrane. The porous bottom membrane is a polysulfone or polyethersulfone polymer layer synergistically modified with microbubble coupled covalent organic framework material, and the composite separation layer is a polyamide layer containing porous cage-like covalent polymer material.

[0048] The security filter is installed between the high-pressure pump and the pretreatment membrane unit, serving as the last particle protection unit before the reverse osmosis system. It intercepts tiny particles and membrane debris leaking from the pretreated product water, protecting the downstream high-pressure pump and the fouling-resistant reverse osmosis membrane elements, preventing particle damage and wear to the reverse osmosis membrane sheets. The security filter is a precision security filter specifically designed for seawater desalination. The shell is made of 316L stainless steel or fiberglass corrosion-resistant material, and the interior is equipped with a PP melt-blown filter element or a PP pleated filter element. The filtration accuracy is 1-5μm, preferably 5μm; the design pressure is 0.6-1.6MPa. Its function is to intercept fine solid particles and membrane material debris carried in the pretreated product water, preventing hard particles from entering the high-pressure pump and reverse osmosis membrane components, thus avoiding wear on the high-pressure pump and damage to the reverse osmosis membrane elements caused by particles. During operation, the inlet and outlet pressure difference can be monitored; the filter element should be replaced when the pressure difference reaches 0.05-0.1MPa. Preferably, the security filter is a precision security filter for seawater, with a shell made of 316L stainless steel or fiberglass, and an internal PP melt-blown filter element or PP pleated filter element, with a filtration accuracy of 1-5μm and a design pressure of 0.6-1.6MPa.

[0049] The antifouling reverse osmosis membrane consists of a porous substrate membrane and a composite separation layer laminated on the surface of the porous substrate membrane. The composite separation layer is a polyamide layer embedded with a porous cage-like covalent polymer material. For example... Figure 3 As shown, the preparation steps of the antifouling reverse osmosis membrane are as follows: Step 1: Preparation of Schiff base-type covalent organic framework materials First, a Schiff base-type covalent organic framework (COF) material is prepared. This COF material is a functional filler polymerized from phthalaldehyde and aromatic amine monomers via a Schiff base condensation reaction for 3-5 hours. The phthalaldehyde monomers are terephthalaldehyde and / or o-phthalaldehyde, and the aromatic amine monomers are trimesoamine and / or p-phenylenediamine. The molar ratio of aldehyde monomers to amine monomers is 1.2-1.5:1, preferably 1.5:1. The reaction solvent is any one of 1,4-dioxane, mesitylene, an ethanol-water system, or an acetic acid-water system.

[0050] Step 2: Preparation of modified casting solution for porous substrate membrane Next, 15%-18% polysulfone or polyethersulfone resin and 1%-3% PVP are added to N,N-dimethylformamide solvent, and stirred at 60°C for 6-8 hours to prepare a homogeneous initial solution. Then, a mixture of pre-dispersed sodium bicarbonate and Schiff base-type covalent organic framework material powder (NaHCO3 to COF functional filler mass ratio of 10:1-30:1) is added, and the mixture is stirred and degassed for 6-16 hours to obtain a modified casting solution. The preferred mass ratio of NaHCO3 to covalent organic framework material is 20:1, and the total addition amount of both accounts for 0.5%-1.5% of the casting solution mass.

[0051] Step 3: The modified casting solution is coated onto the nonwoven fabric support layer and immersed in a slightly acidic coagulation bath for phase inversion to prepare a modified porous bottom membrane, which is then stored for later use.

[0052] The porous substrate membrane is prepared through phase inversion in a slightly acidic coagulation bath system with a pH of 4.5-5.5 and a temperature of 15-25℃. In this system, the growth and expansion of CO2 microbubbles generated by the reaction of sodium bicarbonate in the casting solution are restricted by the space occupancy of the COF (carbon dioxide-air-fiber) structure, resulting in an orderly and uniform distribution of microbubbles. This increases the substrate membrane's support strength and resistance to high-pressure compaction. Simultaneously, the covalent organic framework material generates more interconnected effective pore structures under the physical confinement of the microbubbles, thereby increasing the effective porosity of the porous substrate membrane and reducing its permeation resistance.

[0053] The mass ratio of sodium bicarbonate to covalent organic framework material is 10:1-30:1, preferably 20:1. This ratio range ensures sufficient bubble generation (mainly sodium bicarbonate) while also providing enough nanoscale space within the COF to confine and encapsulate the bubbles. Too much sodium bicarbonate can lead to excessively large bubbles, potentially causing film rupture or macroscopic defects; too much COF will cause a sharp increase in the viscosity of the casting solution, making film scraping impossible.

[0054] Simultaneously with steps 1-3: Preparation of porous cage-like covalent polymeric materials: The porous cage-like covalent polymer is a ketone-enamine covalent polymer, prepared by Schiff base condensation reaction of an oil phase solution of 2,4,6-tricarboxymethyl phloroglucinol (Tp) and an aqueous phase solution containing hexamethylenediamine and polylysine at room temperature for 6-10 h; the mass ratio of 2,4,6-tricarboxymethyl phloroglucinol, hexamethylenediamine and polylysine is 1:(4-8):(0.01-0.1), preferably 1:6:0.05; the oil phase solvent is dichloromethane.

[0055] Polylysine, with its long-chain macromolecular backbone and numerous repeating primary amino groups, acts as a giant multidimensional crosslinking node (nucleation site) at the water / oil interface. Hexamethylenediamine provides bifunctional groups as a connecting backbone. When they react with the oil phase of Tp, the steric hindrance of polylysine forces the polymer chains to coil and aggregate, blocking the infinite extension of the two-dimensional plane, thus forcing the formation of a 3D porous cage-like microsphere microstructure with internal cavities. The mass ratio of Tp, hexamethylenediamine, and polylysine is 1:4-8:0.01-0.1. Polylysine is extremely expensive and has a large molecular weight; even a small amount can induce cage formation.

[0056] Step 4: Prepare an aqueous phase material containing polyamines and porous cage-like covalent polymers. Immerse the porous bottom membrane prepared in step 3 in the aqueous phase material, and blow off the excess liquid on the surface after wetting.

[0057] The aqueous phase material is an aqueous solution containing 1.0-3.5 wt% polyamine and 0.05-0.1 wt% porous cage-like covalent polymer material, wherein the polyamine is selected from at least one of m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, and piperazine, preferably 2.0 wt% m-phenylenediamine.

[0058] Step 5: Interfacial polymerization is performed on the pyromellitic trimethylol chloride oil phase material. After polymerization, the membrane undergoes deep crosslinking via heat treatment at 70-120℃ for 3-8 minutes to obtain an antifouling reverse osmosis membrane. The oil phase material is a hydrophobic organic solvent solution containing 0.05-0.25 wt% polyacrylamide chloride. The hydrophobic organic solvent is selected from at least one of n-hexane, cyclohexane, heptane, and isooctane, preferably n-hexane.

[0059] The antifouling reverse osmosis membrane utilizes the microporous structure of the porous cage-like covalent polymer material to enhance ion hydration and retention, and relies on surface amino groups to enhance the hydrophilicity of the membrane layer. The porous cage-like covalent polymer material not only reduces the resistance of the water molecule functional separation layer, but also promotes ion hydration through its cage-like microporous structure, making it difficult for ions to pass through and enhancing the retention effect of the reverse osmosis membrane. At the same time, the surface amino groups enhance the hydrophilicity of the membrane, thereby improving the membrane permeation flux and antifouling properties.

[0060] A significant difference between seawater and freshwater resources is that seawater contains a large number of negatively charged membrane fouling sources (such as extracellular polymeric substances (EPS) like humic acid and alginate). The S2 pretreatment membrane of this invention, through the complexation of polyethyleneimine (PEI), not only solves the mechanical brittleness of organometallic polyhedra in the base membrane, but also introduces high-density amino groups that give the membrane surface a strong positive charge. This allows for targeted electrostatic adsorption and retention of the vast majority of negatively charged macromolecular organic matter in the S2 stage, greatly protecting the downstream S3 reverse osmosis membrane unit and preventing fouling layer deposition on the cage-like micropore separation layer of S3. This significantly extends the chemical cleaning cycle and service life of the reverse osmosis membrane.

[0061] Specifically, the antifouling reverse osmosis membrane can be prepared according to the following steps: ① Preparation of porous bottom film casting solution and control of bubble confinement: By mass percentage, 15%-18% of polyethersulfone (PES) resin and 1%-3% of defoamer (PVP, K30 medium molecular weight) are added to N,N-dimethylformamide (DMF) solvent, and the mixture is mechanically stirred in a 60°C water bath for 6-8 hours to form a homogeneous initial liquid.

[0062] Sodium bicarbonate and covalent organic framework material are mixed at a mass ratio of 10:1-25:1 (the total amount added accounts for 0.5%-1.5% of the casting solution mass), and pre-dispersed in a small amount of DMF by ball milling and ultrasonication. Then, it is added dropwise into the homogeneous initial solution, and the mixture is stirred and degassed for 6-16 hours to obtain the modified casting solution.

[0063] ②Preparation of porous bottom membrane: The modified casting solution was uniformly coated onto the nonwoven fabric support layer using a doctor blade (blade gap set at 150 μm). It was then immediately and gently immersed in a slightly acidic coagulation bath (typically a 0.1 M acetate-sodium acetate buffer system or a finely adjusted water bath with extremely dilute hydrochloric acid) at a temperature of 15℃-25℃ and a pH of 4.5-5.5. During this slightly acidic phase transition, the sodium bicarbonate embedded within the liquid film reacted rapidly with the slightly acidic aqueous phase that had permeated through the phase transfer, releasing micro-CO2 bubbles in situ. Due to the spatial confinement and anchoring effect of the porous framework of the COF material, the expansion size of the CO2 bubbles was forcibly stripped and locked at the submicron level. These confined microbubbles acted as "gaseous porous templates" before PES coagulation, constructing highly ordered three-dimensional interconnected microchannels. This significantly reduced the resistance of pure water penetrating the substrate membrane, and its dense and ordered framework greatly improved its resistance to pressure densification under high pressure (e.g., 50-80 bar).

[0064] ③ Preparation of highly separable porous cage-like covalent polymeric materials: This porous cage-like covalent polymer material, prepared based on a water / oil interface condensation reaction, plays a crucial role in regulating the porosity and enhancing the water channels of the reverse osmosis layer. The specific preparation steps are as follows: Preparation of aqueous solution: Accurately weigh hexamethylenediamine (providing flexible connecting segments) and polylysine (macromolecule inducer) in a mass ratio of 6:0.05, dissolve them in deionized water, and prepare an amine aqueous solution with a total mass concentration of 2.0 wt%. Preparation of oil phase solution: Weigh a specific amount of 2,4,6-tricarboxymethyl phloroglucinol (Tp), dissolve it in dichloromethane (where the mass ratio of Tp to hexamethylenediamine is 1:6), and prepare an oil phase solution with a total mass concentration of 0.1 wt%. Cage polymerization and separation: At room temperature, the oil phase solution was slowly spread on top of the aqueous phase solution and allowed to stand. At the dichloromethane-water interface, Tp rapidly underwent a Schiff base condensation reaction with hexamethylenediamine / polylysine. After 6-10 hours of reaction, the light yellow polymer at the interface was collected, washed with anhydrous ethanol to remove impurities, and then vacuum dried to obtain porous cage-like nanomaterials.

[0065] In this mass ratio (1:6:0.05), hexamethylenediamine is in severe excess, while polylysine is present in trace amounts. When rigid Tp molecules polymerize with the flexible excess hexamethylenediamine, the large steric hindrance of the polylysine macromolecules (steric hindrance effect) prevents the infinite extension of the polymer's two-dimensional plane. Polylysine acts as a "nodule," forcing the growing polymer chains to undergo three-dimensional curling and three-dimensional aggregation, ultimately closing through self-crosslinking to form a porous cage-like covalent polymer material with a hollow interior and an amino-rich exterior, rather than a typical dense mass.

[0066] ④ Implant porous cage-like covalent polymeric material into the polyamide separation layer: After obtaining the above-mentioned porous cage-like covalent polymer material and modified porous bottom membrane, a reverse osmosis separation layer is prepared through the following steps: Interfacial aqueous phase coating: Prepare an aqueous phase material containing 2.0 wt% m-phenylenediamine (MPD) and 0.05 wt%-0.1 wt% of the above porous cage-like covalent polymer material. After being ultrasonically dispersed evenly, it is dip-coated onto the surface of a porous substrate membrane (PES membrane). After contact for 2 minutes, excess droplets on the surface are blown away with an air knife. Molding of the separation layer (polyamide layer): Prepare a hexane oil phase material containing 0.1 wt% trimesoyl chloride (TMC). Cover it onto a substrate film containing a prepolymerized cage-like material and leave it for 30 seconds. At this time, TMC will undergo rapid interfacial polymerization with MPD in the aqueous phase to form a thin film (TFC layer). At the same time, it will also covalently crosslink with the free primary amino groups (provided by polylysine residues) on the surface of the porous cage-like covalent polymer material embedded in the aqueous phase, firmly anchoring the cage-like material into the polyamide network.

[0067] Thermal curing: Pour out the excess oil phase, and then place the membrane in an oven at 80°C for 5 minutes to complete the deep cross-linking of the interface, thus obtaining the final high-permeability, pressure-resistant reverse osmosis composite membrane.

[0068] like Figure 4 The diagram shown is a schematic representation of the integrated membrane seawater desalination device of the present invention, which includes a membrane tank 1, a security filter 2, a high-pressure pump 3, and an anti-fouling reverse osmosis membrane device 4, which are connected in sequence by pipelines.

[0069] Membrane tank 1 is equipped with a pretreatment membrane device 5. Membrane tank 1 is used to store raw seawater and is equipped with a dosing assembly for adding disinfectant to complete in-situ disinfection of seawater. The pretreatment membrane device 5 includes a submerged filter membrane 51, which is a composite filter membrane with an organic-metal polyhedral structure material on its surface and body and a polyethyleneimine complex modified layer. The antifouling reverse osmosis membrane device 4 is equipped with an antifouling reverse osmosis membrane 41, which includes a polysulfone or polyethersulfone porous bottom membrane synergistically modified with microbubble coupled Schiff base type covalent organic framework material and a polyamide composite separation layer covered with a porous cage-like covalent polymer material.

[0070] Seawater in membrane tank 1 enters pretreatment membrane unit 5 through the inlet of the pretreatment membrane unit. Water from the product outlet of pretreatment membrane unit 5 sequentially passes through security filter 2 and high-pressure pump 3, connecting to the inlet 401 of antifouling reverse osmosis membrane unit 4. Desalinated water is output from the freshwater product outlet 402 of antifouling reverse osmosis membrane unit 4. The concentrate outlet 403 of antifouling reverse osmosis membrane unit 4 connects to the high-pressure inlet 601 of energy recovery unit 6. The low-pressure outlet 602 of energy recovery unit 6 connects to the reverse osmosis concentrate post-treatment unit 7. The high-pressure outlet 603 of energy recovery unit 6 is recirculated back to the inlet 401 of antifouling reverse osmosis membrane unit 4, achieving energy recovery and recycling of residual pressure in the concentrate. Energy recovery unit 6 is used to reduce energy consumption in seawater desalination.

[0071] High-pressure pump 3 consumes a significant amount of energy, accounting for 60% to 80% of the total energy consumption of the seawater desalination system. Since the actual recovery rate of seawater desalination is typically around 40%, this means that over half of the seawater in the system is converted into concentrate and discharged through concentrate outlet 403. This concentrate still contains extremely high residual static pressure energy (only slightly lower than the inlet pressure by 1-2 bar). This invention addresses this by connecting an energy recovery device 6, allowing the high-pressure concentrate with residual static pressure energy to enter the device through the high-pressure inlet 601. Inside the device, this high-pressure concentrate, through a work exchange mechanism, transfers its pressure kinetic energy via a mechanical work exchange principle (such as a hydraulic turbine or isobaric rotor) to a portion of the pre-treated low-pressure seawater simultaneously introduced into the energy recovery device 6. This low-pressure raw water absorbs the pressure kinetic energy and is converted into high-pressure raw water. Subsequently, this high-pressure raw water flows out through the high-pressure outlet 603 and merges with the main water output from high-pressure pump 3, entering the inlet 401 of the anti-fouling reverse osmosis membrane device 4. The system design eliminates the need for high-pressure pump 3 to operate on some of the raw water, thereby significantly reducing the power consumption per ton of water in the entire system.

[0072] To further clarify the present invention and its technological advancements, the following description is provided in conjunction with specific embodiments and technical effects.

[0073] All embodiments and comparative examples of this invention use the same initial seawater quality: conductivity 40.8 mS / cm, turbidity 53.5 NTU, COD 13.6 mg / L; and the same process flow: in-situ disinfection of membrane tank → submerged pretreatment membrane filtration → security filter (5 μm) → high-pressure pump → anti-fouling reverse osmosis membrane device → energy recovery concentrate recirculation.

[0074] The zirconium-based MOP material, Schiff base covalent organic framework material, porous cage-like covalent polymer material, modified submerged filter membrane, microbubble modified porous bottom membrane, and porous cage-like doped polyamide separation layer preparation methods used in this invention all employ the standard processes defined in this invention. Each embodiment only replaces some variable parameters; the remaining process parameters, unless otherwise specified, are completely consistent with the reference embodiments. In the following embodiments or comparative examples, PEI is a branched molecule with a molecular weight (Mw) of 15 kDa-25 kDa. In seawater desalination operation, a lower reverse osmosis permeate conductivity indicates a lower salt content in the permeate and a better seawater desalination effect. A higher permeate flux indicates lower resistance to water molecule permeation. A higher reverse osmosis system recovery rate indicates higher freshwater production efficiency.

[0075] Example 1: A highly fouling-resistant, long-lasting membrane seawater desalination process includes the following steps: S1. Seawater disinfection treatment: Sodium hypochlorite is used to disinfect the original seawater, inhibiting the growth of microorganisms and algae in the seawater and avoiding biofouling during long-term operation of the equipment.

[0076] S2. Pretreatment filtration: Disinfected seawater is passed into the submersible filtration membrane device built into the membrane tank for filtration to obtain pretreated permeate. The submersible filter membrane uses a zirconium-based MOP material / PEI inorganic-organic composite modified PVDF membrane, and the specific preparation process is as follows: (1) Preparation of zirconium-based MOP material: Terephthalic acid and organozirconium metallocene salt (dicyclopentadienyl zirconium chloride) were reacted at a molar ratio of 1:2 at 50℃ for 8h to prepare zirconium-based MOP material; (2) Preparation of modified pre-dispersion: Zirconium-based MOP material and polyethyleneimine (PEI) were ultrasonically dispersed in anhydrous ethanol at a mass ratio of 1.5:1 to form a uniformly coated modified suspension; (3) Preparation of casting solution (mass percentage): PVDF resin 15%, DMAc solvent 78%, PVP-K30 pore maker 4.5%, zirconium-based MOP material / PEI inorganic-organic composite modifier 0.5%; stir at 65℃ for 10h until completely dissolved, add modified suspension dropwise and continue stirring for 3h, vacuum degas at room temperature for 18h. (4) NIPS phase conversion film formation: with a doctor blade gap of 200 μm, it is coated on a non-woven fabric support layer and immediately immersed in a 20°C pure water coagulation bath for curing; after thorough cleaning with deionized water, it is stored in a 0.8 wt% sodium bisulfite aqueous solution for later use.

[0077] S3, Reverse Osmosis Desalination: The pretreated water passes through a 5μm security filter and a high-pressure pump for pressure stabilization before entering the anti-fouling reverse osmosis membrane device for desalination treatment to obtain desalinated water.

[0078] The antifouling reverse osmosis membrane is composed of a microbubble COF-modified PES porous bottom membrane and a porous cage-like covalent polymer material-doped polyamide separation layer. (1) Preparation of porous bottom membrane: 16% PES resin, 2% PVP-K30, 82% DMF solvent; sodium bicarbonate and aromatic Schiff base COF (mass ratio 20:1) were added, and the total amount of composite powder was 1wt%; the COF was prepared by reacting terephthalaldehyde and trimethylamine at a molar ratio of 1.3:1 at room temperature for 5h; the casting solution was degassed for 12h, the membrane was scraped with a 150μm doctor blade, and immersed in a slightly acidic acetic acid-sodium acetate coagulation bath at pH=5.0 and 20℃ for phase conversion and molding.

[0079] (2) Preparation of porous cage-like covalent polymer material: 2,4,6-tricarboxymethyl phloroglucinol (Tp), hexamethylenediamine and polylysine were used as raw materials in a mass ratio of 1:6:0.05; Tp was dissolved in dichloromethane to prepare a 0.1wt% oil phase, and hexamethylenediamine and polylysine were dissolved in water to prepare a 2.0wt% aqueous phase; the interface Schiff base reaction was carried out at room temperature for 8 hours, and the porous cage-like nanomaterial was obtained by washing with ethanol and vacuum drying.

[0080] (3) Interfacial polymerization membrane preparation: The aqueous phase is 2.0wt% m-phenylenediamine + 0.08wt% porous cage material aqueous solution, dip-coated for 2min, and the residual liquid is blown off; the oil phase is 0.1wt% trimesoyl chloride n-hexane solution, interfacial polymerization for 30s, and heat curing at 80℃ for 5min to obtain composite reverse osmosis membrane.

[0081] The desalination operation results of this embodiment are as follows: pretreatment permeate flux is 196 LMH, reverse osmosis system recovery rate is 48%, and reverse osmosis permeate conductivity is 185.2 μS / cm.

[0082] Example 2: This embodiment is basically the same as the process in Example 1, except that: chloramine is used for seawater disinfection in S1; the raw material for the synthesis of zirconium-based MOP material is replaced with bis(methylcyclopentadienyl)zirconium chloride, the synthesis temperature is 60℃, and the reaction time is 8h; the dosage of MOP material / PEI inorganic-organic composite modifier in the casting solution is 1.5wt%; the molar ratio of COF raw materials is 1.3:1, and the reaction time is 4h at room temperature; the interfacial polymerization reaction time of porous cage material is 6h. All other formulations and process parameters are completely identical.

[0083] The desalination operation results of this embodiment are as follows: pretreatment permeate flux is 201 LMH, reverse osmosis system recovery rate is 47%, and reverse osmosis permeate conductivity is 193.6 μS / cm.

[0084] Example 3: This embodiment is basically the same as the process in Example 1, except that: chlorine dioxide is used for seawater disinfection in S1; the raw material for the synthesis of zirconium-based MOP material is replaced with bis(pentamethylcyclopentadienyl)zirconium chloride, the synthesis temperature is 70℃, and the reaction time is 8h; the dosage of MOP material / PEI inorganic-organic composite modifier in the casting solution is 5wt%; the molar ratio of COF raw materials is 1.2:1, and the reaction time is 3h at room temperature; the interfacial polymerization reaction time of the porous cage material is 10h. All other formulations and process parameters are completely identical.

[0085] The desalination operation results in this embodiment are as follows: pretreatment permeate flux is 208 LMH, reverse osmosis system recovery rate is 46%, and reverse osmosis permeate conductivity is 172.4 μS / cm.

[0086] Example 4: This embodiment is basically the same as the process in Embodiment 1, except that: S1 uses ultraviolet disinfection to treat seawater; the raw material for the synthesis of MOP material is replaced with bis(indenyl)zirconium chloride, the synthesis temperature is 65℃, and the reaction time is 8h; the dosage of MOP material / PEI inorganic-organic composite modifier in the casting solution is 3wt%; the molar ratio of COF raw material is 1.4:1, and the reaction time is 4h at room temperature; the interfacial polymerization reaction time of porous cage material is 8h. The remaining formulations and process parameters are completely the same.

[0087] The desalination operation results of this embodiment are as follows: pretreatment permeate flux is 206 LMH, reverse osmosis system recovery rate is 47%, and reverse osmosis permeate conductivity is 190.3 μS / cm.

[0088] Example 5: This embodiment is basically the same as the process in Example 1, except that: sodium hypochlorite is used for sterilization in S1; the synthesis temperature of MOP material is 60℃ and the reaction time is 8h; the dosage of MOP material / PEI inorganic-organic composite modifier in the casting solution is 2wt%; the molar ratio of COF raw materials is 1.5:1 and the reaction time is 3h at room temperature; the interfacial polymerization reaction time of porous cage material is 9h. The rest of the formulation and process parameters are completely the same.

[0089] The desalination operation results of this embodiment are as follows: pretreatment permeate flux is 205 LMH, reverse osmosis system recovery rate is 48%, and reverse osmosis permeate conductivity is 182.1 μS / cm.

[0090] Example 6: This embodiment is basically the same as the process in Example 1, except that in the "preparation of zirconium-based MOP material" in S2, aminoterephthalic acid is used as the ligand, and it is reacted with an organozirconium metallocene salt (dicyclopentadienyl zirconium chloride) at a molar ratio of 1:2 at 50°C for 8 hours to prepare the zirconium-based MOP material. Other steps and process parameters are the same as in Example 1.

[0091] The desalination operation results of this embodiment are as follows: pretreatment permeate flux is 198 LMH, reverse osmosis system recovery rate is 49%, and reverse osmosis permeate conductivity is 188.4 μS / cm.

[0092] Example 7: This embodiment is basically the same as the process in Example 1, except that in the "preparation of zirconium-based MOP material" in S2, a mixed ligand of terephthalic acid amino and terephthalic acid in a 1:1 molar ratio is used, and an organozirconium metallocene salt (dicyclopentadienylzirconium chloride) is reacted at a molar ratio of 1:2 at 50°C for 8 hours to prepare the zirconium-based MOP material. Other steps and process parameters are the same as in Example 1.

[0093] The desalination operation results of this embodiment are as follows: pretreatment permeate flux is 198 LMH, reverse osmosis system recovery rate is 47%, and reverse osmosis permeate conductivity is 184.7 μS / cm.

[0094] Comparative Example 1 (without MOP material / PEI modified pretreated membrane): The process of this comparative example is exactly the same as that of Example 1, except that no MOP material / PEI inorganic-organic composite modifier is added to the casting solution of the immersion filtration membrane, and a pure PVDF blank membrane is used.

[0095] Operational results: Pretreatment permeate flux 146 LMH, reverse osmosis system recovery rate 42%, reverse osmosis permeate conductivity 224.4 μS / cm, permeate quality fluctuates greatly and organic pollutant retention capacity is weak.

[0096] Comparative Example 2 (bottom film without microbubbles + COF modification): The process of this comparative example is exactly the same as that of Example 1, except that sodium bicarbonate and aromatic Schiff base-type COF material are not added to the porous bottom membrane casting solution of the antifouling reverse osmosis membrane, and conventional PES bottom membrane is used.

[0097] Operational results: Pretreatment permeate flux was 191 LMH, and the reverse osmosis system recovery rate was 34%. The bottom membrane was prone to compaction and decay under high pressure, resulting in high operating energy consumption and significant permeate flow reduction. The conductivity of the reverse osmosis permeate was 280.5 μS / cm.

[0098] Comparative Example 3 (Polyamide layer without porous cage-like covalent polymer): The process of this comparative example is exactly the same as that of Example 1, except that: no porous cage-like covalent polymer material is added to the interfacial polymerized aqueous phase material of the antifouling reverse osmosis membrane, and a pure polyamide reverse osmosis separation layer without porous cage-like covalent polymer material is prepared.

[0099] Operational results: Pretreatment permeate flux 196 LMH, reverse osmosis system recovery rate 37%, reverse osmosis permeate conductivity 487.9 μS / cm; poor membrane surface hydrophilicity and weak antifouling ability, water quality deteriorates rapidly with long-term operation, and chemical cleaning cycle is significantly shortened.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. These modifications or substitutions, or combinations of technical features in the above embodiments that do not conflict with each other, can be made in accordance with the manner described in the embodiments. These modifications, substitutions or combinations do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A membrane-based seawater desalination method, characterized in that, Includes the following steps: S1. Disinfect the seawater; S2. The disinfected seawater is passed into a pretreatment membrane device for filtration to obtain pretreatment membrane permeate. The pretreatment membrane device uses an immersion filter membrane, which is a composite filter membrane with an organic-metal polyhedral structure material and a polyethyleneimine complex modified layer on its surface and body. S3. The pretreated membrane permeate is sequentially passed through a security filter and a high-pressure pump into a reverse osmosis membrane device for desalination treatment to obtain reverse osmosis permeate, which is seawater desalination water. The reverse osmosis membrane device employs an anti-fouling reverse osmosis membrane, which includes a porous bottom membrane and a composite separation layer on the surface of the porous bottom membrane. The porous bottom membrane is a polysulfone or polyethersulfone polymer layer synergistically modified with microbubble coupled covalent organic framework materials, and the composite separation layer is a polyamide layer containing porous cage-like covalent polymer materials. The porous cage-like covalent polymer is a ketone-enamine covalent polymer, prepared by Schiff base condensation reaction of an oil phase solution of 2,4,6-tricarboxymethyl phloroglucinol and an aqueous phase solution containing hexamethylenediamine and polylysine at room temperature for 6-10 hours.

2. The membrane-based seawater desalination method according to claim 1, characterized in that, In step S2, the organometallic polyhedral structure material is a zirconium-based MOP material prepared by reacting terephthalic acid and / or aminoterephthalic acid with an organozirconium metallocene salt at 50-70°C. The molar ratio of terephthalic acid and / or aminoterephthalic acid to zirconium ions in the organozirconium metallocene salt is 1:(1-3). The organozirconium metallocene salt is one or a combination of several of the following: dicyclopentadienyl zirconium chloride, bis(methylcyclopentadienyl) zirconium chloride, bis(pentamethylcyclopentadienyl) zirconium chloride, bis(indenyl) zirconium chloride, and dicyclopentadienyl dimethyl zirconium. In the immersion filter membrane, the mass ratio of the organometallic polyhedral structure material to polyethyleneimine is 1.5-2:

1.

3. The membrane-based seawater desalination method according to claim 2, characterized in that, The method for preparing the submersible filter membrane includes: First, an organometallic polyhedral structure material and polyethyleneimine were co-dissolved in anhydrous ethanol and ultrasonically dispersed to prepare a modified pre-dispersed suspension, thereby allowing the flexible long chains of polyethyleneimine to coat the rigid zirconium-based MOP material. Then, a membrane substrate polymer and a hydrophilic pore-forming agent were added to an organic solvent and stirred at 60-80℃ for 8-12 hours to prepare a homogeneous first casting solution. The polyethyleneimine was a branched molecule with a Mw of 10 kDa-30 kDa. The modified pre-dispersed suspension was added dropwise to the first casting solution, stirred for 2-4 hours, and then degassed under vacuum at room temperature for 12-24 hours to obtain the modified casting solution. The modified casting solution is used to prepare a nascent membrane by scraping with a doctor blade or spinning machine. The membrane is then immersed in a coagulation bath at 15-25°C for non-solvent-induced phase transformation and solidification. After cleaning, soaking and storage, the submerged filter membrane is obtained. The coagulation bath is pure water or an aqueous solution containing 2%-5% DMAc. The total mass content of the organic-metal polyhedral structure material and polyethyleneimine in the submerged filter membrane is 0.5-5 wt%.

4. The membrane-based seawater desalination method according to claim 3, characterized in that, The modified casting solution comprises, by mass percentage: 12%-18% membrane substrate polymer, 70%-80% organic solvent, 2%-6% hydrophilic pore-forming agent, and 0.5%-5% inorganic-organic composite modifier; The inorganic-organic composite modifier is a composite of an organic-metal polyhedral structural material and polyethyleneimine (PEI); the membrane substrate polymer is polyvinylidene fluoride or polyethersulfone; the organic solvent is N,N-dimethylacetamide or N-methylpyrrolidone; and the hydrophilic pore-forming agent is polyvinylpyrrolidone or polyethylene glycol.

5. The membrane-based seawater desalination method according to claim 1, characterized in that, In step S3, the raw materials for preparing the porous substrate membrane include polysulfone / polyethersulfone, sodium bicarbonate, and Schiff base covalent organic framework material. The Schiff base covalent organic framework material is a COF functional filler prepared by polymerization of phthalaldehyde and aromatic amine monomers. The mass ratio of sodium bicarbonate to covalent organic framework material is 10:1-30:1, and the total amount of both added accounts for 0.5%-1.5% of the mass of the casting solution. The porous bottom membrane is prepared by phase inversion through a slightly acidic coagulation bath system with a pH of 4.5-5.5 and a temperature of 15-25℃.

6. The membrane-based seawater desalination method according to claim 5, characterized in that, The Schiff base type covalent organic framework material is prepared by the Schiff base condensation reaction of aldehyde monomers and amine monomers at room temperature for 3-5 hours. The aldehyde monomer is terephthalaldehyde and / or o-phthalaldehyde, and the amine monomer is trimethylamine and / or p-phenylenediamine; the molar ratio of the aldehyde monomer to the amine monomer is 1.2-1.5:1; the reaction solvent is any one of 1,4-dioxane, mesitylene, ethanol-water system or acetic acid-water system.

7. The membrane-based seawater desalination method according to claim 1, characterized in that, The composite separation layer of the antifouling reverse osmosis membrane is prepared by interfacial polymerization reaction, and the interfacial polymer material includes aqueous phase material and oil phase material. The aqueous phase material is an aqueous solution containing 1.0-3.5 wt% polyamine and 0.05-0.1 wt% porous cage-like covalent polymer material; The concentration of polyacrylamide chloride in the oil phase material is 0.05-0.25 wt%, and the oil agent is at least one selected from n-hexane, cyclohexane, heptane, and isooctane.

8. The membrane-based seawater desalination method according to claim 7, characterized in that, The specific preparation steps of the antifouling reverse osmosis membrane are as follows: Step 1: Add 15%-18% polysulfone or polyethersulfone resin and 1%-3% polyvinylpyrrolidone to N,N-dimethylformamide solvent, stir at 60℃ for 6-8h to prepare a homogeneous initial liquid, add pre-dispersed sodium bicarbonate and covalent organic framework material mixed powder, stir and degas for 6-16h to obtain modified casting solution. Step 2: The modified casting solution is coated onto the nonwoven fabric support layer and immersed in a slightly acidic coagulation bath for phase inversion to prepare a modified porous bottom membrane. Step 3: Immerse the porous substrate in an aqueous phase material containing aromatic diamine and porous cage-like covalent polymer material, and blow off the excess liquid on the surface after immersion. Step 4: Cover the oil phase material with pyromellitic methyl chloride for interfacial polymerization. After polymerization, perform deep crosslinking by heat treatment at 70-120℃ for 3-8 minutes to obtain an antifouling reverse osmosis membrane.

9. An integrated membrane-based seawater desalination device, characterized in that, The membrane-based seawater desalination method according to any one of claims 1-8 includes a membrane tank, a security filter, a high-pressure pump, an anti-fouling reverse osmosis membrane device, an energy recovery device, and a reverse osmosis concentrate post-treatment device. The membrane tank is used to store raw seawater. The membrane tank is equipped with a pretreatment membrane device and a dosing component for adding disinfectant, which can perform in-situ disinfection and pretreatment filtration of raw seawater. The pretreatment membrane device has a built-in submersible filter membrane, which is a composite filter membrane with an organic-metal polyhedral structure material and a polyethyleneimine complex modified layer on its surface and body. The anti-fouling reverse osmosis membrane device is internally equipped with an anti-fouling reverse osmosis membrane, which includes a polysulfone or polyethersulfone porous bottom membrane synergistically modified by microbubble coupled aromatic Schiff base type covalent organic framework material and a polyamide composite separation layer containing porous cage covalent polymer material. The pretreatment membrane device is equipped with an inlet and a product outlet. Seawater in the membrane tank enters the pretreatment membrane device through the inlet to complete filtration. The product outlet of the pretreatment membrane device is connected to a security filter and a high-pressure pump in sequence through pipelines, and then connected to the inlet of the anti-fouling reverse osmosis membrane device. The anti-fouling reverse osmosis membrane device is equipped with a freshwater product outlet and a concentrate outlet. The freshwater product outlet is used to output desalinated seawater. The concentrate outlet is connected to the high-pressure inlet of the energy recovery device. The low-pressure outlet of the energy recovery device is connected to the reverse osmosis concentrate downstream treatment device. The high-pressure outlet of the energy recovery device is recirculated into the inlet front pipeline of the anti-fouling reverse osmosis membrane device to realize the recovery of residual pressure and stable recycling of reverse osmosis concentrate.

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