A modified process for coating the surface of a water purification membrane

Abstract

The application relates to the technical field of water treatment and separation membrane material modification, and discloses a modification process for coating the surface of a water purification membrane. A polyamide-based membrane is immersed in a buffer modification solution, the buffer modification solution contains 30-70 mmol / L of tris-hydroxymethyl aminomethane and 1.0-2.0 g / L of dopamine hydrochloride, and the pH value of the system is adjusted to carry out a mild immersion reaction under constant temperature conditions; and a complete polydopamine base anchoring layer is constructed on the membrane surface by using the oxidative self-polymerization of dopamine. The composite modified water purification membrane has excellent structural durability and long-acting antifouling efficiency, and highly meets the development requirements of complex water quality separation engineering and high-end water treatment materials.

Description

Technical Field

[0001] This invention relates to the field of water treatment and separation membrane material modification technology, specifically, a modification process for coating the surface of a water purification membrane. Background Technology

[0002] With the development of the water treatment industry, the development of antifouling composite membranes for water purification has become an important means to improve separation efficiency. In complex water quality conditions, water purification membranes not only need to have high-efficiency retention and separation capabilities, but also excellent surface antifouling properties and long-term physical stability to ensure the efficient and stable operation of the system.

[0003] However, traditional surface coating and extensive modification processes, when attempting to improve hydrophilicity, often suffer from weak physical adsorption that is insufficient to withstand the long-term shearing and peeling of water flow. Non-precise cross-linking can easily lead to polymer network stacking and blockage, making it difficult to maintain basic water flux. Furthermore, this type of interfacial coating system has a narrow process window: improper bonding during the bottom anchoring stage can easily cause the entire antifouling barrier to detach; and a lack of precise cross-linking control during the outer coating stage can result in an excessively dense polymer network that leads to a sharp increase in mass transfer resistance. These problems—easy coating peeling, severe attenuation of permeability flux, and insufficient long-term antifouling performance—limit its application in complex water purification scenarios. Summary of the Invention

[0004] In view of the aforementioned existing problems, the present invention is proposed.

[0005] Therefore, this invention provides a modification process for coating the surface of water purification membranes, solving a common technical problem faced in the field of existing water treatment and separation membrane material modification technology.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention provides a modified process for coating the surface of a water purification membrane, comprising the following steps: S1. The polyamide-based membrane is immersed in a buffer modification solution containing 30-70 mmol / L of tris(hydroxymethyl)aminomethane and 1.0-2.0 g / L of dopamine hydrochloride. The pH of the system is adjusted and a mild impregnation reaction is carried out under constant temperature conditions. The complete polydopamine substrate anchoring layer is constructed on the membrane surface by the oxidative self-polymerization of dopamine. After the S2 substrate treatment, the membrane surface was continuously and parallelly rinsed with deionized water to remove free aggregated particles that were not physically adsorbed. Then, a mixed coating precursor solution was prepared by adding 3.0~5.0 g / L of polyvinyl alcohol and 0.5~1.0 g / L of glutaraldehyde to the deionized water and continuously stirring to homogenize the system to achieve microscopic uniformity and stability. S3 places the polyamide-based film that has completed substrate rinsing into a dynamic cross-flow coating system. Under a certain operating pressure, the mixed coating precursor solution is pumped in for dynamic circulation coating, so that the hydrophilic macromolecules can be fully spread on the surface of the base coating and form initial adhesion. After the S4 cycle coating is completed, the precursor residue in the system is emptied, and mild clean air is introduced into the membrane surface for continuous purging and curing. Through thermal drive, the crosslinking agent and the main polymer undergo a deep acetalization reaction, resulting in a dense and locked coating network, thus producing an antifouling composite modified water purification membrane.

[0007] In a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the concentration of tris(hydroxymethyl)aminomethane in step S1 is 50 mmol / L; the concentration of dopamine hydrochloride is 1.5 g / L. In step S2, the concentration of polyvinyl alcohol is 4.0 g / L; and the concentration of glutaraldehyde is 0.8 g / L.

[0008] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the pH value of the system in step S1 is 7.8~8.5; the constant temperature condition is 20~30℃; and the mild immersion reaction time is 3.0~5.0 hours.

[0009] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the pH value of the system in step S1 is 8.2; the constant temperature condition is 25°C; and the mild immersion reaction time is 4.0 hours.

[0010] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the deionized water rinsing flow rate in step S2 is 0.8~1.2L / min; the continuous parallel rinsing time is 4~6 minutes; and the continuous stirring and homogenization time is 20~40 minutes.

[0011] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the deionized water rinsing flow rate in step S2 is 1.0 L / min; the continuous parallel rinsing time is 5 minutes; and the continuous stirring and homogenization time is 30 minutes.

[0012] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the operating pressure in step S3 is 0.10~0.20MPa; and the dynamic cyclic coating time is 1.0~2.0 hours.

[0013] As a preferred embodiment of the modified process for coating the surface of a water purification membrane according to the present invention, wherein: the operating pressure in step S3 is 0.15 MPa; and the dynamic cyclic coating time is 1.5 hours.

[0014] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the temperature of the mild clean air in step S4 is 40~50℃; and the continuous purging and curing time is 1.5~3.0 hours.

[0015] As a preferred embodiment of the modification process for coating the surface of a water purification membrane according to the present invention, wherein: the temperature of the mild clean air in step S4 is 45°C; and the purging and curing time is 2 hours.

[0016] The beneficial effects of this invention are as follows: Based on a dopamine oxidative self-polymerization to construct a bottom-layer physical anchoring network, and supplemented by polyvinyl alcohol acetalization crosslinking to construct a hydrophilic barrier, the composite modification system significantly improves the coating's resistance to water flow shear and its long-term antifouling performance while maintaining the original membrane's basic separation efficiency. It overcomes the limitations of traditional coating processes, which often lead to easy peeling of the modified layer and excessive network density causing a sharp decline in flux. This allows the composite coating layer to achieve stable bonding at the substrate interface and exhibit excellent hydration repulsion and fouling flux recovery capabilities on the outer surface, broadening its application window in complex operating conditions. Furthermore, the gentle dynamic cross-flow coating effectively slows down disordered aggregation on the membrane surface, and the optimized process parameters retain appropriate hydration porosity of the polymer network, resulting in good structural durability and overall permeability performance in long-term operation. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1a The graph shows the effect of the polyvinyl alcohol / glutaraldehyde concentration ratio on the hydration swelling rate of the coating.

[0019] Figure 1b The diagram shows the optimization of the polyvinyl alcohol / glutaraldehyde concentration ratio on the contact angle of pure water.

[0020] Figure 1c The graph shows the effect of the polyvinyl alcohol / glutaraldehyde concentration ratio on the retention rate of pure water flux.

[0021] Figure 1d The graph shows the effect of the polyvinyl alcohol / glutaraldehyde concentration ratio on the dynamic pollution flux recovery rate.

[0022] Figure 1e This is an optimization diagram showing the effect of the polyvinyl alcohol / glutaraldehyde concentration ratio on the coating thickness retention rate after rinsing.

[0023] Figure 2 This is a diagram showing the optimal ratio for dopamine hydrochloride concentration.

[0024] Figure 3 This is a diagram showing the optimization of the impregnation reaction time ratio.

[0025] Figure 4 This is a temperature optimization diagram for the reaction system.

[0026] Figure 5 This is a diagram showing the optimization of buffer pH.

[0027] Figure 6 This is a diagram showing the optimized flow rate for surface rinsing after the reaction.

[0028] Figure 7 This is a flow chart of a modified process for coating the surface of a water purification membrane. Detailed Implementation

[0029] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0030] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0031] Secondly, the term "one embodiment" or "example" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. The appearance of an embodiment in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that mutually excludes other embodiments.

[0032] Example 1 This embodiment aims to provide a water purification membrane surface coating system that combines stable resistance to water flow erosion with excellent hydrophilic and antifouling performance, in order to solve the technical bottlenecks such as the easy occurrence of organic matter and microbial fouling in traditional reverse osmosis membrane modules under complex water quality conditions, and the easy peeling and failure of the modified layer in conventional surface coating modification processes during operation.

[0033] 1.1 Screening and Determination of Coating Substrate Materials To construct a composite coating system for water purification membranes suitable for complex water quality conditions, it is first necessary to determine the substrate material as a continuous transition phase. This stage aims to evaluate the stability of the microscopic bonding and the retention of macroscopic separation properties when different types of adhesives and coupling agents are introduced onto the polyamide surface, thereby screening for substrate materials with resistance to water flow erosion and the potential for a hydrophilic-antifouling balance.

[0034] Experimental Groups: Ten candidate materials were selected, including dopamine hydrochloride, tannic acid, silane coupling agent KH-550, silane coupling agent KH-560, polyethyleneimine, polyacrylic acid, polyacrylamide, chitosan, epoxy resin primary polymer, and glutaraldehyde. Treatment solutions with a mass concentration of 2 g / L were prepared for each group, and these solutions were used to impregnate and coat the polyamide reverse osmosis flat sheet membranes to be tested. According to relevant standards such as "HY / T107-2008 Test Method for Spiral Wound Reverse Osmosis Membrane Modules," the pure water contact angle of the modified membrane surface was measured using a contact angle meter at 25℃ to evaluate the hydrophilicity of the material. The treated membrane sheet was placed in an ultrasonic cleaning tank (power 200W, frequency 40kHz) and vibrated for 30 minutes. The atomic molar ratio of characteristic elements on the membrane surface before and after ultrasonication was measured using X-ray photoelectron spectroscopy, and the coating adhesion retention rate was calculated to determine the substrate material's resistance to shearing under fluid conditions. Under an operating pressure of 0.15 MPa, a 1 g / L bovine serum albumin solution was used to conduct a 12-hour dynamic cross-flow fouling cycle, and the flux recovery rate after fouling was measured to evaluate the anti-fouling ability of the coating under operating conditions. The change in initial desalination rate under a pure water test environment was measured to evaluate the impact of the modified layer on the separation performance of the original membrane.

[0035]

[0036] Tannic acid and polyacrylic acid exhibit low pure water contact angles and good hydrophilicity, but their coating adhesion retention rates are low, and their bonding stability is insufficient, failing to meet the adhesion requirements under long-term water flow erosion. Epoxy resin primary polymers show high adhesion retention rates, but their large pure water contact angles indicate hydrophobicity, and their flux recovery rate after fouling is low. Silane coupling agents KH-550 and KH-560 demonstrate good adhesion, but their flux recovery rate improvement after fouling is limited, and they cause a significant decrease in the initial desalination rate, affecting the core separation performance of the original membrane. Polyethyleneimine, polyacrylamide, chitosan, and glutaraldehyde show only average performance in terms of adhesion retention and flux recovery rates, failing to achieve a good balance among various properties.

[0037] Dopamine hydrochloride exhibits excellent overall performance. While maintaining a relatively stable desalination rate of the original membrane, it also has a high coating adhesion retention rate and flux recovery rate, demonstrating a balanced characteristic without any obvious shortcomings. Therefore, it has been established as a coating substrate material.

[0038] 1.2 Selection and Determination of Main Materials for the Functional Covering Layer Having established dopamine hydrochloride as the substrate adhesive, the next step is to select a host polymer that can stably bind to it and form a highly hydrophilic antifouling barrier on its surface. This stage aims to evaluate the ability of different hydrophilic polymers to form a surface hydration layer and their impact on the overall membrane permeation and separation performance when used as functional coating layers, thereby screening for host materials that offer a good balance between antifouling stability and water flux maintenance.

[0039] Experimental groups: polyvinyl alcohol (PVA); polyethylene glycol (PEG); polyvinylpyrrolidone (PVP); polyacrylic acid (PAA); sodium alginate (SA); carboxymethyl cellulose (CMC); polyacrylamide (PAM); hydrophilic modified polyethersulfone (PES); polyvinyl butyral (PVB); and a self-polymerized control group with only increased dopamine coating thickness. Standard polyamide reverse osmosis membranes were pre-treated using the dopamine hydrochloride process determined in Example 1. Subsequently, the above 10 candidate polymer materials at a mass concentration of 4 g / L were coated onto the membrane surface for 1.5 hours at an operating pressure of 0.15 MPa using a dynamic circulation system, and cross-linked and cured using 0.8 g / L glutaraldehyde. The pure water contact angle of the modified membrane surface was measured at 25°C according to standards such as GB / T32373-2015 "Reverse Osmosis Membrane Test Method". The membrane was immersed in deionized water for 72 hours, and the hydration swelling rate of the coating was determined by weighing and using a thickness gauge to evaluate the structural dimensional stability during underwater operation. The membrane was continuously operated for 48 hours using simulated raw water containing organic contaminants, and the long-term water flux decay rate was measured. The change in the membrane's rejection rate to a standard sodium chloride test solution was measured to evaluate the auxiliary effect of the functional coating on the basic desalination performance.

[0040]

[0041] Polyethylene glycol (PEG) and polyacrylic acid (PAA) exhibited low pure water contact angles (32.1° and 36.4°, respectively), demonstrating good hydrophilicity. However, their coating hydration swelling rates were high, resulting in insufficient structural dimensional stability and failing to meet the requirements for long-term continuous underwater operation. Polyacrylamide (PAM) negatively impacted the original membrane rejection rate (decreasing it by 0.4%), and its water flux decay rate was only moderate. Polyvinyl butyral (PVB) and hydrophilically modified polyethersulfone (PES) had low hydration swelling rates, but large pure water contact angles, weak surface hydration barriers, and significantly high long-term water flux decay rates.

[0042] Polyvinyl alcohol (PVA) combines long-term water flux maintenance with retention rate enhancement, exhibiting balanced characteristics without obvious shortcomings. Therefore, it has been established as the main material for the functional layer of the coating system.

[0043] 1.3 Screening and Determination of Crosslinking Agents for the Coating System To ensure the stability of the composite coating system under long-term dynamic water flow, after establishing dopamine hydrochloride as the base material and polyvinyl alcohol as the main functional layer material, it is necessary to determine a suitable crosslinking agent to network-cur the polymer molecular chains. This stage aims to evaluate the degree of consumption of hydrophilic groups and the effect on improving the overall structural strength of the membrane by different types of chemical crosslinking agents when treating polyvinyl alcohol coatings, thereby screening out crosslinking materials with the best potential for balancing crosslinking curing efficiency and hydrophilic permeability.

[0044] Experimental groups: glutaraldehyde; formaldehyde; glyoxal; epichlorohydrin; isocyanate; boric acid; citric acid; genipin; ethylene glycol diglycidyl ether; polycarbodiimide. A total of 10 candidate crosslinking agents were established. Standard polyamide reverse osmosis membranes were pre-coated using a predetermined dopamine hydrochloride and polyvinyl alcohol process. Subsequently, solutions of the 10 candidate crosslinking agents (0.8 g / L) were circulated at an operating pressure of 0.15 MPa for 1 hour. According to relevant standards such as GB / T32373-2015 "Test Methods for Reverse Osmosis Membranes", the gelation rate of the modified layer was determined by Soxhlet extraction at 25°C to assess the density of the crosslinked network. The pure water contact angle of the modified membrane surface was measured using a contact angle meter to assess the weakening effect of the crosslinking reaction on hydrophilicity. The membrane was placed in a crossflow test tube and continuously flushed for 48 hours at a pressure of 0.3 MPa and a high flow rate. The coating thickness retention rate after flushing was measured to determine the resistance of the crosslinked coating to physical peeling under strong water flow shear. The change in initial desalination rate under a pure water test environment was measured to assess the potential chemical damage of the crosslinking agent to the polyamide desalination layer.

[0045]

[0046] Citric acid and boric acid exhibited low contact angles with pure water (38.5° and 39.2°, respectively), demonstrating good hydrophilicity retention. However, their coating gelation rate and thickness retention were low, and the cross-linked network stability was insufficient, failing to meet the structural requirements under long-term water flow erosion. Genipin had the least impact on the initial desalination rate of the original membrane (0.0%), but its gelation rate and thickness retention were only average. Isocyanates and epichlorohydrin, while having high gelation rates, exhibited large contact angles with pure water, displaying hydrophobic characteristics and causing a significant decrease in the initial desalination rate, thus affecting the core separation performance of the original membrane.

[0047] In comparison, while glutaraldehyde has a higher contact angle with pure water than citric acid and is slightly inferior to genipin in maintaining the initial desalination rate, its coating gelation rate reaches 92.4%, and the coating thickness retention rate after rinsing reaches 89.6%, both of which are the best among the groups in terms of key stability indicators. Glutaraldehyde, while maintaining a basically stable desalination rate of the original membrane, also possesses high crosslinking network strength and long-term structural stability, exhibiting balanced characteristics without obvious weaknesses. Therefore, it was established as the crosslinking agent for the coating system.

[0048] 1.4 Screening and Determination of Reaction Buffer for the Coating System To ensure the smooth progress of dopamine's self-polymerization reaction and polyvinyl alcohol crosslinking in the composite coating system, after determining the substrate material, functional layer main material, and crosslinking agent, a suitable liquid-phase buffer reaction environment needs to be identified. This stage aims to evaluate the ability of different acid-base adjustment systems to control the polymerization rate of the precursor liquid and their intervention effect on the microstructure of the final coating when providing a weakly alkaline environment, thereby screening out the buffer system with the best potential for reaction kinetic control and film quality balance.

[0049] Experimental groups: Tris(hydroxymethyl)aminomethane buffer (Tris); phosphate buffer (PBS); hydroxyethylpiperazine ethanesulfonic acid buffer (HEPES); morpholine propanesulfonic acid buffer (MOPS); carbonate buffer; bicarbonate buffer; borate buffer; dilute sodium hydroxide solution; ammonia solution; deionized water (without alkaline adjuster). Ten candidate reaction media groups were established. At 25°C, the pH of all ten liquid groups was uniformly adjusted to 8.2 (except for the deionized water group, which maintained its natural pH). Equal volumes of dopamine hydrochloride and polyvinyl alcohol were added to each group to prepare precursor solutions. The absorbance of the solutions at 420 nm was continuously monitored using a UV-Vis spectrophotometer. The inflection point at which the absorbance showed a sharp change was defined as the polymerization stabilization period of the precursor solution to assess the system's resistance to macroscopic aggregation. Standard polyamide reverse osmosis membrane sheets were immersed in each group of precursor solutions for coating reaction. After the reaction, in accordance with relevant standards such as "HY / T107-2008 Test Method for Spiral Wound Reverse Osmosis Membrane Modules", the concentration of unreacted dopamine monomer in the residual liquid was determined by ultraviolet spectrophotometry, and the dopamine deposition conversion rate was calculated to evaluate the effective film formation efficiency. Five micro-regions were randomly selected on the membrane surface using atomic force microscopy (AFM) to scan and calculate the coating thickness uniformity deviation to determine the influence of the reaction system on the coating density and smoothness. The modified membrane was tested in pure water at an operating pressure of 0.15 MPa, and the initial desalination rate change was recorded to assess the potential chemical damage of the alkaline buffer system to the polyamide desalination layer.

[0050]

[0051] Among them, the deionized water group had no effect on the initial desalination rate of the original membrane and had a long precursor solution stability period. However, due to the lack of a necessary weakly alkaline environment, its dopamine deposition conversion rate was too low, and an effective coating could not be formed. The polymerization reaction initiated by dilute sodium hydroxide solution and ammonia solution was too fast, resulting in rapid macroscopic agglomeration of the solution in the bulk phase, increasing the deviation of coating thickness uniformity, and causing significant chemical degradation of the polyamide desalination layer (a significant decrease in desalination rate). Hydroxyethylpiperazine ethanesulfonic acid buffer (HEPES) and borate buffer showed a long precursor solution polymerization stability period and good solution maintenance ability, but their dopamine deposition conversion rate was low, and the film formation efficiency was average.

[0052] Although Tris (tris(hydroxymethyl)aminomethane) buffer has a shorter polymerization stabilization period than HEPES and is slightly inferior to deionized water in maintaining the initial desalination rate, its dopamine deposition conversion rate reaches 91.4%, and the coating thickness uniformity deviation is only 4.2%, making it the best performing in these two key film formation quality indicators among all groups. Tris buffer, while maintaining a relatively stable desalination rate of the original film, combines high deposition efficiency with excellent film uniformity, exhibiting a balanced characteristic without obvious shortcomings. Therefore, it was established as the reaction buffer for the coating system.

[0053] Example 2 Referring to Figures 1-2, this is the second embodiment of the present invention. This embodiment aims to optimize the quantitative proportions of the selected superior raw materials through systematic experimental design. Although the basic film-forming and cross-linking functions of each individual component have been confirmed, the antifouling performance and physical stability of the final coating layer are not simply the sum of the effects of each component, but depend on the dynamic balance between the dopamine self-polymerization rate and the degree of polymer cross-linking and curing, as well as the synergistic regulatory effect of the concentration ratio of each raw material on the interfacial reaction kinetics.

[0054] 2.1 Optimization of the concentration ratio of polyvinyl alcohol and glutaraldehyde. To investigate the dynamic balance between microstructure construction, physical structural stability, and surface interface properties of polyvinyl alcohol (PVA) and glutaraldehyde during dynamic cyclic coating and cross-linking curing, under the premise of a dopamine-based anchoring network, and to provide experimental data support for establishing a modified layer with both long-lasting antifouling performance and good water flux, demonstrating the synergistic regulatory effect of the hydrophilicity of the main polymer and the density of the cross-linked network, a full cross-matrix experiment was conducted with PVA in increments of 1.0 g / L (from 1.0 g / L to 8.0 g / L) and glutaraldehyde in increments of 0.2 g / L (from 0.4 g / L to 1.8 g / L). A systematic evaluation was performed on five core inspection items: coating hydration swelling rate, pure water contact angle, pure water flux retention rate, dynamic pollution flux recovery rate, and coating thickness retention rate after rinsing, to determine the appropriate process ratio.

[0055] Test results are as follows Figure 1a As shown in -e, the hydration swelling ratio of the coating shows a continuous upward trend with increasing polyvinyl alcohol (PVA) concentration when the glutaraldehyde (PVA) concentration is constant; however, it decreases significantly with increasing glutaraldehyde (PVA) concentration when the PVA concentration is constant. The relatively stable values ​​are concentrated in the range of 3.0–5.0 g / L PVA and 0.6–1.0 g / L glutaraldehyde. The high proportion of main materials and low crosslinking degree lead to a risk of coating loosening and detachment. The pure water contact angle decreases with increasing PVA concentration, exhibiting good hydrophilicity, but increases with increasing glutaraldehyde concentration. Its lower and balanced value is distributed in the range of 4.0–6.0 g / L PVA and 0.6–0.8 g / L glutaraldehyde. The pure water flux retention rate decreases negatively with increasing PVA and glutaraldehyde concentrations. Thicker coatings and denser crosslinking networks significantly hinder water molecule penetration, and the better-performing areas fall within the low-to-medium concentration combination range. The coating thickness retention rate after rinsing showed a positive response to the increase of glutaraldehyde concentration. The higher degree of crosslinking brought good resistance to physical shearing. The optimal region of this indicator was concentrated in the medium-high crosslinking agent concentration band. The dynamic pollution flux recovery rate is the result of the multi-dimensional synergy of physical stability, surface hydrophilicity and permeability. Its value is not a linear summation of a single variable, but is highly dependent on the physicochemical balance between the coating hydration layer thickness and the rigidity of the three-dimensional crosslinking network. The high recovery rate region and the medium crosslinking balance band (polyvinyl alcohol 3.0~5.0 g / L, glutaraldehyde 0.6~1.0 g / L) overlapped well, while the low recovery rate region fell into the hydrophobic dense band and the loose and easily peelable band.

[0056] By systematically normalizing the data of the five key process indicators mentioned above, and then combining them with multi-dimensional weighted calculations based on interfacial physicochemical feedback, the aim was to simulate and find the comprehensive balance point between the construction of the hydrophilic antifouling barrier, the mechanical strength of the cross-linked network, the permeation mass transfer resistance, and the fluid shear stability. The theoretical balance point of the comprehensive performance of this surface coating system was found to be polyvinyl alcohol: glutaraldehyde = 4.12 g / L: 0.78 g / L. Considering the hydrodynamic control requirements of the precursor liquid viscosity in industrial-scale dynamic cyclic coating, the uniformity of mass transfer mixing in the internal channels of the spiral wound membrane module, and the robust requirements of continuous production in multiple batches, a fixed ratio close to the theoretical point was finally selected as the process formulation, namely: polyvinyl alcohol 4.0 g / L, glutaraldehyde 0.8 g / L.

[0057] 2.2 Optimization of Dopamine Hydrochloride Concentration Ratio Dopamine forms a polydopamine transition layer with strong adhesion on the surface of polyamide membranes through oxidative self-polymerization. However, if the dopamine concentration is too low, a complete covering network cannot be formed on the membrane surface, resulting in insufficient anchoring points for subsequent functional layers. If the concentration is too high, dopamine not only reacts on the membrane surface but also undergoes vigorous self-polymerization in the bulk solution phase, forming large polymer particles that are randomly deposited on the membrane surface, leading to increased coating roughness and membrane pore blockage. This experiment aims to find a suitable balance between anchoring strength, coating smoothness, and permeation resistance through quantitative evaluation.

[0058] In this experiment, the reaction medium was fixed as a Tris buffer solution (pH adjusted to 8.2, concentration 50 mmol / L). Eight concentration gradients of dopamine hydrochloride were set up, ranging from 0.5 g / L to 4.0 g / L in 0.5 g / L increments.

[0059] The cut standard polyamide reverse osmosis membrane sheets were immersed in the eight groups of dopamine precursor solutions of different concentrations and gently immersed in a constant temperature shaker at 25°C for 4 hours. After substrate treatment, they were continuously rinsed with deionized water at a constant flow rate of 1 L / min for 5 minutes. Subsequently, all membrane samples were uniformly coated and cross-linked with the functional layer using the determined optimized parameters (polyvinyl alcohol 4.0 g / L, glutaraldehyde 0.8 g / L). During the testing phase, the modified membrane surface was scanned in a 5 μm × 5 μm microarea using atomic force microscopy (AFM) to measure the average surface roughness (Ra, unit: nm) to evaluate the deposition degree of bulk self-polymerized particles. Referring to the pre-set ultrasonic vibration evaluation method, the atomic molar ratio of characteristic elements before and after 30 minutes of ultrasonic treatment was measured, and the coating adhesion retention rate (%) was calculated to evaluate the bonding strength of the anchoring layer. The sample membrane was loaded into the cross-flow evaluation device, and the pure water flux was tested at 0.15 MPa. The initial pure water flux retention rate (%) was calculated by comparing it with the blank original membrane to quantitatively evaluate the resistance effect of the substrate on permeation mass transfer.

[0060] Test results are as follows Figure 2As shown, the coating adhesion retention rate in the low addition range is low, failing to form a complete and dense polydopamine anchoring network. This does not meet the basic requirements for the density of the underlying sites when subsequent functional layers are polymerically bonded, and there is a significant risk of overall peeling under strong water flow shear. In the high addition range, as the mass concentration of dopamine hydrochloride continues to increase, the coating adhesion retention rate does not show a significant improvement, but instead shows a clear downward trend. Excessively high concentrations of dopamine solution will trigger violent oxidative polymerization within the bulk phase, generating a large number of polymer particles that deposit disorderly. This thick particle stack not only leads to a significant increase in surface roughness, easily generating additional water flow mass resistance within the coating, causing a decline in the initial pure water flux retention rate; at the same time, the relatively weak cohesive force between the large particles weakens the overall structural stability of the coating.

[0061] By normalizing and weighted fitting analysis of three core performance indicators—average surface roughness, coating adhesion retention rate, and initial pure water flux retention rate—the theoretical equilibrium point for the system's substrate anchoring performance was calculated to be 1.54 g / L. Considering the accuracy tolerance of the liquid preparation and weighing system during industrial-scale dynamic cyclic coating, the limitations of fluid dynamic uniformity of the reaction on a large-area spiral wound membrane surface, and the robustness of continuous production in multiple batches, a value close to this theoretical equilibrium point and easily operable and controlled was ultimately selected: the determined dosage of dopamine hydrochloride was 1.5 g / L.

[0062] Example 3 Reference Figure 3-6 This is the third embodiment of the present invention. This embodiment aims to scale up the previously determined core coating materials and optimized proportioning parameters to a pilot-scale operation. Considering that in industrial-scale continuous coating processes, the complex flow channel structure inside large-area spiral-wound membrane modules easily induces fluid shear gradients and mass transfer polarization effects, this will lead to differences between the dynamic interfacial reactions, cross-linking curing rates, and the density of the polymer network within the module and the laboratory flat-sheet membrane test results. Therefore, it is necessary to conduct pilot-scale amplification experiments to finely correct and optimize core engineering parameters such as circulation flow rate, operating pressure, reaction temperature, and cross-linking curing time during the dynamic cyclic coating process. This ensures that under large-scale production conditions, both the uniform distribution of the coating liquid within the narrow flow channels and the acquisition of a structurally uniform antifouling barrier can be maintained, thereby guaranteeing the performance reproducibility and batch consistency of the composite water purification membrane product in large-scale preparation.

[0063] 3.1 Precursor solution preparation steps Each coating raw material was pretreated using a phased dissolution and gentle mixing method. Taking the preparation of 100L of precursor solution as an example, 100L of deionized water was added to a clean preparation vessel, and stirring was started to maintain dynamic flow of the liquid surface. Tris(hydroxymethyl)aminomethane buffer was slowly added to achieve a concentration of 50 mmol / L, and the pH was adjusted to 8.2 to ensure complete dissolution and establish an initial weakly alkaline physicochemical microenvironment. Maintaining a low shear state, 400g of polyvinyl alcohol powder (equivalent to a system concentration of 4.0g / L) was uniformly sieved in to allow it to fully swell and hydrate, initially constructing a hydrophilic polymer dispersion system, and then gently heated to promote complete dissolution. After the system cooled to room temperature of 25°C, 150g of dopamine hydrochloride (equivalent to 1.5g / L) and 80g of crosslinking agent glutaraldehyde (equivalent to 0.8g / L) were slowly added sequentially, utilizing good fluidity to prevent excessive local crosslinking and aggregation of polymer chains. Finally, the rotation speed was adjusted and the system was continuously homogenized for 30 minutes to achieve microscopic uniformity and stability, completing the preparation of the precursor solution.

[0064] 3.2 Optimization of Impregnation Reaction Time Ratio The self-polymerization and deposition of dopamine in a weakly alkaline environment is a dynamic process that accumulates over time. If the reaction time is too short, a sufficiently covered and firmly bonded anchoring network cannot be formed on the membrane surface; if the reaction time is too long, it not only prolongs the production cycle but also easily leads to excessive aggregation and disordered deposition of polydopamine in the bulk phase, resulting in increased coating roughness and membrane pore resistance. This experiment quantitatively evaluates the microstructure and macroscopic properties of the coating under different time gradients to explore the appropriate balance between film formation efficiency, physical bonding, and permeation mass transfer resistance. The concentration of dopamine hydrochloride was fixed at 1.5 g / L. The impregnation reaction time was set from 1.0 h to 8.0 h in 1.0 h increments.

[0065] The cut standard polyamide reverse osmosis membrane sheets were immersed in the aforementioned precursor solution and subjected to gentle immersion treatment for corresponding durations in a constant-temperature shaker at 25°C. After the set time, the membrane sheets were removed and continuously rinsed with deionized water at a constant flow rate of 1 L / min for 5 minutes. Subsequently, all sample membranes were uniformly coated and crosslinked using the previously optimized main material and crosslinking agent parameters (polyvinyl alcohol 4.0 g / L, glutaraldehyde 0.8 g / L). The surface of the modified membrane was scanned using atomic force microscopy (AFM) to measure the average surface roughness (Ra, unit: nm). Referring to the ultrasonic vibration evaluation method, the atomic molar ratio of characteristic elements before and after 30 minutes of ultrasonic treatment was measured, and the coating adhesion retention rate (%) was calculated. The sample membranes were loaded into a cross-flow evaluation device, and the pure water flux was tested at 0.15 MPa, and the initial pure water flux retention rate (%) was calculated.

[0066] Test results are as follows Figure 3As shown, the coating adhesion retention rate was low in the short reaction time range, indicating that a sufficiently dense polydopamine anchoring network could not be formed within the short deposition time, resulting in insufficient physical stability of the coating under water flow shear. In the long reaction time range, as the immersion time continued to increase, the coating adhesion retention rate did not significantly improve after reaching the plateau region, but instead showed a certain degree of decline. The extended reaction time led to excessive deposition of polydopamine on the film surface and triggered large particle agglomeration in the solution phase. This thick deposition layer caused a significant increase in the average surface roughness (reaching 112.5 nm at 8.0 h), which not only created additional water flow mass transfer obstacles within the coating, causing a significant decline in the initial pure water flux retention rate, but also weakened the overall structural stability due to the weak cohesion within the large particle stack layer.

[0067] By normalizing and weighted fitting analysis of three core performance indicators—average surface roughness, coating adhesion retention rate, and initial pure water flux retention rate—the theoretical equilibrium point for the system's impregnation and deposition performance was calculated to be 3.95 h. Considering the production cycle scheduling patterns, fluid displacement operation time tolerance, and robustness of continuous multi-batch production during industrial-scale dynamic cyclic coating, an integer value that is close to this theoretical equilibrium point and easy to operate and control was finally selected; that is, the impregnation reaction time for dopamine substrate treatment was determined to be 4.0 h.

[0068] 3.3 Temperature Optimization of the Reaction System Temperature directly affects the oxidation rate of catechol groups in dopamine molecules and the Brownian motion of polymer chain segments in the liquid phase. If the reaction temperature is too low, the polymerization reaction is slow, making it difficult to form a dense anchoring network within the set time. If the reaction temperature is too high, it not only increases the energy consumption for temperature control in large-scale production but also causes dopamine to undergo excessively rapid disordered self-aggregation in the solution phase, forming large agglomerates that deposit on the membrane surface, leading to increased roughness and permeation resistance. This experiment aims to find a suitable balance between reaction efficiency, coating quality, and operating energy consumption through multidimensional data evaluation.

[0069] The concentration of dopamine hydrochloride was fixed at 1.5 g / L, and the impregnation reaction time was uniformly set at 4.0 h. The reaction system temperature was set from 10℃ to 45℃ in 5℃ increments. Standard polyamide reverse osmosis membrane sheets were immersed in the above precursor solution and placed in a constant-temperature shaker at the corresponding set temperature for gentle impregnation. After the reaction, the membrane sheets were removed and continuously rinsed with deionized water at a constant flow rate of 1 L / min for 5 minutes. Subsequently, all membrane samples were uniformly coated and crosslinked using the previously optimized main material and crosslinking agent parameters (polyvinyl alcohol 4.0 g / L, glutaraldehyde 0.8 g / L). The average surface roughness (Ra, unit: nm), coating adhesion retention rate (%), and initial pure water flux retention rate (%) were measured.

[0070] Test results are as follows Figure 4 As shown, the coating adhesion retention rate was low in the low-temperature range, indicating that the lower temperature limited the oxidative polymerization kinetics of dopamine, failing to form a sufficiently cross-linked anchoring network within the predetermined 4 hours, resulting in relatively weak physical stability of the coating under water shear. Conversely, in the high-temperature range, the coating adhesion retention rate did not further improve with the continuous increase of the reaction system temperature, but instead showed a gradual decline. The higher thermodynamic environment caused dopamine to undergo violent and uncontrollable rapid aggregation in the solution phase, resulting in a large amount of large polymer particles deposited on the film surface. This disordered stacking significantly increased the average surface roughness (reaching 125.4 nm at 45 °C), not only constructing a large water molecule mass transfer barrier within the modified layer (flux retention rate decreased to 61.3%), but also weakening the overall cohesive adhesion of the coating due to the loose interparticle structure.

[0071] By normalizing and weighted fitting analysis of three core performance indicators—average surface roughness, coating adhesion retention rate, and initial pure water flux retention rate—the theoretical equilibrium point for the interfacial polymerization reaction was calculated to be 26.5℃. Considering that in industrial-scale production, operating at ambient temperature (typically between 20 and 25℃) eliminates the need for additional heating or cooling equipment, and that the dopamine reaction rate is stable and controllable within this temperature range, which is beneficial for the robustness of continuous multi-batch production, a value close to the theoretical equilibrium point and with energy-saving advantages was ultimately selected: the reaction system temperature for dopamine substrate treatment was determined to be 25℃.

[0072] 3.4 Optimization of buffer pH Having established the feed concentration (1.5 g / L), impregnation time (4.0 h), and reaction temperature (25 °C) for dopamine hydrochloride, this experiment aimed to optimize the pH value of the buffer solution—a key chemical microenvironment parameter in the substrate treatment process. The rate of oxidation and deprotonation of the catechol groups in the dopamine molecule is highly dependent on the pH of the solution. If the pH of the reaction system is too low, the oxidation kinetics of dopamine are suppressed, resulting in a slow film formation rate and difficulty in forming a dense cross-linked network. If the pH is too high, the self-polymerization rate of dopamine in the liquid and bulk phases will accelerate, generating a large number of disordered polymer particles, which not only increases surface roughness and water flow resistance, but also poses a risk of slight hydrolysis of the polyamide desalination layer due to an excessively alkaline environment. This experiment aimed to find a suitable balance between reaction kinetics and film uniformity through multidimensional data evaluation. In this experiment, the reaction medium was fixed as Tris buffer (50 mmol / L), the dopamine hydrochloride concentration was fixed at 1.5 g / L, the immersion reaction time was 4.0 h, and the temperature was constant at 25 °C. The initial pH of the buffer was set from 7.3 to 9.4 in steps of 0.3.

[0073] The cut standard polyamide reverse osmosis membrane sheets were immersed in the above-mentioned precursor solution and subjected to gentle impregnation in a shaker. After the reaction, the membrane sheets were removed and rinsed continuously for 5 minutes with deionized water at a constant flow rate of 1 L / min. Subsequently, all sample membranes were uniformly coated and crosslinked using the previously optimized main material and crosslinking agent parameters (polyvinyl alcohol 4.0 g / L, glutaraldehyde 0.8 g / L). The average surface roughness (Ra, unit: nm), coating adhesion retention rate (%), and initial pure water flux retention rate (%) were measured.

[0074] Test results are as follows Figure 5 As shown, the coating adhesion retention rate was low in the slightly alkaline range (pH 7.3–7.6), indicating that the oxidation and deprotonation rate of dopamine was slow at this pH, failing to polymerize significantly within the predetermined 4-hour timeframe. The resulting anchoring network was sparse, exhibiting relatively weak physical resistance to peeling under water shear. Conversely, in the higher alkaline range (pH 8.8–9.4), the coating adhesion retention rate gradually declined after reaching a plateau as the system pH continued to rise. The strongly alkaline environment accelerated the dopamine oxidation process, shifting the reaction focus from "interfacial deposition" to "bulk aggregation," resulting in a large number of loose particles stacked disorderly on the film surface. This structure not only significantly increased the average surface roughness (reaching 124.6 nm at pH 9.4), causing substantial steric hindrance in mass transfer and reducing the flux retention rate to 52.3%, but also weakened the overall physical stability of the coating due to the weak cohesive forces between particles.

[0075] By normalizing and weighted fitting analysis of three core performance indicators—average surface roughness, coating adhesion retention rate, and initial pure water flux retention rate—the theoretical comprehensive equilibrium point of the system's acid-base buffer microenvironment was calculated to be 8.18. Considering that this theoretical value is highly consistent with the effective buffer zone and conventional calibration value of Tris buffer (pKa approximately 8.06) during industrial solution preparation, and that the structural stability of the polyamide-based membrane can be well guaranteed at this pH, a value close to this theoretical equilibrium point and taking into account conventional reagent preparation practices was selected; that is, the pH of the substrate treatment buffer was determined to be 8.2.

[0076] 3.5 Optimization of surface rinsing flow rate after reaction After dopamine self-polymerization, the membrane surface simultaneously exhibits an anchoring layer firmly bound by covalent and non-covalent bonds, as well as bulk aggregated particles held by weak physical adsorption. If the rinsing flow rate is too low, insufficient fluid shear force makes it difficult to effectively remove loose, large free particles, easily leading to high coating roughness and pore blockage. If the rinsing flow rate is too high, excessive hydraulic shear may strip away the not-yet-fully-stabilized effective cross-linked network, resulting in insufficient anchoring points for subsequent functional layers. This experiment quantitatively evaluates and explores the appropriate balance between fluid shear cleaning capacity and anchoring layer structure retention. 1.5 g / L of dopamine hydrochloride was added to a 50 mmol / L Tris buffer solution at pH 8.2, and the mixture was incubated at 25°C for 4.0 h. Eight flow rate gradient experimental groups were established, with deionized water surface rinsing flow rates ranging from 0.4 L / min to 1.8 L / min in 0.2 L / min increments. The rinsing time was uniformly set to 5 minutes.

[0077] The cut standard polyamide reverse osmosis membrane sheets were subjected to the aforementioned substrate reaction and corresponding flow rate rinsing procedures. Subsequently, all membrane samples were uniformly coated with the functional layer and cross-linked cured using pre-optimized parameters (polyvinyl alcohol 4.0 g / L, glutaraldehyde 0.8 g / L). During the testing phase, the modified membrane surface was scanned using atomic force microscopy (AFM) to measure the average surface roughness (Ra, unit: nm). Referring to the pre-defined ultrasonic vibration evaluation method, the atomic molar ratio of characteristic elements before and after 30 minutes of ultrasonic treatment was measured, and the coating adhesion retention rate (%) was calculated. The membrane samples were loaded into a cross-flow evaluation device, and the pure water flux was tested at 0.15 MPa, and the initial pure water flux retention rate (%) was calculated.

[0078] Test results are as follows Figure 6As shown, the membrane surface morphology and coating stability exhibit significant range-dependent responses to the flushing hydrodynamics. In the low-flow-rate flushing range, the parallel shear force of the water flow is weak, failing to effectively wash away the weakly adsorbed polydopamine bulk agglomerates at the interface. These residual loose large particles maintain a high average surface roughness; simultaneously, because the subsequent functional layers are partially anchored to these easily detachable particles, the overall coating adhesion retention rate is generally poor. As the flow rate increases to the high-shear range of 1.6–1.8 L / min, the surface roughness decreases significantly and the flux retention rate rebounds sharply. This indicates that the high-intensity water flow flushing removes almost all bulk deposits. However, the excessive fluid shear force also disrupts the relatively stable effective dopamine network originally bound to the membrane surface, leading to a significant loss of substrate anchoring points. Macroscopically, this manifests as a significant decline in coating adhesion retention rate after ultrasonic vibration testing (dropping to 65.2% at 1.8 L / min), losing the anti-peeling capability required for long-term industrial operation.

[0079] By normalizing and weighted fitting analysis of three core performance indicators—average surface roughness, coating adhesion retention rate, and initial pure water flux retention rate—the theoretical comprehensive equilibrium point between the system's interface cleaning efficiency and structure maintenance was finally calculated to be 1.04 L / min. Considering the flow control accuracy range of the cleaning water pump in industrial-scale production, the hydraulic fluctuation compensation factors at the pipeline end, and the environmental protection requirements for water conservation and energy saving, a nominal value that is close to this theoretical equilibrium point and easy to set for equipment control was finally selected, namely, the surface rinsing flow rate after substrate treatment was determined to be 1.0 L / min.

[0080] Example 4 refer to Figure 7 This is the fourth embodiment of the present invention. This embodiment aims to fully demonstrate the entire process of preparing a high-performance composite water purification membrane using the determined suitable ratios and optimized process parameters, and to comprehensively test the physicochemical properties and long-term separation stability of the final product. To objectively verify the effectiveness and synergistic effect of the full-process optimization scheme of the present invention, we systematically compared the performance of the final prepared sample with four representative comparative examples. This ensures that, under standard preparation conditions, the composite membrane optimized through multi-dimensional processes possesses excellent potential for industrial application in terms of antifouling ability, structural durability, and permeability.

[0081] First, the substrate anchoring layer was treated. Deionized water was added to a reaction vessel, along with 50 mmol / L of Tris buffer, and the pH was adjusted to 8.2. Dopamine hydrochloride was slowly added at a concentration of 1.5 g / L. A standard polyamide reverse osmosis membrane was immersed in this solution and reacted at 25°C for 4.0 hours. After the reaction was complete, the membrane surface was continuously rinsed with deionized water at a rate of 1.0 L / min for 5 minutes. Subsequently, the functional layer precursor solution was prepared and coated. In another reaction vessel, a polymer aqueous solution containing 4.0 g / L polyvinyl alcohol was prepared. After cooling to room temperature, 0.8 g / L glutaraldehyde crosslinking agent was added, and the mixture was homogenized for 30 minutes. The substrate-treated membrane was placed in a dynamic coating system and coated using the precursor solution at 0.15 MPa for 1.5 hours. Finally, the coated membrane was placed in a 45°C, clean air environment and allowed to cure for 2 hours to complete the cross-linking and condensation of the polymer network, thus obtaining the final composite modified water purification membrane.

[0082] To verify the results, four comparison models were set up simultaneously: Comparative Example 1 (Blank Control Group): Untreated polyamide film. Comparative Example 2 (No Substrate Group): Polyvinyl alcohol and glutaraldehyde were directly coated and cured on the original film without using dopamine as a primer. Comparative Example 3 (Conventional Coarse Process Group): Unoptimized conventional parameters were used (dopamine 3.0 g / L, reaction time 8 hours; polyvinyl alcohol 8.0 g / L, glutaraldehyde 1.8 g / L coating). Comparative Example 4 (Single-Layer Substrate Group): Only dopamine substrate treatment and rinsing were performed; no subsequent polyvinyl alcohol functional layer coating was applied. Specific test data are shown in the table below:

[0083] By comparing the above test data, the modified membrane prepared by the fully optimized process described in this invention exhibits significant synergistic advantages in various comprehensive properties.

[0084] Comparing Example 4 with Comparative Example 1, it can be seen that the membrane treated by the present invention has a significantly reduced pure water contact angle. Although a small portion of the initial water flux is sacrificed (retention rate 85.4%), the dynamic fouling flux recovery rate is increased from 55.4% to 92.6%, which improves the problem of easy fouling and clogging of the original membrane under complex water quality.

[0085] Comparative Example 2 (without substrate) revealed significant structural defects. It lacked a dopamine anchoring network, and only weak physical adsorption existed between the hydrophilic layer and the substrate. After ultrasonic vibration and fluid scouring, both the coating adhesion retention rate and thickness retention rate decreased significantly, resulting in a loss of long-term antifouling performance.

[0086] Comparative Example 3 (conventional, extensive process) demonstrates the negative effects of unoptimized parameters. Excessive concentration and time accumulation resulted in an overly dense coating with a rough, uneven surface. Its pure water flux retention rate significantly decreased to 45.3%, and due to the disordered aggregation of large particles, its antifouling ability (recovery rate 60.2%) did not reach the expected level. This confirms the engineering value of the parameter optimization and balancing method of this invention.

[0087] Comparative Example 4 (single-layer substrate) exhibited good adhesion, but the hydrophilic antifouling barrier provided by the single polydopamine layer was relatively limited (recovery rate 75.8%). Example 4 introduced a cross-linked polyvinyl alcohol network, further improving the antifouling performance to a better range without significantly increasing mass transfer resistance.

[0088] In summary, this invention, by systematically establishing materials at each level and precisely controlling fluid dynamics and chemical crosslinking parameters, achieves a coating layer with excellent physical stability and long-lasting antifouling ability while maintaining controllable permeation flux concession, demonstrating promising prospects for industrial applications.

[0089] In summary, the composite modification system, based on the physical anchoring network constructed by dopamine oxidative self-polymerization and supplemented by the hydrophilic barrier constructed by polyvinyl alcohol acetalization crosslinking, significantly improves the coating's resistance to water flow shear and long-term antifouling performance while maintaining the original membrane's basic separation efficiency. It overcomes the limitations of traditional coating processes, which often lead to easy peeling of the modified layer and excessive network density causing a sharp decline in flux. This allows the composite coating layer to achieve stable bonding at the substrate interface and exhibit excellent hydration repulsion and fouling flux recovery capabilities on the outer surface, broadening its application window in complex operating conditions. Furthermore, the gentle dynamic cross-flow coating effectively mitigates disordered aggregation on the membrane surface, and the optimized process parameters retain appropriate hydration porosity of the polymer network, resulting in good structural durability and overall permeability performance in long-term operation.

[0090] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A modification process for coating the surface of a water purification membrane, characterized in that... Includes the following steps: S1. The polyamide-based membrane is immersed in a buffer modification solution containing 30-70 mmol / L of tris(hydroxymethyl)aminomethane and 1.0-2.0 g / L of dopamine hydrochloride. The pH of the system is adjusted and a mild impregnation reaction is carried out under constant temperature conditions. The complete polydopamine substrate anchoring layer is constructed on the membrane surface by the oxidative self-polymerization of dopamine. After the S2 substrate treatment, the membrane surface was continuously and parallelly rinsed with deionized water to remove free aggregated particles that were not physically adsorbed. Then, a mixed coating precursor solution was prepared by adding 3.0~5.0 g / L of polyvinyl alcohol and 0.5~1.0 g / L of glutaraldehyde to the deionized water and continuously stirring to homogenize the system to achieve microscopic uniformity and stability. S3 places the polyamide-based film that has completed substrate rinsing into a dynamic cross-flow coating system. Under a certain operating pressure, the mixed coating precursor solution is pumped in for dynamic circulation coating, so that the hydrophilic macromolecules can be fully spread on the surface of the base coating and form initial adhesion. After the S4 cycle coating is completed, the precursor residue in the system is emptied, and mild clean air is introduced into the membrane surface for continuous purging and curing. Through thermal drive, the crosslinking agent and the main polymer undergo a deep acetalization reaction, resulting in a dense and locked coating network, thus producing an antifouling composite modified water purification membrane.

2. The modification process for coating the surface of a water purification membrane according to claim 1, characterized in that, In step S1, the concentration of tris(hydroxymethyl)aminomethane is 50 mmol / L; the concentration of dopamine hydrochloride is 1.5 g / L; in step S2, the concentration of polyvinyl alcohol is 4.0 g / L; and the concentration of glutaraldehyde is 0.8 g / L.

3. The modification process for coating the surface of a water purification membrane according to claim 1, characterized in that, In step S1, the pH value of the system is 7.8~8.5; the constant temperature condition is 20~30℃; and the mild impregnation reaction time is 3.0~5.0 hours.

4. The modification process for coating the surface of a water purification membrane according to claim 3, characterized in that, In step S1, the pH value of the system is 8.2; the constant temperature condition is 25°C; and the mild impregnation reaction time is 4.0 hours.

5. The modification process for coating the surface of a water purification membrane according to claim 1, characterized in that, In step S2, the deionized water rinsing flow rate is 0.8~1.2L / min; the continuous parallel rinsing time is 4~6 minutes; and the continuous stirring and homogenization time is 20~40 minutes.

6. The modification process for coating the surface of a water purification membrane according to claim 5, characterized in that, In step S2, the deionized water rinsing flow rate is 1.0 L / min; the continuous parallel rinsing time is 5 minutes; and the continuous stirring and homogenization time is 30 minutes.

7. The modification process for coating the surface of a water purification membrane according to claim 1, characterized in that, The operating pressure in step S3 is 0.10~0.20MPa; the dynamic cyclic coating time is 1.0~2.0 hours.

8. The modification process for coating the surface of a water purification membrane according to claim 7, characterized in that, The operating pressure in step S3 is 0.15 MPa; the dynamic cyclic coating time is 1.5 hours.

9. The modification process for coating the surface of a water purification membrane according to claim 1, characterized in that, The temperature of the mild clean air in step S4 is 40~50℃; the continuous purging and curing time is 1.5~3.0 hours.

10. The modification process for coating the surface of a water purification membrane according to claim 1, characterized in that, The temperature of the mild clean air in step S4 is 45°C; the purging and curing time is 2 hours.

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